castor oil saponification and lye calculator

Saponifying one gram of castor oil takes about 0.1286 g of sodium hydroxide, or 0.1804 g of potassium hydroxide for liquid soap. Superfatting leaves a deliberate excess of oil so no free lye survives. A lather booster. Above about 10% it makes soap sticky.

Weight of castor oil

Superfat

Water as a share of oil

Lye

Lye purity

Lye to weigh out

61.09g

= Exact castor oil lye calculation Show in

Water: 165.0
Lye at 100% purity: 61.09
Lye with no superfat: 64.30
Lye solution concentration Most recipes run 28 to 33%. Above 35% the solution is aggressive and traces very fast.: 27.02 %
Water to lye ratio: 2.701
Total batch weight: 726
Approximate 110 g bars: 6.6
Oil left unsaponified: 25.0

Show the working

Weight of castor oil = 500
Superfat = 5
Water as a share of oil = 33
Lye = 1
Lye purity = 100
sap = 0.1286 = 0.1286
Lye with no superfat = 500 * 0.1286 * 1 = 64.3
Lye at 100% purity = 64.3 * (1 - 5/100) = 61.085
Lye to weigh out = 61.085 / (100/100) = 61.085
Water = 500 * 33/100 = 165
Lye solution concentration = (165 + 61.085) > 0 ? 61.085 / (165 + 61.085) * 100 : 0 = 27.0186
Water to lye ratio = 61.085 > 0 ? 165 / 61.085 : 0 = 2.701 15
Total batch weight = 500 + 165 + 61.085 = 726.085
Approximate 110 g bars = 726.085 / 110 = 6.600 77
Oil left unsaponified = 500 * 5/100 = 25

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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The formula

lye = oil weight x saponification value x (1 - superfat)
potassium hydroxide needs 1.403 times the sodium hydroxide weight, the ratio of their molar masses
lye concentration = lye / (lye + water)

Sodium hydroxide is caustic and generates heat on contact with water. Always add lye to water, never the reverse, and wear eye protection and gloves. Run any recipe through a second calculator before making it.

Saponification values are measured properties of a natural product and vary by a few percent between batches, pressings and suppliers. A 5% superfat covers that variation as well as leaving the soap gentle.

Potassium hydroxide is often sold at 90% purity. Set the purity field accordingly or the soap will be lye-heavy.

A lather booster. Above about 10% it makes soap sticky.

Frequently asked

What is the SAP value of castor oil?

0.1286 g of sodium hydroxide per gram of oil, or 0.1804 g of potassium hydroxide.

Why superfat?

Superfatting leaves a small excess of oil so that natural variation in the saponification value can never leave free lye in the finished bar.

Where the numbers come from

Saponification values are the standard published figures for this oil; treat them as approximate.