Water as % of oils, lye concentration or water : lye ratio: what's the difference?
All three describe the same thing, how much water dissolves the lye; they just measure it against different things. Lye concentration is lye ÷ (lye + water), the water : lye ratio is water ÷ lye, and water as % of oils is water ÷ oil weight. The first two follow the lye, so they convert exactly (a 2 : 1 ratio is a 33.3% concentration); % of oils ignores how much lye the oils need.
The three formulas
- Lye concentration = lye ÷ (lye + water) × 100. Higher means less water. Water = lye × (100 ÷ concentration − 1).
- Water : lye ratio = water ÷ lye. Higher means more water. Concentration = 100 ÷ (1 + ratio).
- Water as % of oils = water ÷ total oil weight × 100. Common in older printed recipes and the default in some calculators.
Worked example: one recipe, three settings
1000 g of oils (Olive oil 50%, Coconut oil 25%, Shea butter 20%, Castor oil 5%), 5% superfat, 100% NaOH. Only the water setting changes:
| Water set by | NaOH | Water | Lye concentration | Water : lye | % of oils |
|---|---|---|---|---|---|
| 33% lye concentration | 138.5 g | 281.3 g | 33% | 2.03 : 1 | 28.1% |
| 2 : 1 water : lye | 138.5 g | 277.1 g | 33.3% | 2 : 1 | 27.7% |
| 38% of oils | 138.5 g | 380.0 g | 26.7% | 2.74 : 1 | 38% |
The lye is the same in every row. A 33% concentration and a 2 : 1 ratio are almost the same amount of water; 38% of oils is noticeably more water for this recipe.
Why % of oils behaves differently
Oils need different amounts of lye, but % of oils doesn't know that. At the same 38% of oils, 1000 g of Coconut oil (20% superfat) needs 146.3 g of NaOH, giving a 27.8% lye concentration, while 1000 g of Olive oil (5% superfat) needs 128.7 g, giving 25.3%. Setting the water by concentration or ratio keeps the lye solution the same strength whatever the oils.
Which should you use?
Any of them works if you know what you're asking for. Lye concentration or ratio is easier to compare between recipes. Use % of oils when you're following a recipe written that way. Soap Ledger flags values outside its review bands (lye concentration 20–50%, ratio 1–4 : 1, water 15–45% of oils): they're reminders to double-check a typo, not safety limits.
Most of this water leaves the bar while it cures (Makesy: How long does bar soap need to cure?), so less water can mean a shorter wait; see cure time.
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Try it in the lye calculator. Switch the water setting in the calculator and watch the other two numbers update under the result.
Sources
Written for Soap Ledger in our own words. Numbers in the examples are calculated by Soap Ledger's engine when the page is built. Last updated .