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Liquid soap calculator

Liquid soap is a potassium soap: a paste cooked from oils and potassium hydroxide, then diluted with water. Give the oils and your flakes’ purity for the lye and the paste water, then weigh the paste after the cook for the water that dilutes it.

The oils
%
%
%
%
%
Total — %
The lye and the paste
From the label. Flakes are usually 90 %.
Liquid soap is made at 0 to 3 %; a little more keeps the worst case from being lye-heavy.
The flakes against their water. 25 % makes a stiff paste; more water makes a softer one that is slower to trace.
Diluting
About 10 % for a foaming dispenser, 15 to 25 % for a pump bottle.
Weigh all of it once cooked. Until you do, the batch as mixed stands in.

fill in the oils and the lye

Calculated at soapformulator.com, every value sourced. Check the figures against your own scale.

The formula, and where it comes from 
KOH to weigh (g)   = Σ(oil × SAP ÷ 1000) × (1 − superfat ÷ 100) ÷ (purity ÷ 100)
water (g)          = KOH × (100 − c) ÷ c        # c = lye concentration, %
soap in the paste  = oils + pure KOH            # mass is conserved in saponification
finished soap (g)  = soap ÷ (strength ÷ 100)
water to add (g)   = finished soap − paste

A worked batch

The Liquid body wash: 500 g of oils at a 3.5 % superfat take 111.6 g of 90 % potassium hydroxide flakes, 100.4 g of it pure, in 334.7 g of water. If the cooked paste weighs 920 g, it holds 600.4 g of soap, and at 20 % it makes 3002 g of liquid soap: add 2082 g of water.

How strong to make it

Strength here is the soap in the finished liquid: the oils plus the pure potassium hydroxide, as a share of the whole, because saponification conserves mass. About 10 % suits a foaming dispenser and 15 to 25 % a pump or squeeze bottle; a soap about half oleic acid can be left at 30 to 40 % to thicken it. Add the water hot, hold back the last tenth, and judge the thickness at room temperature.

Why not a 0 % superfat

The lye is worked from each oil’s representative saponification value, and real oils vary within their published ranges. Made at 0 %, the batch above could come out anywhere from −3.1 % to 2.9 % superfat, lye-heavy at one end. From 3.1 % even its worst case is not lye-heavy, so the calculator gives that figure as the lowest superfat that cannot come out lye-heavy.

Already have a paste? The liquid soap dilution calculator works from its weight alone.

Liquid soap from potassium hydroxide paste

Common questions

Why does the potassium hydroxide purity matter so much?
Potassium hydroxide is not sold pure: flakes are usually about 90 %, the rest mostly water. The calculator divides the pure requirement by the purity to give the weight of flakes. Enter 100 % with 90 % flakes and the paste is a tenth short of lye: greasy, cloudy and slow to trace.
What superfat should liquid soap have?
Between 0 and 3 %, because unreacted oil does not dissolve in the soap solution and shows as haze or a film. A little more, as the range check shows, keeps the worst case from being lye-heavy; the liquid soap recipes on this site use 3.5 %.
How much water do I add to the paste?
Enough to bring the soap to the strength you want: the soap in the paste divided by the strength, minus the paste. The paste already holds water from the cook, which is why a ratio such as one part paste to two of water means nothing until you know how much.
Why is my liquid soap thin, and will salt thicken it?
Lauric oils such as coconut make a thin soap that salt hardly thickens; oleic oils make a thicker one that salt thickens readily, best when about half the fatty acids are oleic. Less water thickens any of them.

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All tools
Liquid soap dilution Water to add to a potassium soap paste for the concentration you want
Lye calculator NaOH, KOH or dual lye, water three ways, additives, fragrance, mould sizing — every number explained

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