# How much lye does my soap need?

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.

- Page: https://www.acalculator.org/chemistry/soap-calculator
- JSON spec: https://www.acalculator.org/chemistry/soap-calculator.json
- Version: 2528537543f7

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| lye | Lye | Sodium hydroxide (NaOH) for bar soap, or potassium hydroxide (KOH) for liquid soap. |
| u | Weights in | The unit of every weight you type and every weight shown: grams, ounces or pounds. |
| oils | Oils | One row per oil, butter or wax: which one and its weight. |
| sf | Superfat | The share of oil left unsaponified, for a milder bar: less lye by this percent. |
| pure | Lye purity | How pure your lye is, from its label: about 100% for NaOH; KOH is often sold at about 90%. |
| ratio | Water to lye ratio | Parts of water per part of lye by weight: 2 means twice the lye’s weight in water. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| lye | Lye | The weight of lye to dissolve, in your unit. |
| water | Water | Lye × the water-to-lye ratio, in your unit. |
| total | Oils | All the oils added up, in your unit. |
| batch | Batch weight | Oils + lye + water, in your unit. |
| concentration | Lye concentration | Lye ÷ (lye + water) × 100. |
| text | Lye and water | The lye and water with their unit, in words. |
| breakdown | Lye for each oil | Each oil’s weight × its SAP value, before superfat and purity. |

## Method

Lye = Σ (oil weight × SAP value) × (1 − superfat ÷ 100) ÷ (purity ÷ 100); water = lye × water-to-lye ratio; concentration = lye ÷ (lye + water) × 100.

## Assumptions

- 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

1. lye = naoh, u = g, oils = {"oil":"olive","weight":500} or {"oil":"coconut","weight":300}, sf = 5%, pure = 100%, ratio = 2 gives lye = 142.12, water = 284.24, batch = 1,426.36, text = 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/.
2. lye = koh, u = g, oils = {"oil":"olive","weight":1000} or undefined, sf = 0%, pure = 90%, ratio = 3 gives lye = 211.777778, water = 635.333333, 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/.
3. lye = naoh, u = oz, oils = {"oil":"olive","weight":32} or {"oil":"coconut","weight":20}, sf = 5%, pure = 100%, ratio = 2 gives lye = 9.291, total = 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/.

## FAQ

### 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.

## Sources

- The Soap Kitchen, Saponification values in soap making: saponification chart (sodium hydroxide, NaOH) and saponification chart (potassium hydroxide, KOH); multiply the weight of each oil by its value. https://www.thesoapkitchen.co.uk/guide-to-sap-values/ (retrieved 2026-10-02)
- Modern Soap Making, How to Better Understand Water Discounts When You Make Soap (water as a ratio of the lye; a 33% lye solution is 33% lye and 67% water; multiply the lye by 2 for a 33% solution). https://www.modernsoapmaking.com/blog/lye-solution-in-soapmaking (retrieved 2026-10-02)