Both NaOH and KOH are lye — the difference is the metal ion, and it decides whether you get a hard bar or a soft soap.
NaOH — sodium hydroxide (bar soap)
NaOH makes a hard bar. It's what you use for cold-process and hot-process bar soap. Sold as solid beads/flakes, typically ~97-99% pure.
KOH — potassium hydroxide (liquid/soft soap)
KOH makes a soft paste or liquid soap. It's used for liquid soap, cream soap, and some shaving soaps. Usually sold at ~90% purity, so calculators offer a "90% KOH" setting that adds ~10% more to compensate — leave it on unless your KOH is labeled higher purity.
Why two SAP values?
Every oil has a NaOH SAP value and a KOH SAP value. KOH's is roughly 1.4× the NaOH value because KOH is heavier per molecule, so it takes more grams to saponify the same oil. Always use the SAP that matches your lye. Browse both on any oil page.
Dual lye
Blending NaOH and KOH gives a texture between hard and soft — used in cream soaps and some shaving soaps. The calculator splits the lye by your chosen NaOH:KOH ratio.
Pick your lye type in SoapCalc — it uses the right SAP values and handles 90% KOH and dual lye for you.