Calculate the NaOH lye, water and superfat for a cold or hot-process soap recipe from your oil weights.
⚠ Lye (sodium hydroxide) is caustic. Always add lye to water, never water to lye. Wear gloves and eye protection and work in a ventilated area.
NaOH SAP values used: olive 0.134, coconut 0.178, palm 0.141, shea 0.128, castor 0.128. Verify any recipe against a second lye calculator before mixing.
About this tool
Soap Lye & Saponification Calculator works out the exact sodium hydroxide (NaOH) lye and water for a cold-process or hot-process soap recipe. Enter the weight of each oil — olive, coconut, palm, shea and castor — your superfat percentage and a water ratio, and it returns the lye, water, total oils and finished batch weight. Weigh everything on a scale in grams for accuracy.
Every oil has a saponification (SAP) value — the grams of NaOH that turn one gram of that oil into soap (olive 0.134, coconut 0.178, palm 0.141, shea 0.128, castor 0.128). The raw lye is the sum of each oil weight × its SAP value. Superfat leaves a share of the oils unsaponified for a gentler bar, so lye = raw lye × (1 − superfat ÷ 100). Water = lye × your water ratio (2 by default, i.e. twice the lye weight).
Lye is caustic: always add lye to water (never the reverse), wear gloves and eye protection, and work in a ventilated space away from children and pets. SAP values vary slightly between references and oil batches, so cross-check a recipe with a second lye calculator before mixing, and never guess weights.
Frequently asked questions
What is superfatting?
Superfatting leaves a small percentage of the oils unsaponified so the finished bar is moisturising and never lye-heavy. A 5% superfat is typical; the tool reduces the lye by that percentage, which is far safer than a lye-excess bar.
How much water should I use?
The default water ratio of 2 means water = twice the lye weight, a common full-water amount. Experienced soapers sometimes use a water discount (a lower ratio) for a harder, faster-tracing bar; adjust the ratio to match your recipe.
Does this use NaOH or KOH?
It uses sodium hydroxide (NaOH) for solid bar soap. Liquid soap made with potassium hydroxide (KOH) uses different SAP values and is not covered here, so do not substitute one lye for the other.
Are these SAP values exact?
They are widely cited averages, but real oils vary by source and batch, and references differ in the second decimal. Always verify a new recipe against another trusted lye calculator, and make a small test batch before scaling up.