What does “33% lye concentration” mean?
It describes the lye solution, not the recipe: a 33% solution is one-third sodium hydroxide and two-thirds water by weight. If your recipe needs 143.8 g of lye, a 33% solution means 292.0 g of water (143.8 ÷ 0.33 − 143.8) — 436 g of solution in the jug. Water : lye ratio says the same thing the other way round: 2 : 1 is two grams of water per gram of lye, which is a 33.3% solution. The two scale with the lye, so a recipe written in either makes the same bar whether you pour 500 g or 5 kg.
concentration = 1 ÷ (1 + ratio) and ratio = (1 − concentration) ÷ concentration. So 2 : 1 → 1 ÷ 3 = 33.3%; 1.5 : 1 → 40%; 1 : 1 → 50%. Grams of water = grams of lye × ratio.
Why “water as a percent of oils” is the odd one out
The third convention — SoapCalc's old default of “38% water as percent of oils” — sets water from the oil weight instead of the lye weight. That sounds reasonable until you notice that different recipes need different amounts of lye for the same oils. A castile needs about 129 g of NaOH per kilo; a 100% coconut bar needs 147 g. Give both 380 g of water and you get two quite different solutions:
| Recipe (1,000 g oils, “38% water”) | NaOH | Water | Lye concentration | Water : lye |
|---|---|---|---|---|
| 100% olive (castile) | 128.7 g | 380 g | 25.3% | 2.95 : 1 |
| Classic bar — olive 35 / coconut 30 / palm 30 / castor 5 | 143.8 g | 380 g | 27.5% | 2.64 : 1 |
| 100% lard | 134.1 g | 380 g | 26.1% | 2.83 : 1 |
| 100% coconut, 20% superfat | 146.6 g | 380 g | 27.8% | 2.59 : 1 |
| Shave soap — stearic 50 / coconut 25 / shea 15 / castor 10 | 141.0 g | 380 g | 27.1% | 2.70 : 1 |
5% superfat except where stated; computed by this site's engine. The same “38%” spans 2.5 percentage points of lye concentration — from a slow, wet castile to a coconut bar that will move fast.
None of those bars is wrong, but the recipe writer didn't choose them; the arithmetic did. That is why this site (and most modern soapers) state water as lye concentration or ratio, and why the converter above asks for your lye and oil weights before it will call a “% of oils” figure exact: without them it assumes a typical bar at 0.14 g of lye per gram of oil and says so.
Lye concentration vs water : lye ratio — which should I use?
Either. They are interchangeable, and the builder prints both. Ratio is the older habit on the forums (“2 : 1”, “1.5 : 1”); concentration is what the chemistry says and what most calculators default to now. Pick the one you find easier to picture and keep a chart of the other by the scale:
SoapmakingToolbox.com · water : lye ↔ lye concentration
| Water : lye | Lye concentration | Water per 100 g lye | Used for |
|---|---|---|---|
| 1 : 1 | 50.0% | 100 g | Masterbatch solution; the practical ceiling |
| 1.2 : 1 | 45.5% | 120 g | Steep discount — fast trace, experienced hands |
| 1.5 : 1 | 40.0% | 150 g | Steep discount — fast trace, experienced hands |
| 1.7 : 1 | 37.0% | 170 g | Everyday bars |
| 1.8 : 1 | 35.7% | 180 g | Everyday bars |
| 2 : 1 | 33.3% | 200 g | Everyday bars |
| 2.2 : 1 | 31.3% | 220 g | Everyday bars |
| 2.5 : 1 | 28.6% | 250 g | Slow swirls, fussy fragrances, beginners |
| 2.7 : 1 | 27.0% | 270 g | Slow swirls, fussy fragrances, beginners |
| 3 : 1 | 25.0% | 300 g | “Full water” — slow, wet, prone to soda ash |
concentration = 1 ÷ (1 + ratio). Below 1 : 1 (above 50%) the lye will not fully dissolve at room temperature.
What lye concentration should I use?
The water is only there to dissolve the lye and carry it into the oils; it leaves during the cure. Less water (a higher concentration) means faster trace, a firmer bar sooner, less warping and shrinkage, and less soda ash. More water (a lower concentration) means slower trace and more working time, a softer bar that cures longer, and more soda ash risk. Nothing about the finished, cured soap changes — only the road to it.
| Situation | Concentration | Why |
|---|---|---|
| First batches | 28–30% | More time to work; forgives a slow stick blender. |
| Everyday bars | 33% | The modern default; trace is unhurried, cure is reasonable. |
| Intricate swirls, fragrances known to accelerate | 28–30% | Buys working time. Also chill the oils. |
| High-coconut, salt bars, recipes you want out of the mold fast | 35–40% | Firms quickly; less water to lose. |
| Liquid soap (KOH) | 25% | KOH paste is diluted later anyway; more water keeps the paste workable. |
| Milk or beer as the liquid | Split method | Dissolve lye in the water share at 50% (1 : 1), add the milk to the oils. |
When someone says they used a “10% water discount”, they almost always mean 10% less than the old 38%-of-oils default — about 34% of oils, which for an ordinary bar is roughly a 29% lye concentration. It is imprecise, which is the reason to state the concentration instead: “33%” means the same solution in every kitchen.
Reading an old recipe
If a recipe lists only oils, lye and water in grams, the converter can tell you what convention it was written in. Type the water as a ratio or a concentration (divide water by lye), add the lye and oil weights, and the third figure appears. A castile listed with 1,000 g oils, 129 g lye and 380 g water was written at 38% of oils; it is a 25.3% solution, and you can now rewrite it at 33% with 262 g of water and get the same bar sooner.