How to use it
- Choose grams or ounces. Pick an oil for each row and enter its weight; leave unused rows at 0.
- Choose NaOH for bar soap or KOH for liquid soap, and set the purity (KOH is often about 90 %).
- Set the superfat and the lye concentration.
- Weigh out the lye and water shown. Always add the lye to the water, never the other way round.
How it works
lye = Σ (oil weight × SAP) × (1 − superfat ÷ 100) ÷ (purity ÷ 100)
The SAP values are grams of NaOH per gram of oil; for KOH they are multiplied by 56.106 ÷ 39.997 = 1.403, the ratio of the molar masses.
water = lye × (100 − concentration) ÷ concentration
The same arithmetic works in grams or ounces, as long as all weights are in one unit.
Worked example
500 g olive oil, 300 g coconut oil, 150 g shea butter and 50 g castor oil, NaOH, 5 % superfat, 33 % concentration:
- 500 × 0.135 = 67.5; 300 × 0.183 = 54.9; 150 × 0.128 = 19.2; 50 × 0.128 = 6.4; sum 148 g;
- with 5 % superfat: 148 × 0.95 = 140.6 g NaOH;
- water: 140.6 × 67 ÷ 33 = 285.5 g, a water : lye ratio of 2.03 : 1 and 28.5 % of the oil weight;
- the whole batch weighs 1,000 + 140.6 + 285.5 = 1,426.1 g.
Safety
- Sodium and potassium hydroxide burn skin and eyes. Wear goggles and gloves, work in a ventilated place and keep children and pets away.
- Weigh everything on a scale; measuring cups are not accurate enough for lye.
- Run any new recipe through a second calculator or check the arithmetic before you make it.