Cold Process Soap Calculator
Calculate lye, water, and oil amounts for cold process soap from recipe oils and superfat. Enter values for instant results with step-by-step formulas.
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer
Cold Process Soap Calculator
Calculator
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Formula: NaOH = Sum(Oil_Weight x SAP_Value) x (1 - Superfat/100)
Worked example — NaOH: 122.65g | Water: 249.03g | Total: 1,221.68g (~12 bars)
Formula
NaOH = Sum(Oil_Weight x SAP_Value) x (1 - Superfat/100)
Each oil weight is multiplied by its SAP value (NaOH grams per gram of oil) and summed. The total is reduced by the superfat percentage. Water is calculated from the lye concentration: Water = NaOH x (100 - Lye%) / Lye%.
Worked Examples
Example 1: Classic Beginner Recipe
Problem:Calculate lye and water for: 500g olive oil, 250g coconut oil, 100g palm oil, 5% superfat, 33% lye concentration.
Solution:Olive: 500g x 0.1345 = 67.25g NaOH Coconut: 250g x 0.1910 = 47.75g NaOH Palm: 100g x 0.1410 = 14.10g NaOH Total NaOH (before superfat) = 129.10g With 5% superfat: 129.10 x 0.95 = 122.65g NaOH Water: 122.65 x (67/33) = 249.03g Total batch: 850 + 122.65 + 249.03 = 1,221.68g
Result:NaOH: 122.65g | Water: 249.03g | Total: 1,221.68g (~12 bars)
Example 2: Luxury Moisturizing Bar
Problem:400g olive, 150g coconut, 100g shea butter, 100g castor oil, 8% superfat, 30% lye concentration.
Solution:Olive: 400 x 0.1345 = 53.80g Coconut: 150 x 0.1910 = 28.65g Shea: 100 x 0.1280 = 12.80g Castor: 100 x 0.1286 = 12.86g Total NaOH (before superfat) = 108.11g With 8% superfat: 108.11 x 0.92 = 99.46g Water: 99.46 x (70/30) = 232.07g Total: 750 + 99.46 + 232.07 = 1,081.53g
Result:NaOH: 99.46g | Water: 232.07g | Total: 1,081.53g (~10 bars)
Frequently Asked Questions
What is the SAP value and how is it used in soap making?
SAP (saponification value) represents the amount of lye (sodium hydroxide/NaOH) needed to fully convert one gram of a specific oil or fat into soap. Each oil has a unique SAP value because their fatty acid compositions differ. Olive oil has a SAP value of 0.1345, meaning each gram of olive oil requires 0.1345 grams of NaOH. Coconut oil has a higher SAP value of 0.1910 because it contains more short-chain fatty acids. To calculate the total lye needed, multiply each oil's weight by its SAP value and sum the results. Without accurate SAP values, you risk making soap that is either lye-heavy (caustic) or oil-heavy (soft and greasy).
How does lye concentration affect cold process soap?
Lye concentration refers to the ratio of NaOH to water in your lye solution, expressed as a percentage. A 33% lye concentration means the solution is 33% NaOH and 67% water by weight. Higher concentrations (35-40%) result in faster trace, faster hardening, and less drying time, but they also generate more heat and can accelerate fragrance discoloration. Lower concentrations (25-30%) give you more working time, gentler temperatures, and are better for intricate designs and swirls. The standard recommendation for beginners is 30-33%. Experienced soap makers may go up to 40% for specific techniques like fluid hot process or in-the-pot swirls.
What oil combination makes the best cold process soap?
The best soap recipe balances hardness, lather, conditioning, and longevity. A classic combination is 40-50% olive oil (conditioning and creamy lather), 20-30% coconut oil (hardness, bubbly lather, cleansing), 15-20% palm oil or tallow (hardness and creamy lather), and 5-10% specialty oils like castor (lather boosting), shea butter (luxury conditioning), or avocado oil (moisturizing). Keep coconut oil under 30% to avoid overly drying soap. Keep castor oil under 10% as excess can create sticky bars. This balanced approach produces soap that lathers well, rinses cleanly, conditions skin, and lasts a reasonable time in the shower.
How long does cold process soap need to cure and why?
Cold process soap needs a minimum of 4-6 weeks to cure, though some recipes benefit from 8-12 weeks. During curing, several important chemical and physical changes occur. Saponification continues to completion during the first 48-72 hours, ensuring no free lye remains. Over the following weeks, excess water evaporates from the bars, making them harder, longer-lasting, and more mild. The crystalline structure of the soap molecules continues to organize, improving lather quality and bar consistency. Soap made with high olive oil content (Castile soap) benefits especially from long cures of 6-12 months, becoming progressively milder and producing better lather over time.
References
Background & Theory
SAP (Saponification) Values for Common Soap-Making Oils
SAP value is the grams of NaOH needed to fully saponify one gram of a given fat — it is fixed by the oil's fatty-acid profile, not by the recipe, which is why this calculator (and any accurate lye calculator) must look each oil up individually rather than apply one blended number. The table below lists the NaOH SAP values used in the formula above, plus a few oils soap makers commonly add beyond this calculator's five defaults.
| Oil / Butter | NaOH SAP value | Primary soap property |
|---|---|---|
| Olive oil | 0.1345 | Conditioning, creamy lather (Castile base) |
| Coconut oil | 0.1910 | Hardness, big bubbly lather, cleansing |
| Palm oil | 0.1410 | Hardness, stable creamy lather |
| Shea butter | 0.1280 | Conditioning, luxury feel |
| Castor oil | 0.1286 | Lather boosting, stabilizes bubbles |
| Sunflower oil | 0.1357 | Conditioning, softer bar |
| Cocoa butter | 0.1370 | Hardness, stable but can feel waxy over ~15% |
| Beef tallow | 0.1405 | Traditional hard, long-lasting bar |
SAP values per The Spruce Crafts' saponification value tables and Bramble Berry's published oil data (see References). Always re-check the SAP value printed on your specific supplier's product page — natural oils vary slightly by crop and refining method, and a lye calculator is only as accurate as the SAP values fed into it.
Real-World Recipe Scaling: Test Batch to Full Loaf
A common workflow is to trial a small batch in a 4-cavity silicone mold, then scale the same oil ratios up to a full loaf mold once the properties (hardness, lather, conditioning) test well. Because saponification is purely proportional, scaling a recipe up 3x means every oil weight, and therefore the NaOH and water, also scale by exactly 3x — the percentages entered above stay identical. This calculator lets you re-enter oil weights at any batch size and get correct lye figures immediately, instead of re-deriving ratios by hand each time you resize a recipe for a market batch, a gift set, or a single test bar.
Why Lye Concentration Is a Separate Decision From Superfat
Beginners sometimes conflate "extra water" with "extra superfat," but they solve different problems. Superfat leaves a percentage of oils unreacted for skin conditioning; lye concentration (or its inverse, the "water as % of oils" method some recipes use) only controls how much water dilutes the NaOH solution. A soap can be formulated at 5% superfat with either a 30% or 40% lye concentration — the finished bar's oil-to-lye chemistry is identical, but the 40% version traces faster, gels hotter, and unmolds sooner, while the 30% version gives more working time for swirls and embeds. This calculator exposes both variables independently so you can hold superfat constant while experimenting with water amount, which is how most soap-making reference guides recommend troubleshooting a too-fast or too-slow trace.
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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