- Home
- Soap Calculator
Free tool
Soap calculator (lye calculator)
Work out exactly how much lye (sodium hydroxide), water and fragrance your cold process soap recipe needs. Pick your oils, set your superfat, and the numbers update as you type.
1. Your oils & butters
| Oil or butter | Weight | % | Remove |
|---|
2. Your batch
- Sodium hydroxide (lye)0
- Distilled water0
- Total oils0
- Fragrance / essential oil0
- Total batch weight0
- Approx. 4.5 oz bars0
Safety first. Lye is caustic. Wear goggles and gloves, always add lye to water (never water to lye), and double-check any new recipe in a second calculator before you make it. Saponification values vary slightly by supplier.
How to use the soap calculator
- Choose units. Ounces or grams, whichever your scale uses.
- Add your oils and butters with their weights. The percentage column shows your recipe balance.
- Set a superfat. 5% is a good default. It leaves a little unsaponified oil in the bar for mildness.
- Pick a water ratio. 2:1 water to lye is standard. Less water speeds up trace and shortens cure time.
- Weigh everything. Always measure lye and oils by weight on a digital scale, never by volume.
What is superfat?
Superfat is the percentage of oils deliberately left unsaponified. A 5% superfat means the calculator reduces the lye by 5% so a little free oil remains, which makes the bar gentler and guards against small weighing errors. Most bar soap recipes use between 3% and 8%.
Lye safety checklist
- Wear splash goggles, gloves and long sleeves.
- Always add lye to water, never water to lye, in a well-ventilated space.
- Use heat-safe plastic (PP #5) or stainless steel. Never use aluminum.
- Keep kids and pets out of the room while you work.
- Run every new recipe through a second calculator before you make it.
Common SAP values used (NaOH)
| Oil | NaOH SAP | Typical use |
|---|---|---|
| Olive oil | 0.135 | Up to 100%, conditioning |
| Coconut oil (76°) | 0.183 | 15–30%, big bubbly lather |
| Palm oil | 0.142 | Up to 30%, hardness |
| Castor oil | 0.128 | 5–10%, lather boost |
| Shea butter | 0.128 | Up to 15%, creamy feel |
| Cocoa butter | 0.137 | Up to 15%, hardness |
| Sweet almond oil | 0.139 | Up to 20%, conditioning |
How much lye do I need for soap?
It depends on the oils in your recipe, because each oil needs a different amount of lye to saponify (its SAP value). A classic 32 oz recipe of 60% olive, 30% coconut and 10% castor oil at 5% superfat needs about 4.5 oz of sodium hydroxide.
What superfat should I use?
5% is the most common starting point. Recipes very high in coconut oil often use 15–20% superfat to stay gentle, while bars made for laundry use 0–1%.
Can I use this calculator for liquid soap?
No. This calculator is for bar soap made with sodium hydroxide (NaOH). Liquid soap uses potassium hydroxide (KOH), which needs different amounts.
How many bars will my recipe make?
The calculator estimates bars at 4.5 oz each from your total batch weight. The real count depends on your mold and how you cut, so a multi-bar cutter helps keep bars even.
The Soap Club
Get recipes that just work
Join free and we'll email your welcome code right away.
- 10% off your first order
- Recipes & technique tips, about twice a month
