Every oil has a unique saponification (SAP) value: the amount of lye needed to fully convert that oil into soap. This calculator multiplies each oil's weight by its SAP value, sums them up, then reduces by your superfat percentage to leave some oils unreacted for a gentler bar.
“Water:lye ratio” is the simplest method. A 2:1 ratio means 2 parts water to 1 part lye. “Lye concentration” is the inverse: 33% concentration means lye is 33% of the water+lye solution (equivalent to about 2:1). Use whichever method your recipe calls for.
Once you have your lye amount, check your oil blend with the Oil Property Calculator to see how your bar will perform. Ready to scale up? Use the Batch Calculator.