Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are both alkalis that react with oils to make soap — but they produce very different results. Understanding the difference is essential before you put either one near your oils.
Also called caustic soda or lye. Produces hard, solid bar soap. The standard choice for cold process and hot process soap making.
Also called caustic potash. Produces soft, paste, or liquid soap. Used for liquid hand soap, shaving soap bases, and shampoo formulations.
Why they produce different textures
The sodium ion in NaOH bonds with fatty acids to form sodium soaps — tightly packed molecules that solidify at room temperature. The potassium ion in KOH forms potassium soaps — softer, more water-soluble molecules that remain paste-like or dissolve easily into liquid.
This is why you cannot simply swap one for the other in a recipe. The saponification values are different, the resulting textures are different, and the water percentages commonly used differ as well.
NaOH vs KOH at a glance
| Property | NaOH (Sodium Hydroxide) | KOH (Potassium Hydroxide) |
|---|---|---|
| Result | Hard bar soap | Soft / liquid soap |
| Common purity | 97–99% | 90–92% |
| SAP value (olive oil) | 0.134 | 0.188 |
| Typical water ratio | 2:1 to 2.5:1 | 3:1 to 4:1 |
| Form at room temp | White flakes or beads | White/pale pellets |
| Hygroscopic? | Yes — store sealed | Very — store sealed and dry |
| Availability | Widely available | Less common, often online |
| Common uses | Bar soap, shampoo bars | Liquid soap, shaving soap |
Why KOH purity matters more
NaOH is commonly sold at 97–99% purity and behaves predictably. KOH is usually sold at 90–92% purity because it absorbs atmospheric moisture very aggressively. If you use a KOH calculator entry assuming 90% and your actual KOH is 88% from sitting open, your soap will be oil-heavy.
FormulaAlchemy lets you enter your exact KOH purity percentage and the calculator adjusts the lye weight accordingly. Always check your supplier's specification sheet.
Dual-lye recipes: using both together
Some recipes blend NaOH and KOH to hit a texture between hard bar and full liquid. Shaving soap is the classic example — a cream soap made with a 60–70% KOH / 30–40% NaOH split.
Dual-lye calculation is more complex because you're computing two separate lye amounts at their respective purity levels and SAP values. FormulaAlchemy handles this automatically — you set the NaOH/KOH ratio with a slider and the calculator returns both lye weights.
Shaving soap
Typically 60–80% KOH, 20–40% NaOH. Stiff paste that lathers with a brush. Stearic and coconut oils dominate the recipe.
Liquid soap
100% KOH. Diluted from a paste with distilled water after saponification. Often made with a high-oleic oil base for clarity.
Bar soap
100% NaOH. Cold process or hot process. The most common soap-making format. Cure 4–6 weeks for best results.
Shampoo bar
100% NaOH, conditioning-heavy oil blend. Behaves like bar soap but formulated for hair. pH considerations apply.
Which should you start with?
If you are making your first batch, start with NaOH. It is more widely available, higher purity, easier to work with, and bar soap is the most forgiving format for beginners.
Move to KOH once you are comfortable with lye safety and have a few successful NaOH batches behind you. Liquid soap adds more steps (diluting the paste, adjusting pH, preserving) and fewer beginner resources exist for troubleshooting.
Both NaOH and KOH are highly caustic. Always add lye to water — never water to lye. Wear gloves, goggles, and work in a ventilated space. See our complete lye safety guide for full handling procedures.
Calculate NaOH, KOH, or dual-lye in seconds
FormulaAlchemy supports all three lye types with adjustable purity inputs and dual-lye split control. Free, no sign-up needed.