Soap Calculator — Free NaOH & KOH Lye & Soap Making Studio
The most complete free soap calculator: compute saponification lye amounts, water ratios, superfat, fatty acid profiles, batch sizes, fragrance loads, and profit margins for cold process, hot process, and liquid soap — all in one tool, instant, no sign-up.
Candle Making Calculator — Fragrance & Scent Guide
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Soap Mold Size Calculator
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How to Use the Soap Calculator
Using our soap calculator is straightforward, even for beginners. Follow these steps to generate a safe, balanced soap recipe in under two minutes. For a deeper understanding of the math, see the saponification guide below.
Choose your soap type. Select Bar Soap (NaOH) for a traditional solid bar, or Liquid Soap (KOH) for a soft or pourable soap. The calculator automatically switches to the correct lye type and saponification values.
Add your oils. Type any oil name in the search box — the calculator includes 137+ soap making oils with accurate SAP values. Enter the weight and click Add Oil. Repeat for each oil in your recipe. See the oil properties guide to understand what each oil contributes.
Set your superfat. Use the superfat slider to choose how much oil will remain unsaponified. 5% is ideal for most cold process recipes — it creates a safety buffer and adds conditioning without significantly reducing hardness.
Adjust the water ratio. The water slider controls how much water you'll use relative to your oils. 38% is a reliable default. Lower ratios (33–36%) produce faster-tracing soap that unmolds sooner.
Read your results. Instantly see your NaOH or KOH amount, water amount, total batch weight, lye concentration, fatty acid profile, and quality ratings for cleansing, hardness, conditioning, and lather.
Save or share your recipe. Click Save Recipe to store it in your browser, or Share to generate a URL you can send to anyone. Switch to Cost & Profit, Batch Scaler, or Fragrance tabs to finish planning your batch.
Soap Oil Properties & Fatty Acid Guide
Each oil contributes different properties to soap based on its fatty acid composition. Lauric and myristic acids (found in coconut and palm kernel) create hard bars with big, bubbly lather. Oleic acid (olive, avocado, almond) produces a moisturizing, creamy lather. Ricinoleic acid — unique to castor oil — boosts both lather stability and conditioning. See the FAQ for guidance on which oils to combine for your goals.
Common Soap Making Oils — Saponification Values & Properties
Oil
NaOH SAP
Hardness
Cleansing
Conditioning
Best for
Coconut Oil
0.190
High
High
Low
Lather & hardness
Olive Oil
0.134
Low
Low
High
Conditioning castile
Palm Oil
0.141
Good
Low
Med
Hardness & stable lather
Castor Oil
0.129
Low
Low
High
Lather booster (max 10%)
Shea Butter
0.128
Med
Low
Good
Conditioning & hardness
Lard / Tallow
0.138–0.140
Good
Low
Med
Hard, creamy lather bar
Sweet Almond Oil
0.136
Low
Low
Good
Skin-gentle bars
Avocado Oil
0.133
Low
Low
Good
Luxury conditioning
Cocoa Butter
0.137
Good
Low
Med
Hardness & moisturizing
Canola Oil
0.124
Low
Low
Good
Budget conditioning
SAP values are grams of lye per gram of oil for NaOH. Multiply by 1.403 for KOH at 100% purity. All values sourced from established saponification tables.
Understanding Saponification
Saponification is the chemical reaction between a fat (or oil) and an alkali (NaOH or KOH) that produces soap and glycerin. Every triglyceride molecule in the oil reacts with three molecules of sodium or potassium hydroxide to yield three fatty acid salt molecules (soap) and one glycerol molecule.
The saponification value (SAP value) of an oil is the number of milligrams of KOH required to completely saponify one gram of that oil. This calculator converts those values to usable gram weights of NaOH or KOH. Because every oil has a different fatty acid composition, each has a unique SAP value — coconut oil's high lauric acid content gives it a much higher SAP (0.190) than olive oil (0.134).
The superfat (lye discount) in the formula below ensures a small safety margin of unsaponified oil remains in the finished bar, both to prevent lye-heavy soap and to add conditioning free fatty acids:
This is the exact formula this soap calculator uses. The step-by-step guide above walks you through entering it for your recipe.
Candle Scents & Candle Making Calculator Guide
Our candle making calculator helps you compute exact fragrance amounts for any wax weight, blend multiple candle scents, estimate burn time, and calculate cost per candle — all in one tool. Use the Candle Maker tab above to get started.
How Much Fragrance Do I Add to Candles?
The right amount of candle fragrance oil depends on your wax type. The standard rule is:
Fragrance (g) = Wax weight (g) × Fragrance load %
For example, a 500 g soy wax candle at 8% fragrance load needs 40 g of fragrance oil. Recommended fragrance loads by wax:
Gourmand: Caramel, Coffee, Baked Goods, Coconut — sweet and indulgent
Use our candle calculator's Scent Blend section to combine multiple fragrances and see exact gram amounts for each scent in your blend.
Fatty Acid Profiles in Soap Making: What Each Acid Does
Every soap making oil is a blend of fatty acids — the building blocks that determine how your finished bar looks, feels, and lathers. Understanding fatty acid profiles is the foundation of advanced soap formulation. Our lye calculator displays the complete fatty acid breakdown for every recipe you build.
Saturated Fatty Acids — Hardness & Lather
Lauric acid (C12) is the primary contributor to hard, white bars with big, fluffy, fast-rising lather. Found at 48% in coconut oil and 44% in babassu oil, lauric acid is the reason high-coconut recipes trace quickly and produce abundant bubbles. Overuse (above 30% of recipe) can cause dryness due to its strong cleansing action.
Myristic acid (C14) works synergistically with lauric acid to boost cleansing and lather. Found at 16–19% in coconut and palm kernel oils, it contributes to a hard bar that lathers well even in cold water.
Palmitic acid (C16) adds long-lasting hardness and stable, creamy lather without the harshness of lauric acid. Palm oil (44% palmitic), lard (26%), and tallow (26%) are classic sources. Bars high in palmitic acid resist sweating and have excellent longevity.
Stearic acid (C18 saturated) is the hardest of the common fatty acids. Kokum butter (56%), sal butter (44%), illipe butter (44%), and shea butter (42%) are the richest sources. Stearic acid creates dense, long-lasting bars with stable, creamy lather and no sticky or waxy feel.
Oleic acid (C18:1, omega-9) is the primary conditioning fatty acid in soap. It produces a creamy, moisturizing lather and leaves a silky feel on skin. Olive oil (75% oleic), hazelnut oil (78% oleic), camellia oil (80% oleic), and pataua oil (76% oleic) are excellent oleic sources. High-oleic recipes cure slower but produce some of the most skin-friendly soaps.
Linoleic acid (C18:2, omega-6) is a polyunsaturated fatty acid that contributes lightweight, fast-absorbing conditioning and is particularly beneficial for sensitive, acne-prone, or dry skin. Rich sources include grapeseed oil (72%), safflower oil (75%), and evening primrose oil (72%). Linoleic-rich soaps have shorter shelf lives due to oxidation risk — use antioxidants like rosemary extract (ROE) and keep batches small.
Linolenic acid (C18:3, omega-3) appears in flaxseed (55%), chia seed oil (63%), kiwi seed oil (57%), and perilla oil (57%). While excellent for skin, it oxidizes rapidly and can cause DOS (dreaded orange spots) if used above 5–10% of recipe. Pair with a full water discount and antioxidants.
Ricinoleic acid is unique to castor oil (~90%). It is simultaneously a humectant, lather booster, and conditioning agent. Even at 3–10% of a recipe, castor oil visibly improves lather quality and creaminess. It also attracts moisture from the air to skin — making it genuinely multi-functional.
Balanced Fatty Acid Targets for Cold Process Soap
Hardness (palmitic + stearic + lauric + myristic): 29–54% for bars that unmold cleanly and last
Cleansing (lauric + myristic): 12–22% for good lather without stripping skin
This soap calculator displays all five quality ratings in real time as you add oils, so you can see exactly where your recipe stands against these targets.
Lye Safety: Handling NaOH and KOH Safely
Lye (NaOH or KOH) is a strongly caustic alkali. When dissolved in water, it releases significant heat and produces caustic solution capable of causing serious chemical burns. This guide covers essential lye safety practices for soap makers at every level.
Personal Protective Equipment (PPE)
Safety goggles (not just glasses) — lye splashes can cause permanent eye damage within seconds
Nitrile or rubber gloves — lye rapidly deteriorates skin on contact
Long sleeves & closed-toe shoes — protect arms, legs, and feet from spills
Work in a ventilated area — dissolving lye in water releases fumes; avoid inhaling
Dissolving Lye Safely: Always Add Lye to Water
The universal rule: always add lye to water, never water to lye. Adding water to dry lye causes a violent, explosive heat reaction. Add lye slowly to a full measure of cold or room-temperature water (or frozen milk for milk soaps), stirring continuously. The solution will heat to 60–90°C — use a heat-safe container (HDPE plastic, stainless steel, or glass). Allow it to cool to 40–50°C before combining with oils.
Storage & Disposal
Store dry lye in airtight containers away from moisture — it is hygroscopic and will absorb water from the air, forming a crust and losing potency. Lye solutions should never be stored — mix only what you need per batch. Dispose of small lye spills by neutralizing with white vinegar, then rinse with plenty of water.
Is Lye Still Present in Finished Soap?
No. Properly made soap with a correct lye calculation contains zero free lye. The saponification reaction consumes all lye, converting oils and alkali into soap salts (fatty acid salts) and glycerin. A superfat of 5–8% provides an additional safety buffer of excess unsaponified oil. You can verify a finished bar with a zap test (touch the tip of your tongue — a tingle like licking a battery indicates residual lye) or use phenolphthalein pH drops. See our Safety Disclaimer for full guidance.
Soap Making Methods: Cold Process, Hot Process & Liquid Soap
All soap making methods use the same lye calculation — the differences are in technique, cure time, and final texture. Understanding each method helps you choose the right approach for your recipe and goals.
Cold Process Soap (CP)
Cold process is the most common technique for handcrafted bar soap. Oils and lye solution are combined at around 40–50°C (or room temperature for "room temperature CP"), then stick-blended to trace before pouring into a mold. Saponification continues over 24–48 hours in the mold (gel phase), and the soap is cured for 4–6 weeks before use. The extended cure allows water to evaporate, producing a harder, milder bar. Cold process preserves the most glycerin and allows maximum creative freedom — swirls, layers, embeds, and natural colorants all work best at light trace.
Hot Process Soap (HP)
Hot process uses external heat (slow cooker, oven, or stovetop) to accelerate saponification. The batter is cooked until saponification completes — typically 1–2 hours — then pressed into a mold while still hot and pliable. HP soap can be used within 1–2 weeks (rather than 4–6), but the cooked texture is rougher and less suitable for intricate designs. HP is preferred for recipes with high water content, infused liquids (coffee, beer), or fragrance oils known to accelerate trace. The lye calculation is identical to cold process.
Liquid Soap (KOH)
Liquid soap uses KOH instead of NaOH. KOH produces a softer, more water-soluble soap paste that can be diluted to any consistency from thick body wash to thin hand soap. KOH is typically sold at 90% purity — our calculator corrects for this automatically. Liquid soap requires a full cook (hot process), and the paste is then diluted with distilled water (typically 1 part paste to 2–3 parts water) and optionally preserved and scented after dilution. Use our Lye Calculator set to KOH mode for liquid soap recipes.
Melt & Pour Soap (No Lye Required)
Melt and pour (M&P) uses a pre-made soap base that is melted, scented, colored, and poured. No lye handling is required, making it ideal for beginners and children's projects. However, M&P bases contain additives (surfactants, preservatives, humectants) that affect feel and quality. Our soap calculator is designed for from-scratch soap making — M&P bases do not require a lye calculation.
Frequently Asked Questions
A soap calculator computes the exact amount of lye (sodium hydroxide for bar soap or potassium hydroxide for liquid soap) needed to saponify your chosen oils. It uses saponification values (SAP values) unique to each oil to ensure your recipe is safe, properly balanced, and customized to your desired superfat level.
The lye amount depends on which oils you use and your desired superfat. Each oil has a unique SAP value — for example, 500g of olive oil at 5% superfat requires approximately 63.65g of NaOH. Use this soap calculator: enter your oils and weights, set your superfat, and the lye and water amounts are calculated instantly.
Superfat (also called lye discount) is the percentage of oils intentionally left unsaponified in your soap. A 5% superfat means 5% of the oils remain as free conditioning oils. Most cold process soaps use 5–8% superfat. It provides a safety margin against lye-heavy soap and adds skin-conditioning properties, though higher superfat can reduce hardness and shelf life.
Yes — finished, properly cured soap contains zero free lye. During saponification, the lye chemically reacts with the oils to form soap (sodium or potassium salts of fatty acids) and glycerin. Once the reaction is complete, no lye remains. Always handle raw lye with proper safety gear, but well-made soap is completely safe for skin.
NaOH (sodium hydroxide) produces hard bar soap. KOH (potassium hydroxide) produces soft or liquid soap. KOH creates a softer, more water-soluble soap molecule. KOH is typically sold at 90% purity, so the calculator applies a purity correction. Both require different SAP values — our calculator handles both automatically when you toggle soap type.
Cold process soap typically uses 2–3% fragrance oil by total oil weight. For example, 500g of oils needs 10–15g of fragrance. Always verify the supplier's maximum usage rate and IFRA safety data for each fragrance. Use the Fragrance Calculator tab to compute precise amounts and split multiple scents by percentage.
Yes — the lye calculation is identical for both cold process and hot process soap. The same NaOH amount, superfat, and water ratio apply regardless of the method. Hot process accelerates saponification with heat (slow cooker, oven, or stovetop), allowing the soap to be used sooner, but the underlying chemistry and lye amounts are the same.
A beginner-friendly recipe is 30% coconut oil, 30% palm oil (or lard), 30% olive oil, and 10% castor oil. Coconut provides cleansing and lather, palm/lard adds hardness and stable lather, olive gives conditioning, and castor boosts lather. Try 500g total oils, 5% superfat, 38% water ratio in this calculator to get started. See the oil properties guide for more detail.
Cold process soap is mixed at room temperature and left to saponify over 24–48 hours before unmolding, then cured for 4–6 weeks. Hot process soap is cooked (slow cooker, oven, or stovetop) to accelerate saponification — usable sooner but harder to swirl or layer. The same lye calculation in this soap calculator applies to both. See Understanding Saponification for the chemistry.
A water discount (also called water reduction) means using less water than the full amount in your lye solution. The standard water ratio is about 38% of total oil weight, giving roughly 33% lye concentration. Reducing water to 30–33% of oils produces a higher lye concentration (36–40%), which accelerates trace, speeds unmolding, and reduces gel phase risk — especially useful in hot climates or when using fragrance oils that accelerate. This calculator's water ratio slider lets you dial in your exact water discount.
Yes. Milk (goat, cow, coconut, oat), teas, herbal infusions, aloe vera juice, and coffee can replace all or part of the water in your lye solution. The lye amount stays exactly the same — the lye calculation only changes based on oils. For milk soaps, freeze the milk and add lye to frozen milk slowly to prevent scorching sugars. Coffee and tea add antioxidants and natural colorants. Always calculate with this soap calculator using your oil weights, then substitute your liquid of choice for the water amount shown.
Trace is the point in cold process soap making when the oils and lye solution have emulsified enough that the mixture thickens and holds a drizzle mark on its surface — like a light pudding. Light trace is ideal for detailed swirls; medium trace works for layers; heavy trace is needed for embeds or peaks. Trace is reached faster with stick blenders, high-stearic oils (tallow, lard), certain fragrance oils, or warm temperatures. Castor oil and soft oils like sunflower slow trace.
Soda ash is a white, powdery or feathery layer that forms on the surface of cold process soap when unsaponified soap molecules react with carbon dioxide in the air. It is harmless and doesn't affect the soap's quality. To prevent it: pour at a warmer trace, use sodium lactate, cover the mold with plastic wrap, or insulate to encourage gel phase. Soda ash washes off with the first use or can be removed by steaming, planing, or spraying with 91% isopropyl alcohol immediately after pour.
Conditioning comes from oleic, linoleic, linolenic, and ricinoleic fatty acids. Olive oil (75% oleic), avocado oil (70% oleic), sweet almond oil (69% oleic), and hazelnut oil (78% oleic) are top conditioning oils. Hemp seed oil (57% linoleic, 20% linolenic) and rosehip oil add polyunsaturated acids that are deeply nourishing. Castor oil contributes ricinoleic acid, which is both conditioning and boosts lather. Aim for a conditioning score of 44–69 using this calculator's quality ratings. See the oil properties guide for a comparison.
Measure your mold's interior volume in cubic inches (length × width × depth). Multiply by 0.4 to get approximate oil weight in ounces, or multiply cubic inches by 11.35 to get grams of oil. For example, a 9″ × 3.5″ × 2.5″ loaf mold = 78.75 cubic inches × 11.35 ≈ 894g of oils. Enter that oil weight into this calculator, distribute across your chosen oils, and the lye and water amounts calculate automatically. The Batch Scaler tab can then scale any saved recipe up or down to match your mold exactly.
Most cold process soap makers use a lye concentration between 30–40%. The classic 38% water ratio produces around 33% lye concentration. A 33% water ratio gives roughly 37% lye concentration — this speeds unmolding, reduces water weight, and minimizes DOS (dreaded orange spots) by limiting excess water. For milk soaps and fragrances that accelerate, lower concentration (30–33%) gives more working time. This soap calculator shows lye concentration automatically — adjust the water ratio slider to see how it changes.
Yes — 100% coconut oil soap is a classic, called "shampoo bar" or "Grandma's Lye Soap." It produces a very hard, white bar with excellent cleansing and lather. However, it can be drying for some skin types due to high lauric acid. To compensate, use a higher superfat of 15–20% (vs the usual 5%) when making 100% coconut soap — this leaves enough free oil to condition the skin. Try it in this calculator: add Coconut Oil (76°) at 500g, set superfat to 20%, and see how the quality ratings change.
Saponification is an exothermic reaction — it generates heat. "Volcanoeing" or "overheating" happens when the soap mass reaches temperatures above 82°C (180°F), causing it to bubble, erupt from the mold, and develop a translucent, sticky gel with a harsh lye-heavy surface. It's common with high-sugar additives (honey, milk, beer), insulating the mold too much, or pouring at high temperature. Prevention: pour at room temperature, don't insulate sugary batches, use a water discount, and soap in a cooler environment. The soap is usually still usable after full cure.
Yes. Popular natural colorants include: activated charcoal (black), turmeric (yellow-gold), spirulina (green), madder root (pink-red), indigo powder (blue-grey), rose clay (pink), French green clay (green), paprika (orange-tan), and cocoa powder (brown). Note that plant-based colorants can morph or fade in the high-pH environment of cold process soap — some work better in hot process or melt-and-pour. Micas and iron oxides (technically mineral pigments) are widely used and give consistent, stable color results in cold process.
A masterbatch is a large, pre-mixed batch of lye solution (typically 50% lye concentration: equal weights of lye and water) stored safely for future use. Instead of mixing lye fresh each time, you dilute the masterbatch with additional water to your target concentration. This saves time, reduces fume exposure, and ensures consistent results. To use: calculate your full lye amount with this soap calculator, mix at 50% concentration, store in labeled HDPE or glass containers, and dilute before use. Always label with "DANGER: LYE SOLUTION" and keep out of reach of children.
Handmade cold process soap retains naturally-occurring glycerin (a humectant that attracts moisture to skin), which is typically removed from commercial soap for use in lotions. Commercial "soap" is often a synthetic detergent bar (sodium lauryl sulfate-based) with minimal skin-beneficial ingredients. Handmade soap's quality depends entirely on the recipe — that's why balancing fatty acids with this soap calculator matters. A well-formulated soap with a 5–8% superfat, good conditioning oils, and a 4–6 week cure creates a genuinely skin-friendly bar that many people with dry or sensitive skin prefer.
A shampoo bar uses the same NaOH lye calculation as bar soap but with a recipe that emphasizes cleansing and lather over conditioning. A popular formula: 40% coconut oil (cleansing, lather), 30% castor oil (rich lather), 20% olive oil (conditioning), 10% sweet almond oil (slip). Use a lower superfat of 2–3% to avoid waxy buildup on hair and a water ratio of 33% for a firmer bar. Load this recipe using the "Shampoo Bar" starter recipe in the calculator. Note: some people experience a 2–4 week "transition period" when switching from commercial shampoo to a bar.
Rebatching (also called "hand-milling") involves re-melting a failed or ugly batch to fix or repurpose it. Grate or chop the soap, add a small amount of water (10–20% of soap weight), and melt slowly in a slow cooker or double boiler, stirring until smooth. You can add fragrance, color, or additives at this stage. Rebatching works well for riced soap (from acceleration), soap that seized, or plain unscented batches. Lye-heavy soap (zaps your tongue) can sometimes be saved by adding extra oils calculated with a lye calculator — add the missing superfat oils to the warm soap mass.
This calculator uses verified saponification values sourced from established references including SoapCalc, Lovin Soap Studio, and published saponification tables. SAP values for natural oils can vary slightly between suppliers and growing seasons, which is why a 5% superfat safety buffer is recommended — this accounts for natural variation. For maximum safety, never reduce superfat below 2% and always use a calibrated digital scale (0.01g accuracy) for lye. The calculator is used by thousands of soap makers globally and the results match those of other reputable soap calculators like SoapCalc and Brambleberry's lye calculator.
Oleic acid (C18:1) is the primary conditioning fatty acid — found at 75–80% in olive oil, hazelnut oil, camellia oil, and pataua oil. Linoleic acid (C18:2) adds lightweight, fast-absorbing conditioning especially good for oily or acne-prone skin — rich in grapeseed oil (72%) and safflower oil (75%). Ricinoleic acid from castor oil (90%) is unique: it conditions, boosts lather, and acts as a humectant all at once. For maximum conditioning, aim for a total conditioning score of 44–69 using this soap calculator's quality ratings display. See the Fatty Acid Guide for a complete breakdown.
Essential lye safety gear: (1) safety goggles — not just glasses, lye splashes can cause permanent eye damage; (2) nitrile or rubber gloves — lye burns skin rapidly; (3) long sleeves and closed-toe shoes; (4) work in a ventilated space to avoid inhaling fumes when dissolving lye. Always add lye to water — never the reverse — stir slowly using a heat-safe HDPE, stainless steel, or glass container. Keep white vinegar nearby to neutralize spills. See the full Lye Safety Guide on this page.
Cold process is generally recommended for beginners: no cooking equipment needed, more time to work with the batter, and the best results for swirls and designs. However, it requires a 4–6 week cure before use. Hot process (slow cooker method) gives usable soap in 1–2 weeks and requires no cure, but produces a rougher texture that's harder to mold into neat shapes. Both methods use the exact same lye calculation. Start with cold process, use the beginner recipe in this calculator (30% coconut, 30% palm/lard, 30% olive, 10% castor), and read the Soap Making Methods Guide on this page.