Cold Process Soap Calculator
The cold process soap calculator determines the precise amounts of sodium hydroxide (NaOH lye) and water needed to saponify a batch of oils. Each oil has a specific saponification value (SAP), the grams of NaOH required to convert one gram of that oil into soap. The superfat percentage reduces the lye amount slightly, leaving a proportion of oils unsaponified for a gentler, more moisturising bar. Accuracy is critical in soap making: too much lye produces a caustic bar, too little results in a greasy, soft soap that may go rancid. Always verify your final recipe with a dedicated soap calculator before making.
Soap lye calculation formula
Lye (g) = Oil weight (g) x SAP value x (1 - Superfat / 100)
Water (g) = Lye (g) x Water-to-lye ratio
Total batch (g) = Oil weight + Lye + Water
Lye concentration (%) = Lye / (Lye + Water) x 100
Always add lye to water (not water to lye). The mixture will heat up significantly. Allow it to cool to 100-110 degF before combining with oils.
Frequently asked questions
What is the SAP value in soap making?
SAP (saponification value) is the amount of NaOH (lye) needed to fully saponify 1 gram of a specific oil or fat. Each oil has a unique SAP value. For example, coconut oil has a NaOH SAP of approximately 0.190, and olive oil is approximately 0.134. The total lye needed equals the sum of (oil weight x SAP value) for all oils in the recipe.
What is superfat in cold process soap?
Superfat (also called lye discount) is the percentage of oils that remain unsaponified in the finished soap. A 5% superfat means 5% of your oils are not converted to soap, leaving excess oil that moisturises skin. Most recipes use 5 to 8% superfat. A higher superfat makes gentler but potentially softer soap.
How much water do I use for soap making?
The water amount is typically expressed as a ratio to lye weight. A 2:1 water-to-lye ratio (33% lye concentration) is standard for most recipes. Some soapers use a 38% water as percentage of oils for a thicker trace. This calculator uses a 33% lye concentration (water = lye weight x 2) as default.
Is lye (NaOH) safe to use at home?
Lye is caustic and requires careful handling. Always wear protective gloves, eye protection, and work in a well-ventilated area. Always add lye to water, never water to lye, to prevent dangerous spattering. Keep vinegar nearby to neutralise spills. Once saponified in finished soap, no lye remains in the product.
What is the difference between NaOH and KOH in soap making?
NaOH (sodium hydroxide) makes solid bar soap. KOH (potassium hydroxide) makes liquid or soft soap. They have different SAP values for the same oils. This calculator uses NaOH values suitable for cold process bar soap. KOH SAP values are approximately 1.402 times the NaOH values.
Sources
- NIST Chemistry WebBook: Sodium Hydroxide (NaOH) Physical Properties.
- FDA: FDA - Is It a Cosmetic, a Drug, or Both? Soap Regulation.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.