Free Soap Calculator
A free lye calculator for soap making. Enter your oils in grams or ounces and get the exact NaOH or KOH and water for cold process, hot process or liquid soap. Free, instant, no signup required.
Ready to Calculate
Add your oils and weights on the left, then click Calculate Lye.
How to Use This Soap Calculator
Choose your lye type
Select NaOH (sodium hydroxide) for bar soap or KOH (potassium hydroxide) for liquid soap. NaOH is used in cold process and hot process bar soap making. KOH at 90% purity is standard for liquid soap.
Select your soap making method
Cold process soap is made at room temperature and cured for 4–6 weeks. Hot process soap is cooked and can be used sooner. The lye amount is the same — the method changes how you handle the batter.
Add your oils and weights
Enter each oil or butter in your recipe with its weight in grams. Each oil has a unique saponification value (SAP value) — the calculator uses these to determine exactly how much lye is needed.
Set your superfat percentage
Superfat is the percentage of oils left unsaponified in your finished soap. A 5% superfat is standard for a mild, moisturising bar. Higher superfat means more free oils but a softer bar.
Frequently Asked Questions
What is a soap calculator?
A soap calculator (also called a lye calculator) works out the exact amount of sodium hydroxide (NaOH) or potassium hydroxide (KOH) needed to turn a specific blend of oils and butters into soap. It uses the saponification value (SAP value) of each oil, so no lye is left over in your finished bar.
What is the formula for calculating lye in soap?
Multiply the weight of each oil by its SAP value, add the results together, then reduce the total by your superfat. For example, 700 g olive oil (SAP 0.135) plus 300 g coconut oil (SAP 0.183) needs 94.5 g + 54.9 g = 149.4 g of NaOH. With a 5% superfat that becomes 149.4 × 0.95 = 141.9 g of NaOH. See the full SAP value chart for every oil.
What is superfatting in soap making?
Superfatting is using slightly less lye than required to saponify all oils, leaving a small percentage of free oils in the finished soap. This makes a milder, more moisturising bar. A 5% superfat is the most common choice for handmade soap.
What is the difference between NaOH and KOH?
NaOH (sodium hydroxide) makes hard bar soap and is used for cold process and hot process soap. KOH (potassium hydroxide) makes soft or liquid soap. Most bar soap makers use NaOH. Liquid soap makers use KOH, typically at 90% purity, which this calculator accounts for.
What is the best water-to-lye ratio for soap making?
A 2:1 water-to-lye ratio by weight (about a 33% lye concentration) is the standard starting point for cold process soap, and it is what this calculator uses. Experienced makers sometimes use less water to speed up unmoulding and curing, or more water for hot process soap.
What happens if you use too much lye in soap?
Too much lye leaves unreacted sodium hydroxide in the bar, which makes the soap harsh and can irritate or burn skin. Signs include a white crumbly surface, cracking or a bar that zaps when touched to the tongue. Always weigh lye precisely and run every recipe through a soap calculator before you make it.
Can I use this soap calculator in ounces?
Yes. Switch the units to ounces, enter your oils in ounces and the lye and water results are given in ounces too.
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