Workbench › Soap lye

Soap lye calculator

Build a recipe from 33 oils and butters and get exact lye, water and fragrance weights. Save recipes in your browser and print a batch sheet.

Lye safety. Sodium hydroxide and potassium hydroxide are corrosive and can cause severe burns and blindness. Wear splash goggles, gloves and long sleeves. Work in a ventilated space away from children and pets. Always add lye to water, never water to lye. Use only heat-resistant plastic (PP #5) or stainless steel, never aluminium. Keep vinegar away from skin burns and rinse with plenty of running water instead. Check every recipe twice. This calculator is a planning aid; you are responsible for your own recipe and its safety.

Oils

Saved recipes

    How the lye amount is calculated

    Every oil has a saponification value: the milligrams of potassium hydroxide (KOH) that one gram of it consumes. Sodium hydroxide (NaOH) is lighter per molecule, so the NaOH figure is the KOH figure × 39.997 ÷ 56.106 (0.713). The calculator adds up oil weight × SAP value for each oil, then reduces the total by your superfat, the share of oil you deliberately leave unsaponified for mildness and as a safety margin.

    Example: 500 g olive, 300 g coconut and 200 g palm oil, at 5% superfat and 33% lye concentration, need 143.5 g NaOH and 291 g water.

    Choosing the water amount

    • Lye concentration is lye ÷ (lye + water). 30–35% is a common range for cold process bars. Higher concentrations unmold faster but can trace and heat up quickly.
    • Water : lye ratio of 2:1 is the same as 33.3% concentration.
    • % of oils is an older method. It ties water to oil weight, so the lye concentration changes as your superfat or oils change.

    KOH purity and dual lye

    Commercial potassium hydroxide is usually about 90% pure, and the calculator compensates. Sodium hydroxide is typically 97–99% pure and is treated as 100%, which slightly adds to your superfat. Dual-lye recipes use a small share of KOH, often 5–10%, for a creamier, more soluble lather.

    SAP values and the safety-first option

    Natural oils vary from batch to batch, so published SAP values are ranges (see the SAP value chart). "Typical" uses the middle of each range. "Safety-first" uses the low end, so the real superfat can't end up below the value you chose. Lye-heavy soap happens when an oil needs less lye than assumed, so the result always shows the worst case: your superfat if every oil were at the low end of its range.

    Questions

    What superfat should I use?
    5% is the usual starting point for bar soap; 0–3% for cleaning or laundry soap; higher (8–20%) for very conditioning bars, with a shorter shelf life. Liquid (KOH) soap is often made with 0–3% superfat.
    Why is my lye amount different from another calculator?
    Calculators use different SAP numbers. Ours are midpoints of published standard ranges (Codex Alimentarius, European Pharmacopoeia) and can differ by a few percent – your superfat absorbs that.
    Can I use ounces?
    Yes. Choose ounces as the weight unit; all amounts (oils, lye, water, fragrance) switch to ounces.
    Where are my recipes stored?
    In your browser on this device only. Nothing is uploaded. Use the print sheet to keep a paper copy.