How much lye does my soap need?
Pick bar soap (NaOH) or liquid soap (KOH), add your oils and their weights, and set the superfat. The soap calculator shows how much lye and water to use, the batch weight, and the lye each oil needs.
- Lye
- 142.12
Use 142.12 g of NaOH in 284.24 g of water.
- Water
- 284.24
- Oils
- 1,000
- Batch weight
- 1,426.36
- Lye concentration
- 33.3%
- Lye and water
- 142.12 g of NaOH in 284.24 g of water
- Lye for each oil
- Olive Oil: 67 g; Coconut (refined): 57 g; Shea (Karite) Butter: 25.6 g
Lye: 142.12. Use 142.12 g of NaOH in 284.24 g of water.
How to calculate
Computes the lye (sodium hydroxide or potassium hydroxide) and water for a soap recipe from each oil’s weight and published SAP value, with a superfat, the lye purity, and the water-to-lye ratio.
Example with the default inputs (Lye NaOH (bar), Weights in Grams, Oils [Oil Olive Oil, Weight 500; Oil Coconut (refined), Weight 300; Oil Shea (Karite) Butter, Weight 200], Superfat 5%, Lye purity 100%, Water to lye ratio 2): Use 142.12 g of NaOH in 284.24 g of water.
Method: Lye = Σ (oil weight × SAP value) × (1 − superfat ÷ 100) ÷ (purity ÷ 100); water = lye × water-to-lye ratio; concentration = lye ÷ (lye + water) × 100.
- SAP values are The Soap Kitchen’s published NaOH and KOH charts (retrieved 2026-10-02); a batch of a real oil can differ a little.
- Superfat 5% by default; water twice the lye’s weight by default (a 33% lye solution).
- Always weigh lye and wear eye and skin protection; add lye to water, never water to lye.
Worked examples
Each example is checked against the calculator on every build.
- Lye NaOH (bar), Weights in Grams, Oils Olive Oil 500; Coconut (refined) 300; Shea (Karite) Butter 200, Superfat 5%, Lye purity 100%, Water to lye ratio 2 gives Lye 142.12, Water 284.24, Batch weight 1,426.36, Lye and water 142.12 g of NaOH in 284.24 g of water.Source: The Soap Kitchen, Saponification values in soap making (multiply each oil’s weight by its value; NaOH: olive 0.134, coconut (refined) 0.190, shea 0.128), https://www.thesoapkitchen.co.uk/guide-to-sap-values/
- Lye KOH (liquid), Weights in Grams, Oils Olive Oil 1000, Superfat 0%, Lye purity 90%, Water to lye ratio 3 gives Lye 211.777778, Water 635.333333, Lye concentration 25%.Source: The Soap Kitchen, Saponification values in soap making (KOH chart: olive oil 0.1906), https://www.thesoapkitchen.co.uk/guide-to-sap-values/
- Lye NaOH (bar), Weights in Ounces, Oils Olive Oil 32; Coconut (refined) 20; Palm 12, Superfat 5%, Lye purity 100%, Water to lye ratio 2 gives Lye 9.291, Oils 64, Water 18.582.Source: The Soap Kitchen, Saponification values in soap making (example mixture of 2 lb olive, 1 lb 4 oz coconut and 12 oz palm; NaOH: olive 0.134, coconut 0.190, palm 0.141), https://www.thesoapkitchen.co.uk/guide-to-sap-values/
How it works
For each oil row, the lye it needs = oil weight × its SAP value (the NaOH value for NaOH, the KOH value for KOH). Then:
- Lye = (sum of the rows) × (1 − superfat ÷ 100) ÷ (purity ÷ 100)
- Water = lye × water-to-lye ratio
- Oils = sum of the oil weights
- Batch weight = oils + lye + water
- Lye concentration = lye ÷ (lye + water) × 100
Every weight is in the unit you pick (grams, ounces or pounds); SAP values are grams of lye per gram of oil, so they work in any unit.
SAP values are The Soap Kitchen’s published charts, read on 2 October 2026 (the page has no date of its own). The page lists only oils that are in both its NaOH and KOH charts:
| Oil | NaOH | KOH |
|---|---|---|
| Almond (Sweet) | 0.136 | 0.1925 |
| Apricot kernel | 0.135 | 0.1941 |
| Avocado | 0.133 | 0.1886 |
| Babassu | 0.175 | 0.2463 |
| Beef Tallow | 0.1405 | 0.1999 |
| Beeswax | 0.069 | 0.0970 |
| Castor | 0.1286 | 0.1811 |
| Cocoa Butter | 0.137 | 0.1941 |
| Coconut (refined) | 0.190 | 0.2690 |
| Corn (Maize) | 0.136 | 0.1927 |
| Cottonseed | 0.1386 | 0.1954 |
| Emu oil | 0.135 | 0.1939 |
| Evening Primrose | 0.136 | 0.1918 |
| Flaxseed | 0.1357 | 0.1913 |
| Grapeseed | 0.1265 | 0.1861 |
| Hazelnut | 0.1356 | 0.1928 |
| Hemp Seed | 0.1345 | 0.1914 |
| Jojoba | 0.069 | 0.0979 |
| Lard | 0.138 | 0.1970 |
| Macadamia Nut | 0.139 | 0.1959 |
| Neem | 0.1387 | 0.1932 |
| Olive Oil | 0.134 | 0.1906 |
| Palm | 0.141 | 0.2000 |
| Palm Kernel | 0.156 | 0.2503 |
| Peanut | 0.136 | 0.1925 |
| Poppy Seed Oil | 0.1383 | 0.1960 |
| Pumpkin Seed | 0.1331 | 0.1956 |
| Rape Seed (Canola) | 0.124 | 0.1870 |
| Rice Bran | 0.128 | 0.1808 |
| Safflower | 0.136 | 0.1928 |
| Sesame Seed | 0.133 | 0.1882 |
| Shea (Karite) Butter | 0.128 | 0.1825 |
| Soybean | 0.135 | 0.1914 |
| Sunflower | 0.134 | 0.1903 |
| Walnut | 0.1353 | 0.1900 |
| Wheatgerm | 0.131 | 0.1858 |
Rules. Each row needs an oil from the list and a weight more than 0 and at most 100,000; 1 to 20 rows. Superfat is from 0% to 30%, purity from 50% to 100%, and the water ratio from 1 to 5.
Exact arithmetic. Every step is an exact fraction of the weights you type and the SAP values as printed, rounded once for display.
Output format. Lye, water, oils and batch weight show with at most 2 decimals, and the concentration as a percent with at most 1 decimal, rounded half up from the exact value. The words line and the per-oil list round each weight half up to 2 decimals, with trailing zeros left off, and name the unit (g, oz or lb). Each per-oil amount is before the superfat and purity.
Assumptions
- SAP values are typical values; a real batch of oil can differ a little, which the superfat covers.
- Superfat 5%, purity 100% and water twice the lye’s weight by default.
Worked examples by hand
500 g olive, 300 g coconut, 200 g shea, NaOH, 5% superfat, ratio 2. 500 × 0.134 = 67; 300 × 0.190 = 57; 200 × 0.128 = 25.6; sum 149.6 g; × 0.95 = 142.12 g of NaOH; water × 2 = 284.24 g; batch 1,000 + 142.12 + 284.24 = 1,426.36 g.
1,000 g olive oil, KOH at 90% purity, no superfat, ratio 3. 1,000 × 0.1906 = 190.6; ÷ 0.9 = 211.78 g of KOH; water × 3 = 635.33 g; concentration 1 ÷ (1 + 3) = 25%.
The Soap Kitchen’s mixture in ounces: 32 oz olive, 20 oz coconut, 12 oz palm, NaOH, 5% superfat. 32 × 0.134 = 4.288; 20 × 0.190 = 3.8; 12 × 0.141 = 1.692; sum 9.78 oz; × 0.95 = 9.291 oz of NaOH; water 18.582 oz; oils 64 oz (4 lb).
Other questions people ask
How do I calculate lye for soap?
Multiply each oil’s weight by its SAP value for your lye and add them up. Take off the superfat, and divide by the lye’s purity. 500 g olive (0.134), 300 g coconut (0.190) and 200 g shea (0.128) need 67 + 57 + 25.6 = 149.6 g of NaOH; with a 5% superfat, 142.12 g.
What is a SAP value?
The saponification value: how much lye it takes to turn 1 g of an oil fully into soap. It differs from oil to oil because their fatty acids differ: coconut oil needs more lye than olive oil. This page uses The Soap Kitchen’s published charts for NaOH and KOH.
What is superfat?
Lye held back on purpose, so a little oil stays unsaponified. It makes a milder bar and leaves a margin for small weighing errors and for SAP values that vary from batch to batch. The calculator uses 5% by default.
How much water should I use?
Modern Soap Making works out water as a ratio of the lye: twice the lye’s weight gives about a 33% lye solution. Type your own ratio from 1 to 5; a ratio of 1 is a 50% solution, the strongest the guide lists.
What is the difference between NaOH and KOH?
Sodium hydroxide (NaOH) makes hard bar soap. Potassium hydroxide (KOH) makes soft or liquid soap. They have different SAP values, so pick the right one before you calculate.
Is lye dangerous?
Yes. Lye is caustic. Wear goggles and gloves, weigh it carefully, and add the lye to the water, never water to lye, in a well-aired place.