Lye Calculator

Enter the oils in your recipe and their weights. The lye calculator gives the amount of sodium hydroxide for bar soap or potassium hydroxide for liquid soap, the water to dissolve it and the resulting lye solution strength.

How lye amounts are calculated

Soap forms when fats react with a strong alkali — sodium hydroxide (NaOH) for solid bars or potassium hydroxide (KOH) for liquid and soft soaps. Each oil has a saponification (SAP) value: the weight of alkali needed to convert one unit of that oil completely. Because the value is a ratio, it works the same in grams, ounces or pounds.

1. For each oil, multiply its weight by its NaOH SAP value.
2. Add the results to get the lye needed at 0% superfat.
3. Reduce by the superfat: lye × (1 − superfat ÷ 100).
4. For KOH, multiply the NaOH figure by 1.403; if your KOH is 90% pure, divide that by 0.9 (a combined factor of about 1.559).
5. Water = lye × your water-to-lye ratio.

The reverse calculator answers the opposite question: if you have a fixed amount of lye, how much of a single oil will it saponify at your chosen superfat? That is handy when you are down to the last of a container.

NaOH or KOH?

Some cream and shaving soaps use both lyes together. In that case calculate each portion separately: for example, 80% of the lye requirement as NaOH and 20% as KOH, remembering to apply the KOH conversion and purity to its share.

Superfat and lye discount

Superfat means leaving some oil unsaponified, which makes soap milder and provides a safety margin against small weighing errors. It is applied here as a “lye discount”: the calculator simply uses less lye. A 5% superfat is standard for bar soap. Liquid soap makers often use 0–3% because excess oil can cloud the finished soap; a small amount of extra oil can be added after cooking instead.

How much water to use

Lye must be dissolved before it meets the oils. The amount of water changes how the batch behaves but not how much soap forms.

Lye solution strength is lye ÷ (lye + water). Very strong solutions — above about 45–50% for NaOH — may not fully dissolve at room temperature.

Worked example

A bar recipe uses 500 g olive oil, 300 g coconut oil and 200 g shea butter. The NaOH needed at 0% superfat is 500 × 0.135 + 300 × 0.183 + 200 × 0.128 = 67.5 + 54.9 + 25.6 = 148.0 g. At 5% superfat that becomes 140.6 g. With a 2:1 water ratio you dissolve it in 281.2 g of water, a 33.3% lye solution. The same oils made into liquid soap with 90% KOH at 3% superfat would need 148.0 × 1.559 × 0.97 ≈ 223.8 g of KOH.

Handling lye safely

• Wear goggles and chemical-resistant gloves; lye causes severe burns to eyes and skin.
• Add lye slowly to cold water — never pour water onto lye, which can erupt.
• Use stainless steel or polypropylene (#5) plastic containers; glass can etch and weaken with repeated lye use, and aluminium reacts with lye.
• The solution gets very hot quickly; mix in a sink or on a stable surface with good ventilation.
• Store lye sealed, labelled and away from children, pets and food.
• If lye touches skin, rinse with plenty of running water; for eyes, rinse for at least 15 minutes and get medical help.

Notes for hot process and liquid soap

The lye amount does not change between cold process and hot process; only the handling does. In hot process the soap is cooked until saponification is complete, so some makers add the superfat oil after the cook, which means it stays unsaponified and is not part of the lye calculation. For liquid soap, KOH paste is usually made with extra water or glycerin and then diluted, often at roughly one part paste to two or more parts water, until it reaches the clarity and thickness you want. Test that the paste is fully cooked before diluting — a small sample should dissolve clear in hot water.

Check before you mix

Published SAP values vary slightly, and some suppliers provide their own figures for their oils. For new recipes, recheck weights and oil names and compare with a second calculator. Keep a written batch record with the lye amount, water, superfat and date so you can repeat a good batch or trace a problem.