Cold Process Soap Calculator

Cold process soapmaking is fundamentally a saponification reaction between fats/oils and sodium hydroxide (lye), and getting the lye amount even slightly wrong risks a batch that's either too harsh (lye-heavy) or too soft and prone to spoiling (oil-heavy). This calculator starts from the saponification (SAP) value — the amount of NaOH, in this codebase expressed as a decimal fraction, that fully saponifies one unit weight of a given oil (olive ≈ 0.134, coconut ≈ 0.178, palm ≈ 0.141; blended recipes need a weighted average) — and computes lye needed as oil weight × SAP value × (1 − superfat/100). The superfat percentage deliberately withholds a portion of the oil from saponification, leaving it as free, unreacted oil in the finished bar for skin conditioning; 5% is the standard default, with most recipes ranging 3-8%.

Water is then derived from your target lye concentration, defined as lye ÷ (lye + water) × 100: rearranging that relationship gives water = lye × (100/concentration − 1), so a lower concentration percentage (more water relative to lye) produces a thinner, slower-tracing batter, while a higher concentration speeds up trace and can produce a firmer bar sooner. The calculator sums oil, lye, and water into a total batch weight, reports the "excess oil" (the literal ounces of oil left unsaponified by the superfat), and estimates the yield in standard 4 oz bars. The output is only as accurate as your input SAP value, so always verify it against a trusted saponification chart or established lye calculator for your specific oil blend before mixing lye into water.