Water to lye ratio calculator
Water as a percentage of the oils, lye concentration and the water : lye ratio are three names for one amount of water. Give any one, with your lye, and this gives the others, the least water your lye can dissolve in, and the split for a master batch.
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Results
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The least water
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From a master batch
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How it was worked out
Calculated at soapformulator.com, every value sourced. Check the figures against your own scale.
The formula, and where it comes from
water from % of oils = oils × p ÷ 100 water from concentration = lye × (100 − c) ÷ c water from ratio = lye × r concentration c = lye ÷ (lye + water) × 100 least water that dissolves = lye ÷ 1.08 (NaOH) or ÷ 1.12 (KOH), at 20 °C master batch = lye ÷ (s ÷ 100) of solution, plus water − (solution − lye)
- Dunn, Kevin M. Scientific Soapmaking: The Chemistry of the Cold Process. Clavicula Press, 2010. Lye solution concentration and its effect on trace and gel.
- Community convention: "full water" = 38 % of oil weight, the default that most hobby calculators have used since the 2000s; roughly a 27–28 % lye solution for a typical bar recipe. — A convention, not a chemical constant. Lye concentration is the better control variable because it does not drift with the oil blend.
- Solubility in water at 20 °C: sodium hydroxide 108 g per 100 g of water (OECD SIDS, 2002: it solidifies above 52 % by weight; ILO/WHO ICSC 0360 gives 109 g per 100 mL); potassium hydroxide 112 g (Ullmann’s Encyclopedia, via PubChem). — Near that limit a solution crystallises as it cools. The check only catches lye that cannot dissolve at all.
Three names for one amount of water
For 139 g of lye and 1000 g of oils, a 33 % lye solution, a water : lye ratio of 2.03 : 1 and water at 28.2 % of the oils are the same 282 g of water. Concentration is lye ÷ (lye + water), the ratio is water ÷ lye, and the percentage is water ÷ oils.
Why the concentration is the steadier setting
Water as a % of oils drifts with the recipe: the same 38 % is a stronger or weaker lye solution depending on how much lye the oils take. A concentration, or a ratio, stays the same solution whatever the oils, which is why the lye calculator starts from one. How strong the solution is changes how fast the batter traces and how it gels.
Master batching
A master batch is lye dissolved ahead of time as a stock solution, usually equal weights of lye and water. The solution already holds water, so a recipe takes a weight of solution for its lye and plain water for the rest. Label the bottle as lye and keep it where no one can take it for anything else.
Water in soap: lye concentration, water discounting and master batches
Common questions
What lye concentration is a 2 : 1 water to lye ratio?
33.3 %. Two parts of water to one of lye make three parts of solution, one of them lye. A 1 : 1 ratio is 50 %, and 3 : 1 is 25 %.
Is 38 % water the same as a 33 % lye solution?
No. For a typical bar, 139 g of lye for 1000 g of oils, water at 38 % of the oils is a 26.8 % lye solution, and a 33 % solution is water at 28.2 % of the oils. The 38 % “full water” figure is a convention many calculators start from, not a chemical constant.
What is the strongest lye solution I can make?
About 51.9 % for sodium hydroxide and 52.8 % for potassium hydroxide at 20 °C: 1 g of water dissolves 1.08 g of NaOH or 1.12 g of KOH. So 139 g of sodium hydroxide needs at least 129 g of water. Near that limit a solution crystallises as it cools, so soapmakers work well below it.
How do I use a 50 % master batch?
Weigh twice the lye’s weight of solution, then add the rest of the water as plain water. For 139 g of lye at a 33 % concentration, that is 278 g of the 50 % solution plus 143 g of water.
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