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Soap Calculator
Use this free soap calculator to work out the exact lye and water for a cold-process soap batch. Enter up to four oils with their weights, pick a superfat percentage, and choose a water ratio or a water percentage. The calculator uses the standard SAP value for each oil, so you get the right NaOH amount, plus the KOH equivalent for liquid soap.
Set an oil to 0 to leave it out. Use one row for a single-oil batch, or up to four oils for a blend.
How it works
Cold-process soap is made by mixing oils with a lye solution, and the amount of lye has to match the oils exactly. Every oil has a saponification value, usually called the SAP value, which is the grams of sodium hydroxide (NaOH) needed to turn one gram of that oil into soap. Olive oil needs 0.135 grams of NaOH per gram of oil, coconut oil needs 0.183, palm oil 0.142, castor oil 0.128, shea butter 0.128, sunflower oil 0.134, sweet almond oil 0.136, and avocado oil 0.133. The calculator multiplies each oil's weight by its SAP value and adds the results.
Superfat is the percentage of oil deliberately left unsaponified so the finished bar is gentler on skin. A 5 percent superfat, the default here, means the calculator uses 95 percent of the full lye amount, leaving 5 percent of the oils free in the bar. A higher superfat makes a more moisturizing bar but a softer one that can go rancid sooner, while a 0 percent superfat gives a harder, more cleansing bar with no safety margin if your scale is slightly off.
Water is set in one of two ways. The water-to-lye ratio, default 2:1, means two grams of water for every gram of lye, for a workable batter. A lower ratio such as 1.5:1 speeds up curing and gives a harder bar sooner, but the batter thickens fast and leaves less time for swirls. You can also set water as a percentage of total oils instead, which is how many published recipes are written; 33 to 38 percent of the oil weight is the usual range. Either way, the calculator shows the water weight in the same units as your oils.
The lye type switch covers liquid soap. Solid bar soap uses sodium hydroxide, while liquid soap uses potassium hydroxide (KOH), and KOH takes about 1.403 times as much by weight to saponify the same oils. The lye purity field covers the fact that some lye is sold at less than 100 percent purity; if your lye is 95 percent pure, the calculator increases the weighed amount so the pure lye in the batch still matches the recipe.
Enter your oils in grams or ounces, up to four oils per batch. If you only want one oil, fill in the first row and leave the others at zero. Weigh everything on a kitchen scale, never by volume, because oils and lye have different densities and a cup of oil does not weigh the same as a cup of water. Always add lye to water, never the reverse, wear gloves and eye protection, and let the solution cool to about 100 to 110 degrees Fahrenheit before mixing.
Worked example
A beginner batch uses 500 grams of olive oil, 300 grams of coconut oil, and 200 grams of palm oil, with a 5 percent superfat and a 2:1 water-to-lye ratio.
Step 1: Find the full lye for each oil — olive 500 × 0.135 = 67.5 g, coconut 300 × 0.183 = 54.9 g, palm 200 × 0.142 = 28.4 g.
Step 2: Add them up — 67.5 + 54.9 + 28.4 = 150.8 g of NaOH at zero superfat.
Step 3: Apply the 5 percent superfat — 150.8 × 0.95 = 143.26 g of NaOH.
Step 4: Find the water at a 2:1 ratio — 143.26 × 2 = 286.52 g of water.
Step 5: For a liquid-soap version, the KOH amount is 143.26 × 1.403 = 200.99 g.
The batch needs 1,000 grams of oils, 143.26 grams of NaOH, and 286.52 grams of water, which is exactly what the calculator shows for these inputs. If you make the same batch with KOH instead, weigh out 200.99 grams of KOH and keep the water the same.
FAQ
What is superfat in soap making?
Superfat is the percentage of oil left unsaponified in the finished soap so the bar is milder on skin. A 5 percent superfat means 5 percent of the oils stay free in the bar, which the calculator handles by using only 95 percent of the full lye amount. Most recipes use 3 to 8 percent.
Can I use this calculator for liquid soap?
Yes. Choose KOH as the lye type and the calculator multiplies the NaOH amount by 1.403, which is the standard conversion factor for potassium hydroxide. The water amount stays the same, and the finished soap will be a paste that you dilute into liquid soap.
Why does coconut oil need more lye than olive oil?
Each oil has its own SAP value because its fatty acids differ in size. Coconut oil has shorter fatty acid chains, so one gram of it contains more molecules to saponify and needs 0.183 grams of NaOH, while olive oil needs only 0.135 grams. That is why the calculator uses a separate SAP value for every oil in the blend.
How much water should I use for cold-process soap?
A water-to-lye ratio of 2:1 is a good default that gives a workable batter and a normal cure time. A lower ratio such as 1.5:1 makes the soap harden faster but thickens quickly, while a higher ratio gives more working time and a longer cure. As a percentage of oils, 33 to 38 percent is the usual range.
Do I need to adjust for lye purity?
Only if your lye is not 100 percent pure. Some lye is sold at 95 percent or lower purity, and using the full recipe amount would under-lye the batch. Enter the purity in the calculator and it increases the weighed amount so the pure lye matches the recipe.