Water Discount vs Lye Concentration: What the Numbers Actually Mean (With a Conversion Chart)
Lye math · Water and concentration · Updated 2026-07-22
Lye concentration is the percentage of your lye solution that is alkali: NaOH divided by (NaOH plus water), times 100. Water as percent of oils is a completely different measurement - grams of water per 100 g of fats - and the two numbers are never interchangeable. A 33 percent lye concentration means a 2:1 water:lye ratio, or 203 g of water per 100 g of NaOH, which for a typical bar recipe lands near 29 percent water as percent of oils.
A "water discount" is just the act of using less water than whatever you treat as full water. This guide gives the exact conversion between the three notations, a chart you can work from, and a straight account of what physically changes in the pot when you move the number.
The two formulas, in full
Both notations describe the same jar of caustic solution. Only the denominator changes.
- Lye concentration % = alkali weight / (alkali weight + water weight) x 100. Denominator is the solution.
- Water:lye ratio = water weight / alkali weight. Same information, expressed as a ratio.
- Water as % of oils = water weight / oil weight x 100. Denominator is the oils, and it says nothing about the solution on its own.
- Converting between the first two: water:lye ratio = (100 / concentration) - 1. Concentration = 100 / (1 + ratio).
The third one is the trap. Water as percent of oils only becomes a concentration once you know how much lye the recipe needs, and that depends entirely on the oil blend. This is why quoting one as the other seizes batches.
Lye concentration conversion chart
| Lye concentration | Water:lye ratio | Water per 100 g NaOH | Approx. water as % of oils | Effect on trace and set |
|---|---|---|---|---|
| 25% | 3.00 : 1 | 300 g | ~43% | Slowest trace, longest working time. Hot process, liquid soap paste and shave soap territory. Soft in the mould for cold process. |
| 28.6% | 2.50 : 1 | 250 g | ~36% | Very slow trace. About the lowest concentration commonly used for cold process; suits high-coconut recipes that would otherwise race. |
| 30% | 2.33 : 1 | 233 g | ~33% | Slow, forgiving trace. Long window for intricate swirls; longer wait to unmould. |
| 33% | 2.03 : 1 | 203 g | ~29% | The common all-purpose setting. Comfortable working time, firm enough to cut in 24 to 48 hours. |
| 35% | 1.86 : 1 | 186 g | ~27% | Noticeably brisker. Good for layers and simple in-the-pot swirls. |
| 40% | 1.50 : 1 | 150 g | ~22% | Fast trace, early unmould, fewer glycerin rivers. Standard for high-olive and other slow-saponifying recipes. |
| 45% | 1.22 : 1 | 122 g | ~18% | Heavy discount. Little room for hand-stirring; blend briefly and mould immediately. |
| 50% | 1.00 : 1 | 100 g | ~14% | Maximum practical concentration. The solution runs very hot and NaOH can recrystallise as it cools, so it must be used warm. |
Read the chart left to right and the physics is simple. Water is not a reactant in saponification; it is the solvent that lets the alkali and the fatty acids meet. Take water out and the reaction runs in a more crowded, hotter medium, so trace arrives sooner, the batch firms sooner, and the bar reaches usable hardness sooner because there is less water to evaporate during cure. Add water back and you buy the working time that fine swirls, multi-colour pours and detailed moulds require.
Worked example: 1,000 g of oils
Take 300 g coconut, 300 g palm, 400 g olive at 5 percent superfat. That blend needs about 144 g of NaOH (54.9 + 42.5 + 54.1 = 151.6 g at zero superfat, times 0.95). To hit 33 percent lye concentration, divide the alkali by the concentration to get total solution weight: 144 / 0.33 = 436 g of solution, so water = 436 - 144 = 292 g. Check it against the ratio column: 292 / 144 = 2.03. Correct.
Now run the same recipe at 38 percent water of oils: 380 g of water. Concentration = 144 / (144 + 380) = 27.5 percent. That is 88 g of extra water, a visibly slower trace, a softer loaf at 24 hours and a longer wait before the bar stops sweating. Neither setting is wrong. They are different soaps, and the difference is legible in advance if you convert first.
Recommended concentration band by technique
| Technique or recipe | Typical lye concentration | Water:lye ratio | Why |
|---|---|---|---|
| Intricate swirl, drop swirl, column pour | 28% to 31% | 2.57 : 1 to 2.23 : 1 | Maximum working time before the batter thickens past pouring viscosity. |
| Layered pour, in-the-pot swirl | 31% to 35% | 2.23 : 1 to 1.86 : 1 | Enough body to hold a layer without breaking through, still pourable. |
| Plain bar, all-purpose cold process | 33% | 2.03 : 1 | The common default. Comfortable window, cut at 24 to 48 hours. |
| High-olive and 100% olive castile | 38% to 40% | 1.63 : 1 to 1.50 : 1 | Oleic-heavy blends saponify slowly and stay soft for weeks. A heavy discount is standard practice here, not an advanced trick. |
| Hot process and liquid soap paste | 25% to 30% | 3.00 : 1 to 2.33 : 1 | Water evaporates during the cook, so it has to be there at the start or the paste stiffens before saponification finishes. |
| Salt bar (high coconut, 50%+ salt) | 28% to 33% | 2.57 : 1 to 2.03 : 1 | Coconut plus salt sets extremely fast regardless. The real constraint is cutting within a few hours, not the water setting. |
| Shave soap | 25% to 28% | 3.00 : 1 to 2.57 : 1 | Stearic-heavy formulas need the extra solvent to stay workable while hot. |
Set water as a percentage of oils, or set the concentration. Soap Calculator takes water at any percentage of oils and computes NaOH or KOH against verified saponification values for 202 oils, with superfat adjustable from 0 to 15 percent. Change the water figure and the lye weight, solution weight and predicted bar properties update together, so you can see the recipe you are actually about to make before the caustic goes in. Download Soap Calculator: Lye & Recipe on the App Store.
Where discounts go wrong
- Quoting a concentration as a water percentage: Someone says "I use 33 percent water" meaning 33 percent lye concentration. Applied literally as 33 percent of oils, that is a much weaker solution than intended and the batch stays soupy for hours.
- Discounting an already-fast recipe: High-coconut, high-butter and high-stearic blends trace fast at any water level. Pushing them to 40 percent concentration with a fragrance oil that accelerates is the classic route to a seized, ricing pot.
- Discounting and then adding liquid additives: Milk, beer, purees and aloe all carry water. If you swap part of the water for a liquid additive, count it as water or the concentration you calculated is not the concentration you have.
- Forgetting the heat: Dissolving the same alkali into less water releases the same heat into a smaller mass. A 50 percent solution can exceed the temperature a thin plastic pitcher will tolerate. Use heat-safe containers and full PPE - goggles, gloves, long sleeves - at every concentration.
How Soap Calculator handles the water setting
Soap Calculator is built around water at any percentage of oils, so you enter the figure you already work in and it resolves the rest: NaOH for solid bars or KOH for liquid soap, superfat from 0 to 15 percent, and the seven predicted bar properties - hardness, cleansing, conditioning, bubbly, creamy, iodine and INS - alongside a fatty-acid ribbon that shifts live as you adjust an oil. PPE prompts render in safety red. Batches save with notes and photos, the cure timer runs the full four to six weeks on the Lock Screen and Dynamic Island, and labels export as compliant INCI lists. Everything runs on-device, no account.
Frequently asked questions
What does 33 percent lye concentration mean?
It means 33 percent of the lye solution by weight is NaOH and 67 percent is water: a 2.03:1 water:lye ratio, or about 203 g of water per 100 g of NaOH. For a typical bar recipe at 5 percent superfat that works out near 29 percent water as percent of oils. It is the most commonly used all-purpose setting for cold process.
Is water discount the same as lye concentration?
No. Lye concentration is a fixed property of the solution: alkali divided by alkali plus water. A water discount is the act of using less water than your chosen baseline, so it only has meaning relative to that baseline. Discounting from 38 percent water of oils down to 33 percent is a different move in a castile than in a coconut bar, because the two recipes need different amounts of lye.
What is a 2:1 water to lye ratio?
Two grams of water for every gram of NaOH, which is a 33.3 percent lye concentration. It is the usual shorthand for the standard cold process setting and gives roughly 200 g of water per 100 g of alkali.
Why do people use a heavy water discount for castile soap?
Olive oil is dominated by oleic acid and saponifies slowly, so a 100 percent olive bar can sit soft in the mould for a week or more at full water. Running 38 to 40 percent lye concentration gets the batch to trace and to a cuttable firmness in a workable timeframe and reduces glycerin rivers. The bar still needs a long cure; the discount shortens the wait to unmould, not the cure.