Every oil has a unique SAP (saponification) value — the amount of lye needed to turn that specific oil into soap. A soap calculator uses these values to tell you exactly how much sodium hydroxide your recipe needs.
To use a soap calculator: select your oils by percentage or weight, set your superfat (typically 5–8% for body soap), choose your water amount (usually 25–38% of oil weight), and the calculator gives you the precise lye and water weights.
Superfat means intentionally leaving some oil un-saponified in the final bar, making it more gentle and conditioning. A 5% superfat is standard for most body soaps. Increase to 8–10% for dry skin formulations or 15–20% for 100% coconut oil soap.
Water discount (using less water than the default) speeds trace, reduces cure time, and produces a harder bar. But too much water discount can cause acceleration or a crumbly texture. For beginners, stick with the default water amount or 30% of oil weight.
Always double-check your recipe with a soap calculator before making a batch. Never guess lye amounts. A mistake with lye calculation can result in a lye-heavy bar that is unsafe to use on skin.