Lye calculator
The calculator behind The Cold Process Soap Recipe Book. It uses the same verified SAP values and the same rounding as every printed formula card, so a card and this page always agree.
Formula
The book uses lye concentration, 33% by default (Chapter 4).
Size the batch to my mold
Length × width × pour depth × 0.70 = grams of oils (× 0.40 = ounces in inches). Chapter 7.
Results
Sodium hydroxide causes severe burns and eye damage. Wear splash goggles, gloves and long sleeves, add lye to water and never the reverse, and keep water nearby for first aid. Weigh lye to 0.1 g and check it against the card or this page before mixing. This calculator is a tool: check every figure before use. Full guidance is in Chapter 6 of the book.
Rounding follows the book: lye to 0.1 g (exact ties rounded down, which adds a hair of superfat), water from the rounded lye to 1 g, quality scores to whole numbers. SAP values and fatty acid profiles are the book's verified data (Appendix A and B).