Soap Lye (Saponification) Calculator
Getting the lye right is the most important and least forgiving step in cold-process soap making, and this saponification calculator helps you measure it accurately for a single oil recipe. Enter the oil weight, the oil's saponification value (the SAP value, which is the amount of lye needed to fully saponify one unit of that oil), and the superfat percentage you want, and the tool returns both the base lye amount and the lye after the superfat discount. Saponification is the chemical reaction where lye and oil become soap, so the SAP value links the two precisely; olive oil, for instance, has a sodium hydroxide SAP value of about 0.134. Superfat is the gentle, safe part of the recipe: it leaves a deliberate excess of oil unsaponified for a more moisturizing bar and a margin against excess lye, applied as a percentage reduction on the lye. Because SAP values differ for sodium hydroxide bar soap and potassium hydroxide liquid soap, and vary by oil, the SAP value is left editable so you match it to your exact ingredient and lye type. Always weigh ingredients and follow lye safety. The method is shown in the formula below, with a worked example that reconciles exactly to the calculator defaults.
Lye comes from oil weight and its SAP value: lye = oil weight x SAP value. 16 oz of olive oil at a SAP value of 0.134 needs 2.14 oz of lye at zero superfat, about 2.04 oz at 5% superfat.
Soap lye formula
Base lye = W x S
Lye with superfat = Base lye x (1 - SF / 100)
W = oil weight
S = saponification (SAP) value of the oil
SF = superfat percentage
The SAP value gives the lye needed to fully saponify the oil, and the superfat discount reduces that lye so a portion of the oil stays unsaponified for a gentler bar.
Worked example
You use 16 ounces of olive oil with a SAP value of 0.134, at a 5% superfat.
- Base lye = 16 x 0.134 = 2.144 ounces
- Superfat factor = 1 - 0.05 = 0.95
- Lye needed = 2.144 x 0.95 = 2.04 ounces
At zero superfat you need 2.14 ounces of lye, and 2.04 ounces at 5% superfat. These are the calculator's default inputs, so the result above matches the widget exactly.
Soap lye calculator: frequently asked questions
How do I calculate the lye for soap?
Multiply the weight of each oil by its saponification (SAP) value, which is the amount of lye needed to fully saponify one unit of that oil. Olive oil has a sodium hydroxide SAP value of about 0.134, so 16 ounces of olive oil needs 16 x 0.134, which equals 2.144 ounces of lye at zero superfat. A multi-oil recipe sums the lye for each oil.
What is superfat and why use it?
Superfat is a deliberate excess of oil left unsaponified to make a gentler, more moisturizing bar and to provide a safety margin against too much lye. It is applied as a percentage discount on the lye amount. A 5% superfat means using 5% less lye than the full saponification figure. Most cold-process recipes use a superfat between 5% and 8%.
Is this for sodium hydroxide or potassium hydroxide?
SAP values differ for sodium hydroxide (NaOH), used for solid bar soap, and potassium hydroxide (KOH), used for liquid soap. This calculator uses the value you enter, so make sure your SAP value matches the lye type you are using. The olive oil example value of 0.134 is for sodium hydroxide.
Why must lye soap recipes be precise?
Too little lye leaves a soft, oily bar, while too much lye leaves a harsh, caustic bar that can irritate skin. Accurate weighing, correct SAP values and a sensible superfat keep the chemistry balanced. Always weigh ingredients rather than measuring by volume, and follow lye safety precautions, since sodium hydroxide is corrosive.
What is the soap lye formula?
Lye at zero superfat equals oil weight multiplied by the oil's SAP value. With superfat, multiply that by (1 minus the superfat fraction). For 16 ounces of olive oil at a SAP value of 0.134, the base lye is 16 x 0.134 = 2.144 ounces, and at 5% superfat it is 2.144 x 0.95, about 2.04 ounces.
Official sources
- Weight and measurement standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.