How much KOH for liquid soap?
Liquid soap is made with potassium hydroxide instead of sodium hydroxide because potassium soaps stay soluble; the lye math is otherwise the same as bar soap — each oil's SAP value times its weight — with two differences that catch people out. First, flake KOH is sold at about 90% purity, so the calculator divides by 0.9 and you weigh out more: the all-purpose blend above (olive 65 / coconut 25 / castor 10) needs 203.7 g of pure KOH per kilo of oils but 226.3 g of the 90% flakes. SoapCalc's “90% KOH” box does the same thing; leave it ticked unless your supplier states a higher purity. Second, the superfat is tiny — 0–3% — because unsaponified oil turns liquid soap cloudy. The margin against error is smaller, which is one more reason to weigh carefully.
Blend SAP 0.2058 × 1,000 g × 0.99 ÷ 0.90 = 226.3 g KOH. Water for the paste at 25% concentration = 226.3 × 3 = 679 g. The paste weighs 1,905 g and is 63% soap solids. To make a 20% hand soap: solids 1,204 g ÷ 0.20 = 6,018 g finished, so add 4,113 g of water — about 2.2 parts water to 1 part paste — for 5.8 liters of soap.
How much water to dilute liquid soap paste
This is the question the bar-soap calculators don't answer. The paste you make is roughly two-thirds soap solids (the oils plus the pure KOH), and a pourable hand soap is 15–25% solids, so the paste takes about two to four times its weight in water. The exact figure depends on the oils — olive-heavy pastes thicken at lower concentrations than coconut-heavy ones — and on how thick you like it. The table below is the worked example diluted to different strengths:
| Finished concentration | Water to add | Finished soap | Water : paste | Feels like |
|---|---|---|---|---|
| 15% solids | 6,119 g | 8,025 g | 3.2 : 1 | Thin, foamer-bottle friendly |
| 20% solids | 4,113 g | 6,018 g | 2.2 : 1 | Everyday pump hand soap |
| 25% solids | 2,909 g | 4,815 g | 1.5 : 1 | Richer, slightly viscous |
| 30% solids | 2,107 g | 4,012 g | 1.1 : 1 | Thick; may gel with olive-heavy pastes |
| 40% solids | 1,104 g | 3,009 g | 0.6 : 1 | Concentrate — dilute before use |
Paste from the worked example (1,905 g, 63% solids). Computed by this site's engine. Dilute with distilled water, warm, and give it a day — paste dissolves slowly and the soap thickens as it cools.
Older liquid-soap instructions add borax or citric acid to “neutralize excess lye” after cooking. That was a fix for calculators that assumed pure KOH and recipes run at 0% superfat. With the purity set to 90% and a 1–3% superfat, the paste is not lye-heavy; it tests clear and a zap test confirms it. If a paste does zap after a full cook, the problem is the weighing, and the fix is a little more oil, not an acid. (Citric acid in the recipe, for a chelator, is a different thing — it goes in before the KOH, and the acid additives calculator gives the extra KOH it needs at 90% purity.)
Common liquid soap blends
| Blend (per 1,000 g oils) | KOH (90%) | Water at 25% | Cleansing | Conditioning | Bubbly |
|---|---|---|---|---|---|
| Olive 65 / coconut 25 / castor 10 (all-purpose hand soap) | 226.3 g | 679 g | 17 | 66 | 26 |
| 100% olive (castile liquid soap) | 209.0 g | 627 g | 0 | 82 | 0 |
| 100% coconut (cleaning, laundry, very bubbly) | 282.7 g | 848 g | 67 | 10 | 67 |
| Olive 50 / coconut 30 / sunflower 15 / castor 5 | 230.4 g | 691 g | 20 | 62 | 25 |
1% superfat. Quality indices as in the recipe builder — relative, not skin claims. Castor above 10% makes a stringy, hard-to-dilute paste; coconut above ~40% is drying on hands.
The glycerin method, hot process, and why the paste is clear
Cooking the paste (hot process, in a slow cooker or double boiler) pushes the reaction to completion in a few hours rather than days, which is why it tests clear and zap-free before dilution. Replacing some or all of the lye water with glycerin — the “glycerin method” — speeds the cook and gives a paste that dissolves faster; the calculator's water figure is the liquid you are replacing, and the dilution math is unchanged. Whichever method, the finished soap will be slightly alkaline (pH 9–10) because that is what potassium soap is; “pH balancing” it to 7 with acid breaks it back into fatty acids and cloud.