KOH Lye Calculator
SAP values are already quoted in KOH, so there is no 1402.5 here — only the 1000 that turns milligrams into grams. KOH flakes take up water from the air and are usually sold at 90% purity, so the pure requirement has to be divided by that purity to get the weight you actually put on the scale. Oil at 500 g and SAP 190, 3% superfat, 90% purity comes to 102 g. SAP still belongs to the oil, so blends are worked one oil at a time and summed.
Your numbers
Lye needed
102.4g
Water
307g
Batter total
910g
Formula
Lye needed = Oil weight × SAP value ÷ 1000 × (1 − Superfat ÷ 100) ÷ (Lye purity ÷ 100)
Lye into the water here too — reversed, it boils and spits. Goggles, gloves, and a stainless or HDPE vessel; aluminium is attacked by lye. KOH runs hotter than NaOH, so the peak temperature is higher. Liquid soap keeps superfat low, 0–3%, because unsaponified oil will not dissolve and clouds the finished soap. Short on lye and free oil floats on top; long on lye and the pH climbs past usable.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Oil weight (g)Total oils in the recipe, excluding water and lye. | 500 | 1 and up |
| SAP valueMilligrams of KOH to saponify one gram of that oil; it differs per oil. | 190 | 100 ~ 300 |
| Superfat (%)Oil left unsaponified; the lye is cut by this much. | 3 | 0 ~ 10 |
| Lye purity (%)Purity of the lye — KOH is usually 90%. | 90 | 50 ~ 100 |
Step by step
Quick reference table
Results when only Oil weight (g) changes and everything else stays put.
| Oil weight (g) | Lye needed (g) | Water (g) | Batter total (g) |
|---|---|---|---|
| 250 | 51.2 | 154 | 455 |
| 375 | 76.8 | 230 | 682 |
| 500 | 102.4 | 307 | 910 |
| 750 | 153.6 | 461 | 1,364 |
| 1,000 | 204.8 | 614 | 1,819 |
What each result means
| Result | At default values |
|---|---|
| Lye needed (g)Lye needed; too little leaves it soft, too much leaves it caustic. | 102.4 |
| Water (g)Water to dissolve in; lye goes into water, never the reverse. | 307 |
| Batter total (g)Oils, water and lye together. | 910 |
Common mistakes
Lye into the water here too — reversed, it boils and spits. Goggles, gloves, and a stainless or HDPE vessel; aluminium is attacked by lye. KOH runs hotter than NaOH, so the peak temperature is higher. Liquid soap keeps superfat low, 0–3%, because unsaponified oil will not dissolve and clouds the finished soap. Short on lye and free oil floats on top; long on lye and the pH climbs past usable.
Glossary
- Oil weight
- Total oils in the recipe, excluding water and lye.
- SAP value
- Milligrams of KOH to saponify one gram of that oil; it differs per oil.
- Superfat
- Oil left unsaponified; the lye is cut by this much.
- Lye purity
- Purity of the lye — KOH is usually 90%.
- Lye needed
- Lye needed; too little leaves it soft, too much leaves it caustic.
- Water
- Water to dissolve in; lye goes into water, never the reverse.
- Batter total
- Oils, water and lye together.
Frequently asked questions
Q. How is KOH Lye Calculator calculated?
Lye needed = Oil weight × SAP value ÷ 1000 × (1 − Superfat ÷ 100) ÷ (Lye purity ÷ 100) — SAP values are already quoted in KOH, so there is no 1402.5 here — only the 1000 that turns milligrams into grams. KOH flakes take up water from the air and are usually sold at 90% purity, so the pure requirement has to be divided by that purity to get the weight you actually put on the scale. Oil at 500 g and SAP 190, 3% superfat, 90% purity comes to 102 g. SAP still belongs to the oil, so blends are worked one oil at a time and summed.
Q. Can you walk through an example?
With Oil weight 500g, SAP value 190, Superfat 3%, Lye purity 90%, the answer is Lye needed 102.4g.
Q. What do I need to enter?
Enter Oil weight, SAP value, Superfat, Lye purity. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Oil weight (g) moves the answer to Oil weight (g) 250 → Lye needed (g) 51.2 and Oil weight (g) 1,000 → Lye needed (g) 204.8. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Lye into the water here too — reversed, it boils and spits. Goggles, gloves, and a stainless or HDPE vessel; aluminium is attacked by lye. KOH runs hotter than NaOH, so the peak temperature is higher. Liquid soap keeps superfat low, 0–3%, because unsaponified oil will not dissolve and clouds the finished soap. Short on lye and free oil floats on top; long on lye and the pH climbs past usable.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.