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; use the actual supplier figure, whether higher or lower. 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 step bar soap doesn't have. 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 1.5 to 3.2 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 to dilute before use |
Paste from the worked example (1,905 g, 63% solids). Water to add = solids ÷ the finished concentration, minus the paste's weight. Dilute with distilled water, warm, and give it a day: paste dissolves slowly and the soap thickens as it cools.
Why does my cooked paste need a different amount of water?
This estimate treats paste weight as oils plus KOH flakes plus the water added. It does not subtract evaporation during cooking or include added glycerin. Its solids figure is an approximation of non-water material, including glycerin formed in the reaction and unsaponified oil, not a laboratory measurement of active soap. Weigh the cooked paste and record dilution trials; compare samples at room temperature before thinning more.
Cloudiness can have several causes, and a clear diluted sample does not prove there is no free alkali. A low superfat setting also cannot certify the batch. Recheck the actual oils, weights and purity; the testing guide explains why pH alone cannot answer the question. Do not add acid just to chase clarity or a neutral pH. Acids intentionally used in the recipe need a separate lye adjustment.
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 numbers, not skin claims. Castor above 10% makes a stringy, hard-to-dilute paste; coconut above ~40% puts the cleansing number above the usual 12–22.
Can I replace the lye water with glycerin?
This calculator models water, not a glycerin solvent. Substituting glycerin changes the liquid composition and the dilution calculation; do not treat the printed water weight as a general replacement instruction. Use a complete formulation and method for that process. The process guide explains what cooking changes, and the lye-handling checklist still applies to potassium hydroxide.