Soap Making Lye Calculator

How soap lye calculations work:

Soap is made by combining oils/fats with an alkali (lye). The chemical reaction is called saponification. Each oil has a unique SAP value, which is the grams of lye needed to fully saponify 1 gram of that oil.

Basic formula:

Lye (g) = Oil weight (g) × SAP value × (1 − Superfat %)

Superfat (also called lye discount) is the percentage of oils left unsaponified, giving the soap extra moisturising properties. A 5% superfat is the most common starting point.

Common SAP values (NaOH / bar soap):

• Coconut oil: 0.183
• Palm oil: 0.141
• Cocoa butter: 0.137
• Sweet almond oil: 0.136
• Olive oil: 0.134
• Tallow (beef): 0.140
• Lard: 0.138
• Castor oil: 0.128
• Shea butter: 0.128

Sodium hydroxide or potassium hydroxide? Bar soap uses sodium hydroxide (NaOH). Liquid and cream soaps use potassium hydroxide (KOH), which is a heavier molecule: 56.1 against 40.0, so any oil needs about 1.40 times as much of it by weight. On top of that, flake KOH is normally only 90% pure, and the remaining 10% is mostly potassium carbonate that does not saponify. Divide by 0.90 for the real amount. Between the two corrections, KOH comes out at roughly 1.56 times the NaOH figure.

Worked example - simple 3-oil bar soap:

• 300g coconut oil × 0.183 = 54.9g
• 300g palm oil × 0.141 = 42.3g
• 400g olive oil × 0.134 = 53.6g
• Total lye before superfat = 150.8g
• Apply 5% superfat: 150.8 × (1 − 0.05) = 143.3g NaOH

That works out at 0.151 grams of lye per gram of oil across the whole blend. Round it to about 0.15 and you have a number worth carrying around, because most three-oil bar recipes land within a few percent of it. If a recipe you find online is a long way off 0.15, check it before you make it rather than after.

Water amount, for bar soap: Use 33–38% of total oil weight. For 1,000g oils: 330–380g distilled water. Less water = harder bar faster; more water = easier to mix. Soapers also quote water as a lye concentration instead, which is lye divided by lye plus water. The two describe the same thing from opposite ends.

Water amount, for liquid soap: a different convention entirely, and one worth not getting wrong. Liquid soap quotes water as a multiple of the KOH weight, normally 2.5 to 3 times it, and that makes the paste. Apply the bar-soap rule to a KOH recipe and you end up with a 40% solution, which is far stronger than anything you want to be stirring. The paste is then diluted again with roughly its own weight in water to become pourable soap, so budget for about twice as much water overall as a bar recipe of the same oil weight.

Safety, and these are not optional:

• Add lye TO water, never water to lye: adding water to lye causes a violent reaction
• Lye solution reaches 200°F (93°C) instantly: use heat-safe containers
• Wear goggles and gloves at all times
• Mix in a ventilated area: fumes are caustic
• Lye is safe in cured soap: saponification is complete after 4–6 weeks

Enter up to three oils below. Three covers the overwhelming majority of bar recipes, because the fourth and fifth oils in a recipe are usually there at 5% or less for skin feel rather than structure. If you are running a six-oil formula, add the two or three largest here and treat the answer as a check on the figure your full recipe gave you, not as a replacement for it.