What is the Soap Lye Calculator?
This tool tells cold-process and hot-process soap makers exactly how much sodium hydroxide (NaOH lye) and water to use for a given weight of oils. Getting the lye amount right is the single most important safety and quality step in soap making: too much lye leaves a harsh, caustic bar, while too little leaves it soft and greasy. The calculator works for any single oil or oil blend as long as you know the combined SAP value.
How to use it
Enter the total weight of your oils in grams. Enter the SAP value — the grams of NaOH needed to saponify one gram of oil (e.g. olive oil ≈ 0.135, coconut oil ≈ 0.190). For a blend, use the weighted average of each oil's SAP value. Set a superfat percentage (commonly 5%) to leave a portion of oils unsaponified for a gentler bar, and choose a water-to-lye ratio (around 2.0–2.5 is typical). The calculator returns the lye, water and total batch weight.
The formula explained
The base lye requirement is oil weight × SAP value. Multiplying by (1 − superfat) discounts the lye so a fraction of the oils stays free in the finished soap. Water is then lye × water ratio. For example, a 2.3 ratio means 2.3 g of water per gram of lye.
$$\begin{gathered} \text{Lye} = \text{Oil} \times \text{SAP} \times \left(1 - \frac{\text{Superfat}}{100}\right) \\[1.2em] \text{Water} = \text{Lye} \times \text{Water Ratio} \\[1.2em] \text{Batch} = \text{Oil} + \text{Lye} + \text{Water} \end{gathered}$$
Worked example
For 500 g of oil with a SAP value of 0.138, a 5% superfat and a 2.3 water ratio: lye = \(500 \times 0.138 \times 0.95 = 65.55\) g. Water = \(65.55 \times 2.3 = 150.765\) g. Total batch = \(500 + 65.55 + 150.765 = 716.315\) g.
FAQ
Is this for NaOH or KOH? This calculator uses NaOH (solid bar soap) SAP values. KOH (liquid soap) uses different SAP values and a 1.403 conversion factor.
What superfat should I use? 5% is a safe all-purpose default; 1–3% for laundry/hard bars, 7–8% for facial bars.
Always run a lye calculator before mixing. Lye is caustic — wear gloves and eye protection and add lye to water, never water to lye.
