Lye Calculator (NaOH)
SAP values are published as milligrams of potassium hydroxide per gram of oil, so using one for NaOH means dividing by 1402.5 — the molar ratio of KOH at 56.1 against NaOH at 40.0, scaled from milligrams. Olive oil has a SAP of 190, so 500 g at 5% superfat needs 64 g of lye. The value belongs to the oil, not to soap in general: this tool takes one oil at a time, and a blended recipe is worked oil by oil and the lye added up. Water is shown alongside at twice the lye as a starting point.
Your numbers
Lye needed
64.3g
Water
129g
Batter total
693g
Formula
Lye needed = Oil weight × SAP value ÷ 1402.5 × (1 − Superfat ÷ 100)
Lye goes into the water, never water into the lye — the wrong order can boil up and spit in seconds. Wear goggles and gloves, and mix in stainless steel or HDPE, never aluminium, which lye attacks and turns into hydrogen. The solution heats itself to 80–90°C on contact, so no glass and no thin plastic. Too little lye leaves a soft bar that never firms up; too much leaves a caustic one.
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. | 5 | 0 ~ 20 |
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 | 32.2 | 64 | 347 |
| 375 | 48.3 | 97 | 520 |
| 500 | 64.3 | 129 | 693 |
| 750 | 96.5 | 193 | 1,040 |
| 1,000 | 128.7 | 257 | 1,386 |
What each result means
| Result | At default values |
|---|---|
| Lye needed (g)Lye needed; too little leaves it soft, too much leaves it caustic. | 64.3 |
| Water (g)Water to dissolve in; lye goes into water, never the reverse. | 129 |
| Batter total (g)Oils, water and lye together. | 693 |
Common mistakes
Lye goes into the water, never water into the lye — the wrong order can boil up and spit in seconds. Wear goggles and gloves, and mix in stainless steel or HDPE, never aluminium, which lye attacks and turns into hydrogen. The solution heats itself to 80–90°C on contact, so no glass and no thin plastic. Too little lye leaves a soft bar that never firms up; too much leaves a caustic one.
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 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 Lye Calculator (NaOH) calculated?
Lye needed = Oil weight × SAP value ÷ 1402.5 × (1 − Superfat ÷ 100) — SAP values are published as milligrams of potassium hydroxide per gram of oil, so using one for NaOH means dividing by 1402.5 — the molar ratio of KOH at 56.1 against NaOH at 40.0, scaled from milligrams. Olive oil has a SAP of 190, so 500 g at 5% superfat needs 64 g of lye. The value belongs to the oil, not to soap in general: this tool takes one oil at a time, and a blended recipe is worked oil by oil and the lye added up. Water is shown alongside at twice the lye as a starting point.
Q. Can you walk through an example?
With Oil weight 500g, SAP value 190, Superfat 5%, the answer is Lye needed 64.3g.
Q. What do I need to enter?
Enter Oil weight, SAP value, Superfat. 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) 32.2 and Oil weight (g) 1,000 → Lye needed (g) 128.7. 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 goes into the water, never water into the lye — the wrong order can boil up and spit in seconds. Wear goggles and gloves, and mix in stainless steel or HDPE, never aluminium, which lye attacks and turns into hydrogen. The solution heats itself to 80–90°C on contact, so no glass and no thin plastic. Too little lye leaves a soft bar that never firms up; too much leaves a caustic one.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.