Skip to content
Generated on: July 20, 2026 at 09:33 PM

Soap Lye Calculator

Inputs

Oil weight500 g
Oil typeOlive
Superfat5 %
Water ratio38 %

Sodium hydroxide (lye) is caustic and can cause severe burns; always wear gloves and eye protection, work in a ventilated area, and add lye to water (never water to lye). These results assume NaOH (not KOH) and standard saponification values and cover a single base oil only. Always verify a complete recipe with a dedicated lye calculator before making soap.

Generated with OneCalc · https://onecalc.app/en/lifestyle/soap-lye

Lifestyle

Soap Lye Calculator

Estimate the sodium hydroxide (lye) and water for a single-oil cold-process soap recipe from the oil weight, oil type, superfat, and water ratio.

Inputs

≥ 0.1 g
%
0 – 20 %
%
25 – 50 %

Results

Enter a value to see results.

Saponifying 500 g of Olive oil at a 5 % superfat needs about ... of lye dissolved in ... of water.

Embed this calculator

Preview

Paste this code into your page to show the calculator.

Share this calculation

Anyone who opens this link sees your values already filled in.

Soap Lye

Cold-process soap is made by reacting oils and fats with sodium hydroxide (lye), and this calculator estimates the lye and water a single-oil recipe needs from the oil weight, the oil type, a superfat, and the water ratio. Getting the lye amount right matters because too much leaves a caustic, skin-damaging bar.

Saponification and SAP values

Saponification is the reaction in which lye splits a fat or oil into soap and glycerine. Each oil requires a specific amount of lye to react completely, expressed as its saponification (SAP) value — the mass of sodium hydroxide needed to saponify one unit mass of that oil. Olive oil has a SAP value of about 0.134, coconut about 0.178, and palm about 0.141. The base lye requirement is simply the oil weight multiplied by the SAP value.

Superfat and the water ratio

In practice recipes use slightly less lye than the SAP value calls for. This deliberate shortfall, the superfat or lye discount, leaves a small fraction of the oils unsaponified so the finished bar is gentler and there is a safety margin against excess lye. A 5% superfat is typical. The full lye estimate is therefore:

L=m⋅SAP⋅(1−s)\begin{aligned} L &= m \cdot \text{SAP} \cdot (1 - s) \end{aligned}

where mm is the oil weight, SAP\text{SAP} is the saponification value, and ss is the superfat fraction. Water dissolves the lye and is set as a fraction rr of the oil weight, commonly around 38%; it mostly evaporates during cure and does not change the lye amount.

Worked example

For 450 g of olive oil at a 5% superfat with a 38% water ratio:

L=0.450⋅0.134⋅(1−0.05)=0.0573 kg=57.3 gW=0.450⋅0.38=0.171 kg=171 g\begin{aligned} L &= 0.450 \cdot 0.134 \cdot (1 - 0.05) = 0.0573 \ \text{kg} = 57.3 \ \text{g} \\ W &= 0.450 \cdot 0.38 = 0.171 \ \text{kg} = 171 \ \text{g} \end{aligned}

So the batch needs about 57 g of lye dissolved in 171 g of water.

For 16 oz of olive oil at a 5% superfat with a 38% water ratio:

L=16⋅0.134⋅(1−0.05)=2.04 ozW=16⋅0.38=6.08 oz\begin{aligned} L &= 16 \cdot 0.134 \cdot (1 - 0.05) = 2.04 \ \text{oz} \\ W &= 16 \cdot 0.38 = 6.08 \ \text{oz} \end{aligned}

So the batch needs about 2.0 oz of lye dissolved in 6.1 oz of water.

Lye is caustic and requires gloves and eye protection, and a full multi-oil recipe should always be confirmed with a dedicated lye calculator. To resize a batch up or down while keeping the proportions, see Recipe Scaler.

Frequently Asked Questions (FAQ)

What is superfat (lye discount)?

Superfat is the share of oils deliberately left unsaponified by using slightly less lye than the full saponification value requires. A 5% superfat means about 5% of the oils stay as free, conditioning oils in the finished bar, which makes it milder and provides a safety margin against excess lye. Most cold-process recipes use 3-8%; higher superfat gives a softer, more conditioning bar but reduces lather and shelf life.

Does this use NaOH or KOH?

This calculator uses sodium hydroxide (NaOH), the lye used for solid, cold-process and hot-process bar soap. Potassium hydroxide (KOH) is used for liquid and soft soaps and has different saponification values, roughly 1.4 times the NaOH figure for the same oil. The results here are not valid for KOH recipes.

How much water should I use?

Water is usually set as a fraction of the oil weight, with about 38% being a common starting point. More water gives a longer working time but a softer bar that takes longer to cure; less water (a water discount) speeds up trace and unmolding but leaves less margin for error. The water only carries the lye and largely evaporates during cure, so it does not change the lye amount.

Disclaimer

Sodium hydroxide (lye) is caustic and can cause severe burns; always wear gloves and eye protection, work in a ventilated area, and add lye to water (never water to lye). These results assume NaOH (not KOH) and standard saponification values and cover a single base oil only. Always verify a complete recipe with a dedicated lye calculator before making soap.

Recommended Next

Recipe Scaler

Scale a whole recipe up or down. Enter the original and desired servings, then list each ingredient — every amount is recalculated for the new batch size.

 Learn more
Unit Price Calculator

Compare packages by per-unit cost — enter price and quantity for each item and the cheapest option stands out at a glance.

 Learn more
Pizza Dough Calculator

Calculate flour, water, salt, and yeast for pizza dough using baker’s percentages, from the number of dough balls and the weight per ball.

 Learn more

200+ calculators · 10 languages · 100% free

Was this calculator helpful?

No

How can we improve this calculator?
Powered by OneCalc ↗