Saponification Value Calculator (lye for soap: NaOH or KOH from oil SAP values)

Understanding the soap lye calculator — concept and principle

This calculator works out how much sodium hydroxide (for bar soap) or potassium hydroxide (for liquid soap) is needed to saponify a blend of oils, using each oil's saponification (SAP) value, a superfat and the lye purity, and how much water to dissolve it in. Soap makers use it to formulate safe recipes.

Each oil needs its own SAP amount of alkali; holding back 5 % leaves some oil free for a milder bar.

How it is calculated

KOH needed (g) = Σ oil (g) × SAP ÷ 1,000

NaOH = KOH × 39.997 ÷ 56.106

Lye to weigh = full amount × (1 − superfat) ÷ purity

Water = lye × (100 − c) ÷ c, for lye concentration c (%)

SAP values in mg KOH per g of oil; preset values are mid-points of Codex ranges.

The principle

Saponification splits fats (triglycerides) with a strong base into soap and glycerol. The SAP value measures how much KOH one gram of a particular fat consumes; it depends on the fat's average chain length. NaOH reacts mole for mole with the same fatty acids, so its amount follows from the ratio of molar masses. A superfat deliberately leaves some oil unreacted, making the bar milder and covering natural variation in SAP values.

Worked example

Default inputs: 600 g olive (SAP 190), 250 g coconut (256.5), 150 g palm (199.5); superfat 5%; NaOH 100% pure; 33% lye concentration.

KOH for full saponification = 600 × 0.190 + 250 × 0.2565 + 150 × 0.1995 = 208.1 g; average SAP = 208.1.
NaOH = 208.05 × 39.997 ÷ 56.106 = 148.3 g.
With 5% superfat: 148.3 × 0.95 = 140.9 g of lye to weigh.
Water = 140.9 × 67 ÷ 33 = 286.1 g; total oils 1,000 g; batch 1,427 g.

Each point is the calculator re-run with only “Oil 1 weight (g)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Oil 1 weight (g)	Lye to weigh (g)
150	83
300	102.3
450	121.6
600	140.9
750	160.2
900	179.5
1,200	218.1

How to read the result

Weigh lye and oils precisely on a scale; small errors in lye change the finished soap a lot. Superfats of a few percent are common for bar soap. Coconut oil has a high SAP value because its fatty acids are short, so it needs more lye per gram than olive oil.

Common mistakes

Using KOH SAP values directly for NaOH without the molar-mass conversion.
Forgetting the purity of commercial KOH, which is often below 100%.
Measuring by volume instead of weight.

Keep in mind: Sodium and potassium hydroxide are highly corrosive. Wear eye protection and gloves, add lye to water (never water to lye), work in a ventilated area and keep away from children.

Frequently asked questions

What is a SAP value?

The milligrams of KOH needed to saponify one gram of oil.

Why use a superfat?

It leaves some free oil for a milder soap and covers variation in oils.

Why does coconut oil need more lye?

Its fatty acids are shorter, so each gram contains more molecules to react.

Can I use KOH for bar soap?

KOH makes soft or liquid soap; NaOH makes hard bars.

Sources: Codex Alimentarius, Standard for Named Vegetable Oils (CODEX-STAN 210-1999) and Standard for Olive Oils (CODEX STAN 33), as reproduced by FAO

Checked 9 Nov 2026. Saponification value ranges for crude oils: Codex Alimentarius, Standard for Named Vegetable Oils (CODEX-STAN 210-1999), Appendix Table 2, and Standard for Olive Oils (CODEX STAN 33), as reproduced by FAO (document y2774e); molar masses KOH 56.106 and NaOH 39.997 g/mol (standard values).

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