Free lye calculator
Lye calculator for soap
Work out the lye and water for a soap recipe: NaOH for bar soap, KOH for liquid soap, or both. No account needed, nothing is sent anywhere, and it uses the same engine and sourced oil data as the Soap Ledger app.
Result
| At a glance | Pounds | Ounces | Grams |
|---|---|---|---|
| Water | 0.62 | 9.92 | 281.3 |
| Lye — NaOH | 0.305 | 4.89 | 138.5 |
| Oils | 2.205 | 35.27 | 1000 |
| Total raw batter (before cure) | 3.13 | 50.08 | 1419.8 |
| Oils | % of oils | Weight |
|---|---|---|
| Olive oil | 50% | 500.0 g |
| Coconut oil | 25% | 250.0 g |
| Shea butter | 20% | 200.0 g |
| Castor oil | 5% | 50.0 g |
| Total oil weight | 100% | 1000.0 g |
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Soap qualities
(for comparing recipes)
Each score adds up the share of certain fatty acids in your oils, from published fatty-acid profiles. They let you compare one recipe with another. They don't predict how a bar will feel, lather or last, and they say nothing about safety or skin. Cure time, superfat, water, additives and your actual oils matter too.
Fatty acid breakdown
Every soap is a balance; there's no single perfect profile. Fatty acids not listed (e.g. palmitoleic, erucic) make up the rest.
How this was calculated
Soap Ledger's own calculation engine (the one in the app) works with exact decimals and rounds only for display.
- Lye each oil needs (NaOH): oil weight × its SAP value.
Oil Weight SAP NaOH NaOH Olive oil 500 g 0.135448 67.724 g Coconut oil 250 g 0.182854 45.714 g Shea butter 200 g 0.129602 25.92 g Castor oil 50 g 0.129388 6.469 g Total, before superfat 145.827 g - Superfat 5%: 145.827 g × (1 − 5/100) = 138.536 g pure NaOH.
- Purity 100%: 138.536 g ÷ 100/100 = 138.54 g NaOH to weigh.
- Water, lye concentration 33%: lye as weighed × (100/33 − 1) = 281.3 g.
Where each SAP value comes from
- Olive oil: 190 mg KOH/g (Codex CXS 33 range midpoint (virgin/refined olive oil)), from FAO/WHO Codex Alimentarius Commission. NaOH 0.135448, KOH 0.19 g per g.
- Coconut oil: 256.5 mg KOH/g (Codex CXS 210 range midpoint), from FAO/WHO Codex Alimentarius Commission. NaOH 0.182854, KOH 0.2565 g per g.
- Shea butter: 181.8 mg KOH/g (Median of sources (no Codex/pharmacopoeia standard found)), from Academic Journals (peer-reviewed). NaOH 0.129602, KOH 0.1818 g per g.
- Castor oil: 181.5 mg KOH/g (JP18 pharmacopoeia range midpoint (no Codex standard for castor oil)), from Ministry of Health, Labour and Welfare (Japan). NaOH 0.129388, KOH 0.1815 g per g.
mg KOH/g ÷ 1000 = g KOH per g; × 39.997 ÷ 56.106 = g NaOH per g (rounded to 6 decimals, as in the app).
Keep this recipe. Save it in Soap Ledger with versions, batches and curing reminders. Sign-up is free, with no card.
Save this recipe in Soap LedgerEngine soap-engine-1.1.0. Values are computed with exact decimals and rounded only for display.
How to calculate lye for soap
Multiply each oil's weight by its SAP value, add the results, take off your superfat, and divide by your lye's purity. That gives the lye to weigh. Then set the water from the lye or the oils. The SAP (saponification) value is how many grams of lye one gram of that oil needs; it differs for sodium hydroxide (NaOH, bar soap) and potassium hydroxide (KOH, liquid soap).
Worked example
1000 g of oils, 5% superfat, 100% pure NaOH and a 33% lye concentration:
| Oil | % | Weight | SAP NaOH | NaOH needed |
|---|---|---|---|---|
| Olive oil | 50% | 500 g | 0.135448 | 67.72 g |
| Coconut oil | 25% | 250 g | 0.182854 | 45.71 g |
| Shea butter | 20% | 200 g | 0.129602 | 25.92 g |
| Castor oil | 5% | 50 g | 0.129388 | 6.47 g |
| Total | 100% | 1000 g | 145.83 g |
- Lye before superfat: 145.83 g.
- 5% superfat: 145.83 × (1 − 5/100) = 138.54 g.
- 100% purity: 138.54 ÷ 1 = 138.5 g of NaOH.
- Water at a 33% lye concentration: 138.54 × (100 / 33 − 1) = 281.3 g of water (a water : lye ratio of 2.03 : 1, or 28.1% of the oils).
- Total raw batter: 1419.8 g.
These numbers are worked out by the calculator when this page is built, so they always match it.
Three ways to set the water
- Lye concentration: lye ÷ (lye + water). This calculator starts at 33%; a higher concentration means less water.
- Water : lye ratio: water ÷ lye. 2 : 1 is the same as a 33.3% concentration.
- Water as % of oils: the older way, used by many printed recipes. It depends only on the oil weight, not on how much lye the oils need.
Soap Ledger flags values outside its review bands (lye concentration 20–50%, ratio 1–4, water 15–45% of oils). Those bands are reminders to double-check, not safety limits.
Superfat, purity and dual lye
Superfat (lye discount) leaves a little oil unsaponified; 5% is common for bar soap. Purity matters most for KOH, which is often sold at about 90%: the calculator divides by the purity so you weigh enough. For dual-lye recipes (for example cream soap or a softer bar), each lye's share of the oils is worked out separately with its own purity.
Where the SAP values come from
Every oil's value comes from a published source (Codex Alimentarius food standards, pharmacopoeias, peer-reviewed papers and supplier data), never copied from another soap calculator. Look any oil up on the oil reference pages, which list the sources. Real oils vary by lot, so your supplier's certificate of analysis wins.
Lye safety. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification. Eye protection, gloves and covered skin; ventilation and a clear, uninterrupted workspace.
Save your recipes
This calculator is free with no account. If you want to keep recipes with versions, record batches, track curing and print labels, Soap Ledger is free to start, with a 14-day trial of every feature and no card.
Engine soap-engine-1.1.0, oil catalog catalog-2026.10.1.