How to Calculate Lye for Soap Making by Hand (and How Much Lye per Pound of Oils)
Lye math · Calculation method · Updated 2026-07-22
There is no fixed amount of lye per pound of oils, because every fat saponifies at its own rate: one pound of olive oil takes about 61 g of NaOH at zero superfat, while one pound of coconut oil takes about 83 g - roughly 35 percent more for the same weight. The calculation is always the same three moves: multiply each oil's weight by that oil's NaOH factor, add the results, then multiply the total by (1 minus your superfat).
This guide walks the arithmetic once with real numbers, gives you a per-pound lye chart you can use to sanity-check any calculator before you mix, and shows the full line-by-line math for a four-oil recipe.
The lye calculation formula
Every soap calculator online runs this one equation: total NaOH = (sum of [oil weight x NaOH factor]) x (1 - superfat). The NaOH factor is the grams of sodium hydroxide needed to fully saponify one gram of that oil, derived from the oil's saponification value - the standard analytical number published as a range in Codex Alimentarius CXS 210-1999 (the Standard for Named Vegetable Oils) and measured by methods such as ISO 3657, expressed in mg KOH per gram of fat.
- Convert the SAP value to a NaOH factor: SAP values are published in mg KOH per gram. Sodium hydroxide has a molar mass of 40.00 g/mol against potassium hydroxide's 56.11, so NaOH factor = SAP (mg KOH/g) x 0.713 / 1000. Olive oil's SAP of about 190 becomes a NaOH factor of 0.135.
- Multiply each oil by its own factor: Never apply one factor to the whole batch. Coconut and palm kernel are short-chain and lye-hungry; castor and shea sit at the low end. A blend's requirement is the sum of its parts.
- Add the per-oil lye amounts: That sum is your zero-superfat lye weight: the amount that converts every last triglyceride, leaving no free oil and no free alkali. Nobody actually soaps here.
- Apply superfat last: Multiply the sum by (1 - superfat). Five percent means x 0.95, eight percent means x 0.92. It is a discount on lye, not an addition of oil, so the oil weights never change.
Worked example: how much lye for 32 oz of olive oil
Olive oil's NaOH factor is 0.1353. For a single-oil castile at zero superfat: 32 oz x 0.1353 = 4.33 oz of NaOH. At 5 percent superfat: 4.33 x 0.95 = 4.11 oz. In metric, 907 g of olive oil takes 122.7 g at zero superfat and 116.6 g at 5 percent. These are weight ounces on a scale, never fluid ounces.
Lye per pound of oils: quick reference chart
| Oil | NaOH factor (g/g) | g NaOH per lb, 0% SF | at 5% SF | at 8% SF | oz NaOH per 16 oz oil, 5% SF |
|---|---|---|---|---|---|
| Olive | 0.1353 | 61.4 | 58.4 | 56.5 | 2.06 |
| Coconut (76 deg) | 0.1831 | 83.1 | 79.0 | 76.5 | 2.78 |
| Palm | 0.1418 | 64.4 | 61.2 | 59.2 | 2.16 |
| Palm kernel | 0.1762 | 80.0 | 76.0 | 73.6 | 2.68 |
| Castor | 0.1286 | 58.4 | 55.5 | 53.7 | 1.95 |
| Shea butter | 0.1279 | 58.1 | 55.2 | 53.4 | 1.94 |
| Cocoa butter | 0.1376 | 62.5 | 59.3 | 57.5 | 2.09 |
| Lard | 0.1411 | 64.1 | 60.9 | 58.9 | 2.14 |
| Beef tallow | 0.1425 | 64.7 | 61.5 | 59.5 | 2.17 |
| Avocado | 0.1326 | 60.2 | 57.2 | 55.4 | 2.02 |
| Sunflower | 0.1347 | 61.2 | 58.1 | 56.3 | 2.05 |
| Rice bran | 0.1333 | 60.5 | 57.5 | 55.7 | 2.03 |
Read the spread rather than the individual rows. Across every common soaping fat the answer lands between roughly 55 and 80 g of NaOH per pound of oils at 5 percent superfat. A number outside that band for a normal recipe means a mistyped weight or percentage. Published SAP values differ slightly between suppliers because real oil lots differ, so treat any single factor as accurate to about plus or minus 2 percent - which is exactly why nobody soaps at zero superfat.
Skip the transcription errors. Soap Calculator holds verified saponification values for 202 oils and runs this exact equation - NaOH for solid bars, KOH for liquid soap, superfat 0 to 15 percent, water at any percentage of oil weight. It also renders predicted bar properties and a live fatty-acid blend ribbon, so you see what changing an oil percentage does before you weigh anything. See Soap Calculator. Download Soap Calculator: Lye & Recipe on the App Store.
A full recipe, calculated line by line
| Oil | Weight | % of oils | NaOH factor | NaOH (g) |
|---|---|---|---|---|
| Olive oil | 454 g | 50% | 0.1353 | 61.43 |
| Coconut oil (76 deg) | 227 g | 25% | 0.1831 | 41.56 |
| Palm oil | 181 g | 20% | 0.1418 | 25.67 |
| Castor oil | 45 g | 5% | 0.1286 | 5.79 |
| **Subtotal (0% superfat)** | **907 g** | 100% | - | **134.44** |
| **x (1 - 0.05) superfat step** | - | - | x 0.95 | **127.7** |
| Water at 33% of oil weight | 299 g | - | - | - |
That is the whole job: 127.7 g of NaOH, 299 g of water, 907 g of oils. Water is a separate decision that does not touch the lye math - it only changes how concentrated the solution is and how fast the batch traces. Round lye down rather than up: excess lye leaves a caustic bar, while a small deficit just raises the superfat.
Sanity checks before you mix
- Lye should be 13 to 15 percent of oil weight for a typical bar at 5 percent superfat. On 907 g of oils that is 118 to 136 g. Well outside that band means a data-entry error.
- Check the mode. Liquid soap uses potassium hydroxide, and KOH factors are about 1.40 times the NaOH factor (56.11 / 40.00). Most KOH sold to soapmakers is 90 percent pure, so divide the KOH weight by 0.90 before you weigh it.
- Weigh, do not measure. Every number on this page is a weight. Volume conversions are the most common cause of a lye-heavy batch.
- Re-run the numbers if you swap an oil. Substituting shea for coconut at the same weight cuts the lye needed for that portion by about 30 percent.
How Soap Calculator handles it
Soap Calculator runs the same equation against verified SAP values for 202 oils and shows total fat weight, lye weight and water weight on one screen, with a safety checklist before you pour and every PPE prompt in safety red. Batches save with notes and photos, the cure timer runs the full four-to-six-week cure on the Lock Screen and in the Dynamic Island, and recipes export as FDA-compliant INCI labels on Avery 5160 sheets. It works offline, with no account.
Frequently asked questions
How much lye do I need per pound of oils?
Between about 55 and 80 g of NaOH per pound (454 g) of oils at 5 percent superfat, depending entirely on which oils. Olive needs about 58 g per pound, coconut about 79 g, tallow about 62 g. There is no universal number, which is why every recipe has to be calculated from its own oil list.
How much lye for 32 oz of olive oil?
About 4.33 oz of sodium hydroxide at zero superfat (32 x 0.1353), or 4.11 oz at 5 percent superfat. In grams that is 122.7 g and 116.6 g for 907 g of olive oil. These are weight ounces on a scale, not fluid ounces.
What is the formula for calculating lye in soap?
Total NaOH = (sum of each oil weight x that oil's NaOH factor) x (1 - superfat). The NaOH factor comes from the oil's published saponification value: SAP in mg KOH per gram x 0.713, divided by 1000. For potassium hydroxide, multiply the NaOH factor by 1.4028 and then divide by your KOH purity, usually 0.90.
Can I make soap without a lye calculator?
Yes, the arithmetic is one multiplication per oil plus a superfat step, and doing it by hand is the only way to catch a calculator you have mis-typed. Use published SAP values for the exact oils you own, never a generic average, and always cross-check the result against the 13 to 15 percent of oil weight rule before mixing.