Soap Making Lye Calculator

How to use

Calculate lye and water from your oils with transparent SAP values, explicit purity assumptions and recipe scaling.

Add 1–20 oil rows, choose superfat, NaOH or KOH, and lye concentration. Search filters the native oil selector by common or alternative name. SAP info shows both reference values and explicit overrides. Advanced exposes lye purity. Scale Recipe changes oil masses and recalculates. Copy Recipe includes all assumptions. Reset restores the demo; refresh clears edits. The demo is an arithmetic fixture, not a recipe recommendation.

How SAP values work

Saponification (SAP) values express the mass of hydroxide required per mass of oil at the reference conditions. This curated table stores direct supplier NaOH and KOH values in g/g from Soaper’s Choice, with ingredient variant, source label and review date. No molecular-weight multiplier is applied. SAP values vary by reference, oil composition, processing and ingredient specification; these are reference assumptions, not exact universal values. Coconut 76°F differs from fractionated coconut; refined shea differs from unrefined shea. Match your ingredient documentation.

How lye is calculated

Lye at 0% superfat = Σ(oil weight in grams × selected SAP). After superfat, pure-equivalent lye = theoretical lye × (1 − superfat / 100). Each row contributes exactly once. NaOH uses the stored NaOH SAP; KOH uses the stored KOH SAP. Purity is then applied once. Calculations retain full precision; displayed weights are rounded to two decimals, or six significant digits for positive masses below 0.01 in the selected unit. Display precision is not measurement accuracy.

Superfat

Superfat is applied here as a reduction in calculated lye from the selected reference SAP values: pure-equivalent lye = theoretical lye × (1 − superfat / 100). The supported 0–20% range is a computational limit. The default is 5%, with decimal text and slider controls. This lye discount does not establish a chemically exact free-oil percentage in finished soap.

NaOH vs KOH

NaOH is commonly used for solid/bar soap formulations; KOH is commonly used for liquid/soft soap formulations. Selection changes the SAP basis for every ingredient. It does not alone determine final texture or successful formulation. Both chemicals are highly caustic.

Lye concentration and water

Lye concentration = lye / (lye + water). With concentration c expressed as a fraction, c × (lye + water) = lye, so water = lye × (1 − c) / c. This calculator consistently defines lye as the actual weighed lye product after purity adjustment. This explicit convention is not a universal formulation rule; other references may use pure-equivalent lye. Supported concentration is 20–50%, default 33%, as computational bounds rather than universal safety limits. No alternative water method is included.

Lye purity

Weighed lye product = required pure-equivalent lye / (purity / 100). Purity must be greater than 0 and at most 100%. Theoretical 100% purity — change this to match your product documentation. If your lye product lists a purity, use that documented value. The calculator never guesses commercial KOH or NaOH purity. For the algebraic example 100 g pure-equivalent at 90% purity, product mass is 100/0.90 = 111.111… g; this is not a formulation recommendation.

Scaling a soap recipe

Scale factor = target total oils / current total oils. Every canonical oil mass is multiplied by that factor, then SAP, superfat, purity and water equations run again. Rounded displayed lye is never scaled. The original mass, target and factor remain visible until the recipe changes. Duplicates are combined in the percentage summary while their individual SAP assumptions remain visible.

If you intend to sell a finished product, explore markup from an independently established soap unit cost. This recipe model does not determine cured saleable yield, complete unit cost, product safety or whether sale and labeling requirements are met.

Units

Canonical internal mass is grams. One avoirdupois ounce is exactly 28.349523125 g. The global Grams/Ounces control converts every oil weight, result and valid scaling target. Switching units reads canonical grams rather than previously rounded display values. Numeric inputs accept locale-friendly decimal punctuation such as 425.5 and 425,5. Incomplete or malformed required inputs remove quantities instead of restoring previous values.

Worked examples

All examples are computed by the same engine from the stored source-backed SAP values. They illustrate arithmetic and are not recipe recommendations.

Single oil

500 g refined-A olive oil × NaOH SAP 0.135 = 67.5 g theoretical NaOH at 0% superfat. At 5%, pure-equivalent NaOH is 64.125 g.

Multi-oil fixture

• Olive oil (refined A): 500 g × 0.135 = 67.5 g NaOH at 0% superfat.
• Coconut oil (76°F): 250 g × 0.178 = 44.5 g NaOH at 0% superfat.
• Shea butter (refined): 100 g × 0.131 = 13.1 g NaOH at 0% superfat.

Sum: 125.1 g. At 5% superfat: 125.1 × 0.95 = 118.845 g pure-equivalent NaOH. At theoretical 100% purity this equals weighed product.

Water algebra

At 33% concentration, 118.845 × (1 − 0.33)/0.33 = 241.291364 g water. Initial batch weight before fragrances, additives, and curing losses: 1,210.136364 g.

Scale 850 g to 1,500 g

Scale factor = 1500/850 = 1.764705882×.

• Olive oil (refined A): 882.352941 g.
• Coconut oil (76°F): 441.176471 g.
• Shea butter (refined): 176.470588 g.

Recomputed NaOH product: 209.726471 g; water: 425.808289 g, with the same 5% discount, theoretical 100% purity and 33% concentration.

Assumptions and safety limitations

Lye calculations depend on SAP values, ingredient identity, lye purity, measurement accuracy, and other recipe assumptions. Verify your ingredient data and calculations before making soap.

NaOH and KOH are highly caustic. Consult the authoritative hazard documents linked in Sources. This page provides arithmetic and verification context, not procedural chemical-handling advice. No safe-recipe guarantee or recipe-quality scoring is provided. Fragrances, additives, acid neutralization, other reactants, measurement uncertainty and curing losses are outside this model. Initial batch weight is not final cured weight.

Bounds: 1–20 rows, search up to 64 characters, numeric fields up to 32 characters, per-row oil mass from 0.000001 g and total oil mass at most 100,000 g, superfat 0–20%, purity greater than 0 up to 100%, concentration 20–50%, custom SAP greater than 0 up to 0.5 g/g, and positive scale target at most 100,000 g. These are computational limits, not safety standards. Missing or invalid required values fail closed with no lye quantity; nothing is clamped or silently substituted.

FAQ

How is lye calculated for soap?

Sum each oil mass times its selected SAP, apply the superfat discount, then divide by documented purity. Verify all assumptions before use.

What is a SAP value?

It is a reference hydroxide mass per oil mass. This calculator uses g NaOH/g oil or g KOH/g oil.

Why can SAP values differ between sources?

Oil composition, processing, ingredient specifications and reference methods can differ. A table value is not exact for every batch.

What does superfat mean in a lye calculator?

It is a calculated lye discount: theoretical lye × (1 − superfat/100), not exact finished free-oil percentage.

What is the difference between NaOH and KOH?

They use different SAP values. NaOH is commonly used for bar formulations and KOH for liquid/soft formulations; selection alone does not guarantee texture or success.

How does lye concentration determine water amount?

Concentration = weighed lye product/(weighed lye product + water). Water = product × (1 − c)/c with c as a fraction.

Why does lye purity matter?

Pure-equivalent lye is divided by purity/100 once. Use your documented product purity rather than assuming commercial purity.

Can I change an oil’s SAP value manually?

Yes. Enable the explicit override and provide both positive NaOH and KOH values up to 0.5 g/g. Verify them against your ingredient documentation.

How do I scale a soap recipe?

Enter a target total oil mass and select Scale oils. Every oil is scaled by target/current, then the same model recalculates lye and water.

Why should I verify a lye calculator result before use?

Ingredient identity, SAP variability, purity, measurement accuracy and omitted formulation assumptions affect the result. This calculator does not guarantee a safe recipe.

Sources

Reviewed October 3, 2026. Supplier table values were checked for each named variant. Both hydroxide values are stored directly; no conversion multiplier is used. Table has no published version date. These references do not endorse BuzCalculator. Concentration on weighed-product mass is this calculator’s explicit convention.

• Soaper’s Choice: supplier SAP values, both NaOH and KOH (opens in a new tab)
• NIOSH: sodium hydroxide chemical hazards (opens in a new tab)
• NIOSH: potassium hydroxide chemical hazards (opens in a new tab)
• NIST Handbook 133: exact mass conversion (opens in a new tab)
• Handcrafted Soap & Cosmetic Guild: saponification concepts (opens in a new tab)