Soap Lye Calculator
Calculate sodium hydroxide (lye) amount for cold-process soap from oil SAP values. Get precise NaOH weights for safe, balanced soap recipes.
Specialist tool for Candle Soap1 documented formula8 real workflowsPrivate in your browser
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Inputs that matter
Total Oil Weight, Average SAP Value, Superfat, Lye Type
Output to expect
Soap Lye Calculator
How it works
1 documented formula across 1 calculation mode
- Keep 3 bounded inputs within the limits shown beside the fields.
- Confirm that every value uses the unit and time basis shown before comparing scenarios.
- Change one input at a time to see which assumption materially changes the result.
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Reference & details
How it works
Updated July 2026
How it works
Updated July 2026SAP Value Application
Each oil has a SAP value — grams of NaOH needed to saponify 100 g of that oil (or oz NaOH per oz oil in imperial). Multiply each oil weight by its SAP and sum.
NaOH (g) = Σ (oil weight g × SAP value / 100)Superfat Adjustment
Superfat leaves unsaponified oil for mildness and moisture. Reduce calculated lye by the superfat percentage — typically 5–8% for body bars, 15–20% for salt bars.
Adjusted NaOH = calculated NaOH × (1 − superfat% / 100)Lye Concentration and Water
Lye solution strength is set by water-to-lye ratio. A 33% lye solution (2:1 water:lye by weight) is standard for trace-friendly cold process.
Water (g) = NaOH (g) × (1 − lye% / 100) / (lye% / 100)Updated: July 2026
Example Scenarios
500 g olive (SAP 0.135), 200 g coconut (SAP 0.183), 100 g palm (SAP 0.142): NaOH = 67.5 + 36.6 + 14.2 = 118.3 g; at 5% superfat = 112.4 g NaOH.
1000 g coconut oil × 0.183 = 183 g NaOH, no superfat discount. High cleansing bar — use gloves and cure 4+ weeks.
112 g NaOH needs 224 g distilled water for 33% solution. Always add lye to water, never water to lye, in heat-resistant container.
Common Mistakes to Avoid
Common Mistakes to Avoid
Using volume measurements for lye or oils
All soapmaking calculations require weight in grams or ounces. Volume varies with temperature and oil density — weight is the only reliable basis.
Applying superfat discount twice
Superfat reduces lye once. Do not also reduce oils — the superfat percentage already accounts for leftover unsaponified oil in the formula.
FAQ
Saponification value (SAP) is the amount of NaOH (or KOH for liquid soap) required to fully convert a given weight of oil into soap. Each oil has a unique SAP from its fatty acid profile.
5% is standard for balanced bars. 8% adds extra moisture for dry skin. Higher superfat (15%+) suits salt bars and specialty recipes but reduces lather in some formulas.
KOH makes soft or liquid soap. KOH SAP values differ — typically multiply NaOH SAP by ~1.4 for KOH, or use KOH-specific SAP tables.
Too much lye produces caustic, skin-irritating soap. Too little leaves excess unsaponified oil and risks rancidity. A 0.1 g error matters on small batches.
Water is a solvent, not a reactant. More water slows trace and aids swirling; less water accelerates trace. Lye amount is independent of water — set water by lye concentration.
About Soap Lye Calculator
Cold-process soapmaking requires precise lye calculated from each oil's saponification (SAP) value. This calculator totals NaOH needed for your oil blend with adjustable superfat to produce safe, fully saponified soap.