Glossary
Lye calculator
A tool that calculates the exact lye and water amounts a soap recipe needs based on its oils.
A lye calculator is a tool, often a simple web form, that takes the oils in a recipe and their weights and returns the exact amount of lye and water needed. It does this by applying each oil's SAP value along with your chosen superfat percentage, removing the guesswork from a step where mistakes matter.
Most calculators also let you adjust the water amount as a percentage or a water-to-lye ratio, which affects how quickly the batter thickens and how soon bars can be unmolded. Because oils vary, a calculator has to be rerun any time a recipe's oils, weights, or superfat target change, even slightly.
When using one, double check that you selected sodium hydroxide for bar soap or potassium hydroxide for liquid soap, since the two give very different results for the same oils. Never guess or reuse an old calculation for a new recipe; a common and costly mistake is copying lye amounts from a similar-looking recipe instead of recalculating for the exact oils and weights actually being used.