Getting your lye amount right is the difference between soap and a caustic mess – this calculator does the math for you using published saponification (SAP) values, so you can focus on your recipe instead of a spreadsheet.
Soap Lye (NaOH) & Water Calculator
Enter up to 5 oils and their weights. This calculates the sodium hydroxide (lye) and water needed for cold-process soap, using published saponification (SAP) values per oil.
⚠ Lye (sodium hydroxide) is caustic. Always wear gloves and eye protection, add lye to water (never water to lye), and work in a ventilated area. This calculator estimates quantities only – it does not replace proper safety training.
Why a range of SAP values? Saponification values vary slightly by oil batch, crop, and processing method. The values used here are commonly-cited midpoints from published SAP tables – for a specific bottle of oil, check the supplier's own SAP value if provided.
Every oil saponifies (turns into soap) with a different amount of lye, because every oil has a different fatty acid makeup. That’s what the SAP value represents: grams of sodium hydroxide needed to fully saponify one gram of that oil. Coconut oil, for example, needs roughly 40% more lye per gram than olive oil.
Superfatting means holding back a small percentage of lye so some oil remains unreacted in the finished bar, making it gentler on skin. Most cold-process recipes use 3-8% superfat; this calculator defaults to 5%, a common middle ground.
Lye concentration controls how much water you mix with your lye. A lower concentration (more water) traces more slowly, useful for intricate swirls; a higher concentration speeds up trace. 33% is a commonly used starting point for beginners.
Where These Numbers Come From
SAP values used here are commonly-cited midpoints from published saponification tables (Gary Olds’ saponification chart, cross-checked against soap-making-resource.com and thesoapkitchen.co.uk). Exact SAP values vary slightly by oil batch, crop, and processing method – if your oil supplier publishes its own SAP value, use that for the most accurate result.