Quick Answer — How Much Lye Does a Soap Recipe Need?
NaOH (oz) = Σ(Oil Weight × Oil's SAP Value) × (1 - Superfat %). Every oil or fat has a saponification (SAP) value — the ounces of sodium hydroxide needed to fully convert one ounce of that oil into soap. Multiply each oil's weight by its SAP value, sum the results across your whole recipe, then reduce the total by your chosen superfat percentage.
Superfat deliberately withholds a small percentage of the calculated lye so that percentage of oils remains unreacted in the finished bar, acting as a mild, skin-conditioning excess fat. 5-8% is a typical range for a moisturizing bar; 0% is a fully saponified "no excess fat" soap.
Water is sized to hit a target lye concentration — lye as a percentage of the total lye-water solution: Water = NaOH/(Lye Concentration %) - NaOH. A 30-33% concentration (sometimes called a "full water" or standard discount) is a common cold-process default; higher concentrations use less water and trace faster.