How do I calculate how much lye my soap recipe needs?

For each oil, multiply its weight by that oil's SAP value (grams of lye per gram of oil), then add them up. Multiply the total by (1 − superfat%) to apply the lye discount. For example 1,000 g of coconut oil at SAP 0.190 needs 190 g of NaOH at 0% superfat, or 180.5 g at 5% superfat. Use NaOH SAP for bar soap and KOH SAP for liquid soap. Always run your final recipe through a lye calculator before mixing — lye is caustic.

What is superfat and how does it change the lye amount?

Superfat (also called lye discount) is the percentage of oils you deliberately leave unsaponified so there is never excess lye in the finished bar. A 5% superfat means you use 95% of the full saponification lye, which is the most common cold-process default. Higher superfat gives a more conditioning but softer bar; below about 1% leaves little margin for natural SAP-value variation.

What is the difference between NaOH and KOH in soapmaking?

NaOH (sodium hydroxide) makes hard bar soap; KOH (potassium hydroxide) makes soft or liquid soap. Because KOH has a higher molecular weight (56.11 vs 40.00 g/mol), it takes about 1.4035 times as much KOH as NaOH to saponify the same oil — so every oil's KOH SAP value is roughly 1.4× its NaOH value. This calculator switches SAP tables when you pick the lye type.

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Soap lye calculator

Add your oils by weight and get the exact NaOH or KOH grams, water, and superfat discount — SAP values cross-checked to SoapCalc and AOCS, cited to Kevin Dunn.

Every cold-process bar starts with one non-negotiable number: how much lye. Each oil has a SAP value — the grams of lye it takes to saponify one gram of that oil — so the lye a recipe needs is the sum of (oil weight × SAP), then trimmed by your superfat (lye discount). This calculator does exactly that for any oil blend, in NaOH for bar soap or KOH for liquid soap. The SAP table is public reference data, cross-checked against SoapCalc and AOCS-typical ranges and anchored to Kevin Dunn's Scientific Soapmaking.

Your oils

Lye type

Superfat / lye discount (%)

Water as

Water amount % of oils

Lye purity (%) (opt.)

Safety note. Lye (NaOH and KOH) is caustic — it burns skin and eyes and gives off fumes when it hits water. Always wear goggles and gloves, add lye to water (never water to lye) in a ventilated space, and keep it away from children and pets. This tool is a starting point: always run your final recipe through a second lye calculator before mixing, weigh everything on a gram scale, and never skip the superfat — a bar mixed with too little superfat or a mis-typed SAP value can be lye-heavy and unsafe on the skin.

Want the whole soap studio?

This page gives you the lye. Trace gives you the rest: a live fatty-acid quality analysis (hardness, cleansing, conditioning, bubbly and creamy lather, longevity) with cited target ranges so you can tell if a blend will actually make a good bar, multi-batch cure tracking, brand-aware oil presets, and fragrance/additive dosing with IFRA caps. The full soap studio — qualities, cure tracking, brand oils — lives in the app.

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How to use it

Add each oil in your recipe and type its weight in grams (weigh on a digital gram scale — soapmaking is by weight, never volume).
Pick NaOH for a hard bar or KOH for soft/liquid soap, and set your superfat (5% is the usual default).
Choose how you express water, then read off the lye and water to weigh. Confirm it against a second calculator before you mix.

FAQ

How do I calculate how much lye my recipe needs?
For each oil, oil weight × its SAP value, summed, then × (1 − superfat%). 1,000 g coconut at SAP 0.190 is 190 g NaOH at 0% superfat, or 180.5 g at 5%. NaOH for bar soap, KOH for liquid.

What is superfat?
The share of oils you leave unsaponified so there's never free lye in the bar. 5% is the common default: you use 95% of the full-saponification lye. Higher is more conditioning but softer.

Why is the KOH number bigger than the NaOH one?
KOH is heavier per molecule (56.11 vs 40.00 g/mol), so it takes about 1.4× as much to saponify the same oil. Switching the lye type swaps to each oil's KOH SAP value.

Sources

Kevin M. Dunn — Scientific Soapmaking: The Chemistry of the Cold Process (Clavicula Press, 2010); Hampden-Sydney College — saponification chemistry & SAP method
USDA National Nutrient Database / Agricultural Handbook 8 — fatty-acid composition tables (public reference data)
AOCS Official Methods — SAP value ranges; cross-checked against SoapCalc.net and Bramble Berry lye calculators

SAP values are typical reference figures; real oils vary by source and batch. This is a starting point — always verify your final recipe on a second lye calculator before mixing, and keep a safe superfat.