Wood-ash lye calculator
Before there was bottled lye, soap was made with potash leached from hardwood ashes. This guide walks the old process — leach, test the strength, formulate — and turns it into numbers you can use. Wood-ash lye strength can never be known exactly, so every figure here is an estimate: always test a small batch first. For bottled NaOH/KOH, use the main calculator.
Step 1 · Your ash
Hardwood ash (oak, beech, ash, fruitwood) is the traditional choice — highest in potassium carbonate. Leach with soft or rain water for the strongest lye.
Step 2 · Measure its strength
Estimated strength
EstimateA patch of shell the size of a coin shows above the surface — the classic 'ready' strength makers aimed for. (≈ 70–121 g/L)
From the float test (rough). Raw wood-ash lye is partly carbonate, so its true saponifying power is lower than this hydroxide-equivalent figure — treat it as a ceiling and confirm with a test batch.
Step 3 · Your recipe
How much of your lye to use
EstimateYour lye is dilute, so you'll need a large volume — traditional soft soap is simmered for hours and the excess water cooks off. Boiling the lye down first makes the cook shorter.
Wood-ash lye is potassium-based, so this makes a soft / gel soap, not a hard bar. For a firm bar, traditional makers either “salted out” with common salt or causticised the lye with slaked lime (Ca(OH)₂) to turn the carbonate into true hydroxide. The lye already carries your water — don't add more.