Soap Lye Calculator
Build a soap recipe from up to eight oils and butters. The calculator uses each oil's saponification (SAP) value to work out the sodium hydroxide (NaOH) or potassium hydroxide (KOH) and water you need at your chosen superfat, and estimates hardness, cleansing, lather and conditioning.
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Common: 33–38% of oils, 30–35% lye concentration, or 2 : 1 to 2.5 : 1.
How to use the Soap Lye Calculator
- Choose NaOH or KOH: sodium hydroxide makes bar soap, potassium hydroxide makes soft or liquid soap.
- Select your oils and enter the weight of each in grams, ounces or pounds.
- Set the superfat (5% is a common starting point for bar soap).
- Choose how to set the water and enter the value.
- Weigh the lye and water shown, check the bar properties, and double-check your recipe before making soap.
The lye calculation
- Lye for each oil = oil weight × SAP value (NaOH or KOH)
- Total lye = sum of all oils × (1 − superfat), divided by purity for KOH 90%
- Water = oils × water % of oils, or lye × (100 − concentration) ÷ concentration, or lye × water : lye ratio
- KOH SAP = NaOH SAP × 1.403 (the ratio of their molar masses, 56.11 ÷ 40.00)
Example: 400 g olive, 250 g coconut, 200 g palm, 50 g castor and 100 g shea need 400 × 0.1354 + 250 × 0.1832 + 200 × 0.1418 + 50 × 0.1283 + 100 × 0.1276 = 147.5 g NaOH for full saponification. At 5% superfat that is 140.1 g NaOH, and at 33% of oils, 330 g of water.
SAP values used
Average saponification values (grams of lye per gram of oil) and iodine values. Values from different sources differ slightly.
| Oil / butter | SAP NaOH | SAP KOH | Iodine |
|---|---|---|---|
| Olive oil | 0.135 | 0.190 | 85 |
| Coconut oil (76°F) | 0.183 | 0.257 | 10 |
| Palm oil | 0.142 | 0.199 | 53 |
| Palm kernel oil | 0.176 | 0.247 | 20 |
| Castor oil | 0.128 | 0.180 | 86 |
| Shea butter | 0.128 | 0.179 | 59 |
| Cocoa butter | 0.138 | 0.194 | 37 |
| Mango butter | 0.137 | 0.192 | 45 |
| Sweet almond oil | 0.137 | 0.192 | 99 |
| Avocado oil | 0.133 | 0.187 | 86 |
| Sunflower oil | 0.135 | 0.189 | 133 |
| Sunflower oil, high oleic | 0.135 | 0.189 | 83 |
| Canola (rapeseed) oil | 0.133 | 0.186 | 110 |
| Rice bran oil | 0.128 | 0.180 | 100 |
| Grapeseed oil | 0.129 | 0.181 | 131 |
| Soybean oil | 0.135 | 0.190 | 131 |
| Peanut oil | 0.136 | 0.191 | 92 |
| Apricot kernel oil | 0.139 | 0.195 | 100 |
| Argan oil | 0.136 | 0.191 | 95 |
| Hemp seed oil | 0.138 | 0.193 | 165 |
| Safflower oil, high oleic | 0.135 | 0.190 | 93 |
| Macadamia nut oil | 0.139 | 0.195 | 76 |
| Babassu oil | 0.175 | 0.245 | 15 |
| Lard (pork fat) | 0.141 | 0.198 | 57 |
| Tallow (beef) | 0.143 | 0.200 | 45 |
| Jojoba oil (liquid wax) | 0.066 | 0.092 | 80 |
| Beeswax | 0.067 | 0.094 | 10 |
Bar property ranges (hardness 29–54, cleansing 12–22, conditioning 44–69, bubbly 14–46, creamy 16–48, iodine 41–70, INS 136–170) are the commonly used guideline ranges for bar soap.
Frequently asked questions
How do you calculate how much lye to use in soap?
Multiply each oil's weight by its SAP value (the grams of lye needed to saponify one gram of that oil), add them up, then reduce the total by your superfat percentage. For 1,000 g of olive oil: 1,000 × 0.1354 = 135.4 g NaOH, and at 5% superfat 135.4 × 0.95 ≈ 128.7 g.
What is superfat in soap making?
Superfat is the percentage of oil left unsaponified because you use slightly less lye than needed. It makes the bar milder and gives a safety margin for SAP values that vary. Bar soap is commonly made at 3–8% superfat; liquid soap is often 0–3%.
How much water should I use?
Common settings are water at 33–38% of the oil weight, a lye concentration of 28–35%, or a water : lye ratio between 2 : 1 and 2.5 : 1. Less water (a stronger lye solution) speeds up trace and shortens cure time; more water gives more working time.
What is the difference between NaOH and KOH?
Sodium hydroxide (NaOH) makes hard bar soap. Potassium hydroxide (KOH) makes soft or liquid soap. KOH SAP values are about 1.403 times the NaOH values because of its higher molecular weight. Most KOH sold is about 90% pure, so the calculator divides by 0.9 for KOH 90%.
What do the bar property numbers mean?
They are estimates from typical fatty acid profiles. Hardness and cleansing come from lauric, myristic, palmitic and stearic acids; conditioning from oleic, linoleic, linolenic and ricinoleic acids. Recipes inside the typical ranges usually make a balanced bar, but they are guidance, not rules.
Are SAP values exact?
No. SAP values vary with the source and batch of an oil, which is one reason soap makers use a superfat. This calculator uses widely published average SAP values. Always run a new recipe through a second lye calculator before using it.
Is it safe to make soap at home?
Lye is caustic. Wear goggles and gloves, work in a ventilated area away from children and pets, always add lye to water (not water to lye), and use stainless steel or heat-safe plastic, never aluminium. Cure cold process bars for about 4–6 weeks.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.