How can 64 grams of white flakes turn half a kilo of olive oil into a hard bar of soap? The answer is a number called the saponification value. Every oil has one, and it tells you exactly how much lye that oil will use up. Get it right and the bar is mild. Get it wrong and the bar is greasy or harsh.
Multiply each oil’s weight by its saponification (SAP) value, add the results, then shrink the total by your superfat. For 500 g of olive oil at a SAP of 0.135 and a 5 percent superfat, that is 500 x 0.135 x 0.95 = 64.1 g of sodium hydroxide. Water only sets the strength of the lye solution, not the amount of lye.
What Is a Saponification Value?
A saponification value is the amount of lye that one gram of a fat or oil can react with. Chemists write it in milligrams of lye per gram of fat. Soap makers divide by 1,000 and use grams of lye per gram of oil instead.
Saponification is the reaction between fats and a strong base. The base splits the fat into soap and glycerin. Once every bit of lye has reacted, the reaction stops, so the lye amount controls the whole batch.
Each oil has its own value because oils are built from fatty acids of different lengths. Short fatty acids, common in coconut oil, need more lye per gram. Longer ones, common in olive oil, need less. That is why swapping one oil for another always changes the recipe.
Lab Values Versus Soap Values
Lab tables usually list values for potassium hydroxide (KOH). Olive oil, for example, measures 184 to 196 mg of KOH per gram. Dividing by 1.403, the ratio of the two molecular weights, converts that to about 0.131 to 0.140 g of sodium hydroxide per gram. The 0.135 used below sits inside that range.
Turning SAP Values Into Grams of Lye
The whole method fits on one line: lye = oil weight x SAP value x (1 – superfat). Superfat is written as a decimal, so 5 percent becomes 0.05 and the multiplier becomes 0.95.
Superfat, also called a lye discount, leaves a small share of oil unreacted on purpose. That spare oil softens the bar and acts as a safety margin. It also covers small errors in scales and in the SAP values themselves, which vary from one batch of oil to the next.
| Oil or fat | NaOH SAP (g per g) | Lye for 100 g at 0% | Lye for 100 g at 5% |
|---|---|---|---|
| Coconut oil | 0.178 | 17.8 g | 16.9 g |
| Palm oil | 0.142 | 14.2 g | 13.5 g |
| Lard or tallow | 0.138 | 13.8 g | 13.1 g |
| Cocoa butter | 0.137 | 13.7 g | 13.0 g |
| Olive oil | 0.135 | 13.5 g | 12.8 g |
| Castor oil | 0.128 | 12.8 g | 12.2 g |
Mixing a 500 Gram Olive Oil Batch
Our calculator’s own example uses 500 g of olive oil, a 5 percent superfat and water at 33 percent of the oils. Here is the math, step by step.
- Full lye: 500 x 0.135 = 67.5 g of NaOH to react with every gram of oil.
- After superfat: 67.5 x 0.95 = 64.1 g of NaOH (64.125 before rounding).
- Water: 500 x 0.33 = 165 g of distilled water.
- Batch weight: 500 + 64.1 + 165 = about 729 g before any scent or color.
- Lye concentration: 64.1 / (64.1 + 165) = 28.0 percent.
Switch the same 500 g to coconut oil and the lye jumps to 84.6 g. That is 20.4 g more lye for the same weight of oil.
Adding a Second Oil
Most recipes blend oils, so you run each oil on its own and add the lye amounts. Take 700 g of olive oil and 300 g of coconut oil. Olive needs 94.5 g and coconut needs 53.4 g, for 147.9 g in total.
At a 5 percent superfat, that becomes 147.9 x 0.95 = 140.5 g of lye. To double or halve a trusted recipe, scale every weight by the same factor, as our guide to working with ratios and proportions shows.
The Soap Lye Calculator gives the lye, water, batch weight and lye concentration for nine common oils. Run each oil in a blend separately and add the lye totals.
How Much Water Does the Lye Need?
Water does not change how much lye your oils need. It only dissolves the lye and sets the strength of the solution. Our calculator starts at water equal to 33 percent of the oil weight.
Some makers set water a different way, by lye concentration. The formula is lye / (lye + water). A 30 percent solution for 64.1 g of lye needs 64.1 x 70 / 30 = 149.6 g of water.
| Method | Water | Lye concentration |
|---|---|---|
| 33 percent of 500 g oils | 165 g | 28.0% |
| 30 percent lye solution | 149.6 g | 30.0% |
| 2 parts water to 1 part lye | 128.3 g | 33.3% |
Less water gives a stronger solution, which our calculator notes can speed up trace and shorten cure time.
NaOH or KOH?
Sodium hydroxide (NaOH) makes solid bars. Potassium hydroxide (KOH) makes liquid soap and needs about 1.403 times more by weight. Olive oil at 0.135 for NaOH becomes about 0.189 for KOH. Our calculator handles NaOH only, so never swap the two lyes gram for gram.
Weighing Errors That Ruin a Batch
Almost every failed batch traces back to one of these slips. Each one is easy to catch before the lye touches the oil.
- Measuring by volume. Oils differ in density, so a cup of one oil does not weigh the same as a cup of another. Weigh oils, lye and water in grams.
- Reusing lye across oils. Lye figured for olive oil is wrong for coconut. On 500 g, the gap is 20.4 g.
- Skipping the superfat. A zero superfat leaves no margin for scale error or SAP variation. Our calculator suggests 5 to 8 percent.
- Mixing up NaOH and KOH. Using NaOH values with KOH leaves the soap short of lye and oily.
- Forgetting to tare the scale. Zero the scale with the empty container on it. A 20 g jug left in the reading means 20 g too little lye.
How Do You Handle Lye Safely?
Treat sodium hydroxide as a corrosive chemical: protect your eyes and skin, and add lye slowly to water. The federal pocket guide for this chemical says to prevent all skin and eye contact.
Solid lye and strong lye solutions can cause deep burns and severe eye damage. Federal medical guidance warns that dissolving it in water releases a lot of heat. Always stir the lye into the water, never pour water onto dry lye.
- Wear chemical splash goggles and gloves, with long sleeves over your arms.
- Mix in a ventilated space, away from children and pets.
- Use stainless steel or heat-safe plastic. The pocket guide lists aluminum, tin and zinc as metals lye attacks.
- On skin, flush with water for at least 15 minutes. For eyes, the guidance says to flush for at least 30 minutes and get medical help.
Once your safety gear is ready, run your final weights through the soap making lye calculator one more time. Then weigh, mix and pour with the numbers in front of you.
Common Questions About Soap Lye Calculations
How Do You Calculate Lye for Soap?
Multiply the oil weight by its SAP value, then multiply by one minus the superfat. For 500 g of olive oil at 5 percent superfat, that is 500 x 0.135 x 0.95 = 64.1 g of sodium hydroxide.
What Superfat Should a Beginner Use?
Most soap makers use a superfat of 5 to 8 percent. On 500 g of olive oil, 5 percent gives 64.1 g of lye and 8 percent gives 62.1 g. The higher number leaves more free oil and a softer bar.
Does More Water Mean You Need More Lye?
No. The lye amount depends only on the oils and the superfat. Water just dissolves the lye and sets the solution strength, such as 28 percent with 165 g of water or 30 percent with 149.6 g.
How Do You Calculate Lye for a Blend of Oils?
Work out the lye for each oil separately and add the results before the superfat. For 700 g of olive oil and 300 g of coconut oil, that is 94.5 g plus 53.4 g, or 140.5 g at 5 percent superfat.
Can You Convert NaOH Values to KOH Values?
Yes. Multiply an NaOH value by 1.403, the ratio of the molecular weights of KOH and NaOH. Olive oil at 0.135 for NaOH becomes about 0.189 for KOH.
Why Does Coconut Oil Need More Lye Than Olive Oil?
Coconut oil is rich in shorter fatty acids, so each gram holds more fat molecules for lye to react with. Its SAP value is 0.178 against 0.135 for olive oil, so 500 g needs 84.6 g of lye instead of 64.1 g at 5 percent superfat.
References
References Used in This Article
This article is general soap-making education and does not replace a full recipe check. SAP values match our calculator and fall within published ranges. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 27, 2026.
Author
Shakeel Muzaffar is the Founder and Editor-in-Chief of MultiCalculators.com, bringing over 15 years of experience in digital publishing, product strategy, and online tool development. He leads the platform's editorial vision, ensuring every calculator meets strict standards for accuracy, usability, and real-world value. Shakeel personally oversees content quality, formula verification workflows, and the platform's commitment to publishing tools that are genuinely useful for students, professionals, and everyday users worldwide.
