What saponification actually is, and why the lye number is not negotiable
Soap is the salt of a fatty acid. When you mix a triglyceride with a strong hydroxide, the ester bonds break and each fatty acid chain pairs with a sodium or potassium ion, releasing glycerol. Sodium hydroxide gives a hard, water-tolerant salt that you can cut into bars. Potassium hydroxide gives a much softer, far more soluble salt, which is why liquid and paste soaps are made with KOH.
Every oil demands a different amount of hydroxide because every oil is a different mixture of fatty acids with different chain lengths. Coconut oil is dominated by short lauric and myristic chains, so a gram of it contains more ester bonds than a gram of long-chain olive oil, and needs about 41% more sodium hydroxide. That per-gram demand is the saponification value, and it is measured in a laboratory by the standard AOCS titration method (Cd 3-25), which reports milligrams of KOH consumed per gram of fat. Divide that published figure by 1,403 and you have the grams of NaOH per gram of oil that this calculator uses.
You cannot round the lye figure. Too little hydroxide leaves the batch oily and soft; too much leaves free caustic in the finished bar, which will burn skin. The whole method exists to keep you on the safe side of that line with a deliberate margin.
The formula, term by term
You calculate the lye demand oil by oil, then discount it. For each oil, multiply its weight by its SAP value in grams of NaOH per gram of oil, and add the results. That sum is the lye needed to convert every last ester bond in the pot — a zero-superfat recipe.
Superfat is the deliberate shortfall. A 5% superfat means you supply only 95% of the theoretical lye, so roughly 5% of the oil weight survives unsaponified and stays in the bar as free oil. Soapmakers also call this a lye discount, and the two terms mean the same arithmetic from opposite ends. It exists for three reasons: published SAP values are lot averages rather than measurements of your specific bottle, scales have tolerance, and a small oil surplus improves the feel of the bar.
Lye concentration sets the water. A 33% solution means the lye is 33% of the combined lye-plus-water weight, so the water is the other 67%. That gives water = lye × (100/33 − 1) = lye × 2.030. Older recipes state the same thing as a water-to-lye ratio — a 2:1 ratio is a 33.3% solution — or as a percentage of oil weight, which is the least reliable of the three because it ignores how much lye the oils actually demand.
KOH conversion. Potassium's molar mass is 39.10 g/mol against sodium's 22.99, so KOH (56.11 g/mol) is 1.403 times heavier than NaOH (39.997 g/mol) for the same number of hydroxide ions. Multiply the NaOH figure by 1.403. Then divide by the purity, because flake KOH is typically sold at an assay of 90%: to deliver 100 g of active KOH from a 90% flake you must weigh 111.1 g.
Worked example: a 1,000 g three-oil bar recipe
Take 600 g olive oil, 250 g coconut oil and 150 g shea butter, at 5% superfat, a 33% lye solution and 3% fragrance.
- Lye demand per oil. Olive: 600 × 0.1345 = 80.70 g NaOH. Coconut: 250 × 0.1900 = 47.50 g. Shea: 150 × 0.1280 = 19.20 g.
- Sum. 80.70 + 47.50 + 19.20 = 147.40 g of NaOH at zero superfat.
- Apply the 5% superfat. 147.40 × 0.95 = 140.03 g NaOH. That is what you weigh.
- Water for a 33% solution. 140.03 × (100/33 − 1) = 140.03 × 2.0303 = 284.30 g water. The solution weighs 424.33 g and is 33% lye by weight.
- Fragrance. 3% of the 1,000 g oil weight = 30 g, stirred in at light trace.
- Batch weight. 1,000 + 140.03 + 284.30 + 30 = 1,454.3 g of raw soap into the mould, before any water evaporates during cure.
Run the same recipe as liquid soap and step 3 becomes 147.40 × 1.403 ÷ 0.90 = 229.8 g of 90% KOH flake at zero superfat. Liquid soapmakers usually run 0-3% superfat, because free oil clouds a finished liquid soap.
How to read the numbers you get back
Superfat. Cold-process bars are almost always made at 5%, with 1-2% for laundry and salt-free castile bars that want maximum hardness and 8% for facial bars. Above about 12% the surplus oil starts to soften the bar and shorten shelf life, because the free oil is the part that oxidises and produces the orange rancidity spots soapmakers call dreaded orange spots. Below zero the recipe is lye-heavy and should not be used at all.
Water. The water only carries the lye; none of it stays in the finished bar. A 33% solution is the safe default. Experienced makers run a water discount at 35-40% solution to firm the bar up faster and cut cure time, but a stronger solution reaches a higher temperature when mixed and accelerates trace, which leaves less working time for swirls. Below 25% solution the extra water lengthens the cure and the bar shrinks noticeably in the mould.
Batch weight. Use it to size your mould. Cold-process soap has a density near 0.9 g/cm³ once poured, so a 1,454 g batch fills roughly 1,600 cm³ — near enough a standard 10-inch silicone loaf mould. Expect the cured bars to weigh 10-15% less than the poured batch as water leaves over four to six weeks.
If you are costing bars for sale, feed the finished batch weight and your oil prices into the cost per unit calculator and check your listing fees with the Etsy fee calculator before you set a price.
Saponification values for common soaping oils
| Oil or fat | NaOH (g/g) | KOH, 100% (g/g) | Typical role in a recipe |
|---|---|---|---|
| Coconut oil, 76 deg | 0.1900 | 0.2666 | Hardness and lather, 15-30% |
| Palm kernel oil | 0.1560 | 0.2189 | Coconut substitute, 15-25% |
| Palm oil | 0.1410 | 0.1978 | Hardness, 20-35% |
| Beef tallow | 0.1405 | 0.1971 | Traditional hardness, 20-50% |
| Lard | 0.1380 | 0.1936 | Creamy bar, 20-50% |
| Cocoa butter | 0.1370 | 0.1922 | Brittle hardness, 5-15% |
| Mango butter | 0.1370 | 0.1922 | Conditioning butter, 5-15% |
| Sweet almond oil | 0.1360 | 0.1908 | Conditioning, 5-15% |
| Olive oil | 0.1345 | 0.1887 | Base oil, up to 100% |
| Sunflower oil | 0.1340 | 0.1880 | Base oil, up to 20% |
| Avocado oil | 0.1330 | 0.1866 | Conditioning, 5-20% |
| Canola oil | 0.1324 | 0.1858 | Economy base oil, up to 25% |
| Castor oil | 0.1286 | 0.1804 | Lather booster, 3-8% |
| Shea butter | 0.1280 | 0.1796 | Conditioning butter, 5-20% |
| Rice bran oil | 0.1280 | 0.1796 | Olive substitute, up to 30% |
| Jojoba | 0.0690 | 0.0968 | Wax ester, 1-5% |
These are the mid-range published averages used across soapmaking references. A specific lot can differ by a few percent, which is exactly what your superfat margin absorbs.
Handling hydroxide safely
Solid NaOH and KOH are corrosive solids that cause deep, painful burns and permanent eye injury. Wear splash goggles and nitrile gloves every time. Always add the lye to the water, never water to the lye — the reaction is strongly exothermic and a solution flashing to boiling in a jug throws caustic droplets. Mix in a heat-tolerant HDPE or stainless vessel, never aluminium, which reacts with hydroxide and releases hydrogen. Work where the vapour can escape, and keep the batch away from children and pets until it is fully saponified.
Mistakes that ruin a batch
- Measuring oils by volume. A cup of coconut oil and a cup of olive oil have different masses and very different lye demands. Everything in soapmaking is weighed.
- Using a NaOH figure for KOH. Missing the 1.403 factor underdoses the lye by 29% and leaves you with an oily sludge that never traces.
- Forgetting the KOH assay. Weighing 90% flake as if it were pure underdoses the active hydroxide by 10%, which is a hidden extra superfat on top of the one you chose.
- Counting fragrance, additives or lye water in the oil weight. Superfat and fragrance percentages are both taken on the oil weight alone. Sodium lactate, clays and colourants do not saponify and do not change the lye figure.
- Substituting an oil without recalculating. Swapping shea for coconut at the same weight raises the lye demand by 48% for that portion of the recipe, which turns a 5% superfat into a lye-heavy bar.
- Trusting an old recipe's lye weight. Enter it in the override field and read the effective superfat before you use it; recipes copied between sites frequently carry the wrong figure.
Where this fits: cold process, hot process, liquid and dual lye
The lye arithmetic is identical for cold and hot process. The difference is only in how the reaction is driven: cold process relies on the heat of the reaction itself and a four-to-six week cure, while hot process cooks the batch until saponification finishes in the pot, so the soap is technically usable the next day but still benefits from a cure for hardness. Neither changes the amount of hydroxide the oils demand.
Liquid soap uses KOH and normally a 0% superfat, then a dilution step afterwards: the cooked paste is thinned with distilled water at roughly one part paste to one part water, adjusted to taste. Because dilution comes later, this calculator's water figure covers only the water needed to dissolve the lye for the cook.
Some makers use a dual lye blend — say 90% NaOH and 10% KOH — to get a slightly softer, more soluble bar with better lather. Calculate each hydroxide separately at its share of the total demand and add them. Cream soaps and shaving soaps push that further, running KOH shares of 30-60%.
Two things this calculator deliberately does not do. It does not check whether your oil blend makes a good bar — the fatty-acid profile that controls hardness, lather and conditioning is a separate question from lye demand, and a 100% coconut bar at 5% superfat is perfectly saponified and still harsh on skin. And it does not compute lye for melt-and-pour bases, which are already saponified; you add nothing caustic to those at all. If you are working with other studio chemistry, the pH calculator is a useful companion when you test a cured bar, and the salt percentage calculator covers brine soaps where a saturated salt solution replaces part of the water.
Key terms
- SAP value
- The saponification value of a fat: the mass of hydroxide needed to saponify one unit mass of it. Laboratories publish it as mg KOH per g of fat; soapmakers usually use the g NaOH per g form, which is the published figure divided by 1,403.
- Superfat
- The percentage of oils left deliberately unsaponified by supplying less lye than the theoretical demand. Also called a lye discount.
- Lye concentration
- Lye as a percentage of the lye-plus-water solution by weight. A 33% solution is the common default; 40% is a water discount.
- Trace
- The point at which the emulsion thickens enough that a drizzle from the stick blender leaves a visible trail on the surface. It marks the end of the mixing window.
- Zap test
- Touching a cured bar to the tongue. A sharp electric sting means free hydroxide remains and the bar is lye-heavy. A safe bar tastes only of soap.