How a Soap (Lye) Calculator Works

Soap is made by reacting fats (oils) with a strong base — lye. The reaction is called saponification, and getting the lye amount right is the single most important step: too much lye makes a harsh, caustic bar; too little leaves it greasy.

Saponification (SAP) values

Every oil has a SAP value — the grams of lye (NaOH) needed to fully saponify one gram of that oil. Coconut oil is around 0.178; olive oil is around 0.134. A lye calculator multiplies each oil's weight by its SAP value and adds them up to get the total lye.

Superfat

Then it applies your superfat (lye discount) — usually 5% — reducing the lye so a small amount of oil stays unsaponified. That guarantees no leftover lye and makes the bar more moisturizing.

Water

Finally it works out the water that dissolves the lye, either as a percentage of your oils (≈38%), a lye concentration, or a water:lye ratio. That's your full recipe.