Soap Making Lye Calculator

1. Enter the total weight of your oils in grams.
2. Choose your main oil type to set its saponification value.
3. Set a superfat, usually 5 to 8 percent, to leave a little oil unsaponified.
4. Set the water as a percentage of oils, then read the lye and water your recipe needs.

A lye calculator works out how much sodium hydroxide, known as lye or NaOH, you need to turn a given weight of oils into soap. Every oil reacts with a specific amount of lye, so getting the number right is the core of any safe soap recipe.

Too much lye leaves a harsh, caustic bar; too little leaves an oily one that will not set. The calculator takes the guesswork out by using the saponification value of your oil and your chosen superfat.

1. Each oil has a saponification value, the grams of lye that fully react with one gram of oil.
2. Multiply your oil weight by that value to get the full lye amount.
3. Reduce it by your superfat percentage to leave some oil unsaponified.
4. Set the water as a share of the oils to dissolve the lye.

The result is the lye and water for a cold-process or hot-process bar. Superfat is your safety and conditioning margin, ensuring no free lye remains in the finished soap.

You want a 500 gram olive oil soap at a 5 percent superfat. Olive oil has a saponification value of about 0.135, so the full lye is 500 times 0.135, which is 67.5 grams. Reducing by 5 percent superfat gives 500 times 0.135 times 0.95, about 64 grams of lye. With water at 33 percent of oils you add 165 grams of water. Change the oil to coconut and the lye rises, because coconut saponifies with more lye per gram.

Oil Type

Each oil has its own saponification value, so coconut needs far more lye per gram than olive or castor.

Superfat Level

A higher superfat lowers the lye and leaves more free oil, giving a gentler but softer bar.

Oil Blend

Most recipes mix oils, so the true lye is the sum of each oil weight times its own value; run each oil and add the results.

Water Amount

Water does not change how much lye you need, but it sets the concentration, which affects trace speed and cure time.

The saponification value is the grams of NaOH per gram of oil. Higher means more lye.

Because the values differ, swapping oils without recalculating the lye is the fastest way to ruin a batch.

Formulating a Recipe

Design a new blend and get the exact lye and water before you buy or measure anything.

Scaling a Batch

Resize a trusted recipe up or down while keeping the lye ratio correct.

Substituting an Oil

Swap one oil for another and recheck the lye, since even a small change alters the amount you need.

1. Reusing Lye Across Different Oils

Each oil saponifies differently, so lye figured for one oil is wrong for another.

2. Skipping Superfat

A zero superfat leaves no safety margin, risking a harsh bar with free lye.

3. Confusing NaOH and KOH

This tool uses sodium hydroxide for solid bars; liquid soap uses potassium hydroxide with different values.

4. Measuring by Volume

Soap making is by weight; always weigh oils, lye and water in grams, never by cups.

The calculator applies standard saponification values exactly for a single oil, so the lye figure is reliable for a straightforward recipe. Multi-oil blends need each oil run separately.

Multiply your oil weight by the oil saponification value, then reduce by your superfat. For 500 g of olive oil at 5 percent superfat, that is 500 times 0.135 times 0.95, about 64 g of lye.

Superfat is the small share of oil left unsaponified, usually 5 to 8 percent. It gives a gentler, more conditioning bar and a safety margin against excess lye.

It is the grams of lye needed to fully react with one gram of a given oil. Coconut oil is about 0.178, olive oil about 0.135.

A common starting point is water at about 33 percent of your oil weight, or roughly two parts water to one part lye. Less water speeds trace and cure.

Run each oil separately and add the lye amounts, because every oil saponifies with a different amount of lye.

Sodium hydroxide (NaOH) makes solid bar soap, while potassium hydroxide (KOH) makes liquid soap. They have different saponification values, and this tool uses NaOH.

Yes. Lye is caustic and can cause serious burns. Always wear gloves and eye protection, work in a ventilated area, and add lye to water, never water to lye.

Soap chemistry depends on exact ratios, and oils vary in density. Weighing in grams is the only reliable way to get a safe bar.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Saponification chemistry (Wikipedia).
• Soap and soap making (Wikipedia).
• Sodium hydroxide (lye) (Wikipedia).