🧼 SoapLog · soap & candle batch records
Soap recipe & lye calculator basics
Every batch of soap is a chemical reaction with a precise recipe, and the most important number in it is how much lye to use. Use too little and you get soft, oily soap; use too much and you get a harsh, lye-heavy bar that can burn skin. The good news: you never have to guess. A lye calculator does the math for you — but you have to actually run it, for every recipe, every time, even one you've made before but resized or tweaked.
Open SoapLog — free →What is saponification (SAP)?
Saponification is the reaction where lye and oil become soap and glycerin. Here's the key fact: each oil needs a different amount of lye. Coconut oil needs much more lye per gram than olive oil, for example. That ratio is captured in the oil's SAP value (saponification value) — the amount of lye it takes to fully saponify a given weight of that fat. Because every recipe is a unique blend of oils, every recipe has a unique total lye requirement.
What a lye calculator does
You enter your oils and their weights and pick your superfat %. The calculator looks up each oil's SAP value, adds up the lye needed, applies your superfat (lye discount), and returns:
- The exact lye (NaOH or KOH) weight for your blend
- A recommended water (or other liquid) amount
- Often the resulting fatty-acid profile (hardness, cleansing, conditioning)
Choose the right lye type: NaOH for solid bars, KOH for liquid soap — they have different SAP requirements and are not interchangeable.
Why run it every single time
Lye is unforgiving of small mistakes. Doubling a batch, swapping one oil for another, "rounding" a number, or even using a different lye purity all change the correct lye weight. Trusting a remembered figure is how lye-heavy batches happen. Recalculating takes a minute and is the single most reliable safety step in formulation — pair it with the gear and the lye-into-water rule from a good safety routine.