A soap lye calculator takes the guesswork out of one of the most critical steps in soap making: figuring out exactly how much lye you need. Enter your oils, choose a superfat, and it returns precise NaOH or KOH amounts in seconds. But there are a few things to understand before you start.
Too much lye produces caustic soap that irritates skin. Too little and the soap stays greasy and soft. A lye calculator removes this risk by computing exact amounts from published saponification values for each oil.
What the calculator actually does
Every oil has a saponification (SAP) value — the amount of lye required to fully convert 1 gram of that oil into soap. The calculator multiplies each oil's weight by its SAP value, sums the results, then adjusts for your chosen superfat percentage. That final number is your lye weight.
Grams of lye per gram of oil needed for full saponification. Every oil has a different value based on its fatty acid composition.
The percentage of oils left unsaponified. A 5% superfat means 5% of oils remain as free fatty acids — more conditioning, safer margin.
Sodium hydroxide. Use this for solid bar soap. The most common lye type for cold process and hot process soap.
Potassium hydroxide. Use this for liquid or soft soap. KOH is less concentrated than NaOH, so you need more by weight.
Using the calculator
- 1
Choose your soap type
Select NaOH for a solid bar, KOH for liquid soap, or Dual-Lye if you want a blend of both (common for shaving soap). FormulaAlchemy lets you adjust the NaOH/KOH split ratio for dual-lye recipes.
- 2
Add your oils
Search for each oil by name and enter its weight in grams or ounces. You can add as many oils as your recipe requires. Common starting oils include coconut oil (lather), olive oil (conditioning), and castor oil (fluffy bubbles).
- 3
Set your superfat percentage
For most bar soaps, 5% is a safe default. Increase to 7–8% for a more conditioning bar. Drop to 3% for a harder, longer-lasting bar. Do not go below 2% — the margin against lye-heavy soap gets too thin.
- 4
Choose your water input method
You can enter water as a ratio to lye (e.g. 2:1 is the classic starting point), as a percentage of total oil weight (typically 33–38%), or as a lye concentration (25–33% is common). Any method gives the same result.
- 5
Check the soap property chart
FormulaAlchemy shows a visual fatty acid chart with scores for hardness, cleansing, conditioning, bubbly lather, creamy lather, iodine, and INS. These tell you what kind of soap you'll get before you make it. Aim for a balanced bar: conditioning 50+, cleansing 12–22, hardness 29–54.
- 6
Record your results
Write down or save the lye weight and water weight. Pro users can save recipes to their account for easy access later.
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Weigh everything on a digital scale
A scale accurate to 1 gram minimum — ideally 0.1g — is non-negotiable. Measuring by volume is not accurate enough for lye calculations.
Things beginners often get wrong
Using volume instead of weight
Oil density varies significantly. A cup of coconut oil and a cup of olive oil do not weigh the same. Always weigh in grams or ounces — never use volume measurements for soap lye calculations.
Forgetting lye purity
Consumer-grade NaOH is typically 97–99% pure. If your lye has absorbed moisture from storage, it becomes even less concentrated. FormulaAlchemy lets you enter your lye's actual purity percentage and adjusts the lye weight automatically.
Misunderstanding superfat
Superfat reduces the lye amount — it does not add extra oil to the recipe. A 5% superfat means the calculator computes lye for only 95% of your oil weight. This provides both a conditioning benefit and a safety buffer.
Confusing water ratio and lye concentration
A 2:1 water to lye ratio (by weight) is equivalent to a 33% lye concentration. Both describe the same thing from different angles. More water = easier to work with but longer cure time. Less water = faster trace, firmer bar sooner.
All lye calculators — including ours — use average SAP values from published literature. Real oils vary slightly by crop and supplier. This is why a 3–8% superfat is always recommended: it absorbs the natural variability in your ingredients.
Ready to calculate your first recipe?
FormulaAlchemy gives you NaOH, KOH, and dual-lye calculations with fatty acid charts, mold sizing, and fragrance dosage — all free, no account needed.