Soap Making Calculator
Cold process soap making requires precise lye (sodium hydroxide, NaOH) calculations because too much lye produces a caustic bar that burns skin, and too little means the oils will not fully saponify, leaving a greasy, soft soap. Every oil has a unique saponification value (SAP value): the weight of NaOH needed to convert one unit of that oil into soap. By multiplying each oil weight by its SAP value, summing the results, and applying a superfat discount, you arrive at the exact lye amount for your recipe. Water is calculated separately at 38% of total oil weight, a widely used starting ratio in the craft soap industry. This calculator supports up to three oils per batch and lets you choose your superfat percentage, typically 5% to 7% for most cold process recipes. A safety warning is shown prominently: always add lye to water, never water to lye, and always wear gloves and eye protection when handling lye. No finished soap contains free lye after the saponification process is complete.
Lye needed: -- oz. Water needed: -- oz. Total batch: -- oz.
How the lye calculation works
Each oil has a saponification value: the ounces of NaOH needed to saponify one ounce of that oil. Multiply each oil weight by its SAP value, sum those products, then apply the superfat discount to find the final lye amount. Water is set at 38% of total oil weight.
Raw lye = sum(oil_weight × SAP_value)
Lye (oz) = raw lye × (1 - superfat / 100)
Water (oz) = total oil weight × 0.38
Total batch (oz) = total oil weight + lye + water
Worked example
16 oz olive oil (SAP 0.134) and 8 oz coconut oil (SAP 0.190), 5% superfat:
- Raw lye from olive oil: 16 × 0.134 = 2.144 oz
- Raw lye from coconut oil: 8 × 0.190 = 1.520 oz
- Total raw lye: 2.144 + 1.520 = 3.664 oz
- After 5% superfat: 3.664 × 0.95 = 3.48 oz NaOH
- Water: 24 × 0.38 = 9.12 oz
- Total batch: 24 + 3.48 + 9.12 = 36.60 oz
SAP value reference table
| Oil | NaOH SAP value | Notes |
|---|---|---|
| Coconut oil | 0.190 | High lather, cleansing bar |
| Palm oil | 0.141 | Adds hardness and stable lather |
| Olive oil (pure) | 0.134 | Moisturising, slow lather |
| Sweet almond oil | 0.136 | Skin-conditioning, light feel |
| Sunflower oil | 0.134 | Affordable, moisturising |
| Castor oil | 0.128 | Boosts lather, use at 5 to 10% |
SAP values vary slightly by source and oil batch. These values are standard figures from published saponification tables used across the soap-making industry.
Frequently asked questions
What is a saponification value (SAP value) and how is it used?
A saponification value (SAP value) is the amount of sodium hydroxide (NaOH) in ounces required to fully saponify one ounce of a given oil. Each oil has a unique SAP value because its fatty acid profile differs. For example, coconut oil (SAP 0.190) requires more lye per ounce than olive oil (SAP 0.134) because of its higher proportion of short-chain saturated fatty acids. The lye amount for a batch is the sum of each oil weight multiplied by its SAP value, then reduced by the superfat percentage.
What is superfat and why does it matter?
Superfat (also called lye discount) is the percentage of oils left unsaponified in the finished soap. A 5% superfat means 5% of the oils will not be converted to soap, leaving them free in the bar to add moisturising properties and a safety margin against excess lye. A superfat that is too high (above 10 to 12%) can cause the soap to go rancid faster or feel greasy. Most cold process recipes use 5% to 7% superfat.
Why is the water amount set at 38% of total oil weight?
Water dissolves the lye and provides the liquid medium for saponification. A water amount of 38% of oil weight (sometimes called a 2:1 water to lye ratio) is a common starting point that produces a workable soap batter. Using less water (a water discount) speeds up trace and curing but requires more experience. Using more water makes the batter thinner and extends the time to unmould. 38% is a reliable default for beginners.
Is it safe to make soap with lye at home?
Cold process soap cannot be made without lye (sodium hydroxide, NaOH). However, lye is a caustic alkali that causes serious chemical burns on contact with skin or eyes. Always wear safety goggles, gloves, and long sleeves. Always add lye to water, never water to lye, to avoid a violent exothermic reaction. Work in a well-ventilated area. Keep children and pets away. Store lye in a sealed, clearly labelled container away from moisture.
Can I use this calculator for liquid soap?
No. Liquid soap uses potassium hydroxide (KOH), not sodium hydroxide (NaOH). The SAP values for KOH are different from those for NaOH. This calculator is specifically for cold process bar soap using NaOH. For liquid soap, you will need KOH-specific SAP values and a separate calculation.
Official sources
- Standard saponification tables used across the soap-making industry (NaOH SAP values as published in soap-making reference literature).
- Brambleberry soap supply resource: Brambleberry Lye Calculator reference.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.