NaOH vs KOH lye for soap making

What each alkali produces

Both are strong alkalis, both are corrosive, and both react with the fatty acids in oils to make soap. The difference is the metal ion left attached to the soap molecule. Sodium soaps pack tightly and crystallize into a firm, opaque bar. Potassium soaps are more water soluble and stay soft, which is why KOH is the choice for liquid soap, shower gel, soap paste, whipped soap, and traditional shaving soap.

That solubility is also why potassium soaps rinse away cleanly and are common in industrial hand cleaners. RusticWise describes KOH as smaller at the molecular level and better able to penetrate oil and rinse away, while NaOH gives the durability a bar needs to sit in a dish for weeks.

Cost and availability differ too. NaOH is sold as beads or flakes at 95 to 99 percent purity and is cheap. KOH is sold as flakes or pellets at around 85 to 90 percent purity and costs roughly two to three times as much per unit weight, depending on the supplier and quantity.

The 1.403 conversion factor

Saponification values (SAP values) are published as milligrams of KOH needed to saponify one gram of oil. The molecular weight of KOH is about 56.1 and that of NaOH is about 40.0. Their ratio, 56.1 divided by 40.0, is 1.403. One mole of either alkali saponifies the same amount of oil, but a mole of KOH weighs 1.403 times more.

So to convert a KOH SAP value to NaOH, divide by 1.403 (or multiply by 40 divided by 56.1, about 0.713). To convert an NaOH recipe to KOH, multiply the lye weight by 1.403. Example from a shaving soap reference: a recipe calling for 10.00 g NaOH needs 14.03 g KOH instead. Published tables sometimes list the NaOH factor and sometimes the KOH factor, so always check which alkali a column refers to before multiplying.

Typical multiplication factors from The Soap Kitchen's chart: coconut oil 0.190 NaOH and 0.269 KOH, olive oil 0.134 NaOH and 0.1906 KOH, palm oil 0.141 NaOH and 0.200 KOH, castor 0.1286 NaOH and 0.1811 KOH. Multiply the oil weight by the factor for the alkali you are using, then apply the superfat.

Worked example: the same oils, both alkalis

Take 500 g of a blend: 300 g olive oil, 150 g coconut oil, 50 g castor oil. For NaOH: 300 times 0.134 is 40.2 g, 150 times 0.190 is 28.5 g, 50 times 0.1286 is 6.4 g, total 75.1 g NaOH at 0 percent superfat. At 5 percent superfat: 75.1 times 0.95 is 71.3 g.

For KOH, either multiply 75.1 by 1.403 to get 105.4 g, or use the KOH factors directly: 300 times 0.1906 is 57.2, 150 times 0.269 is 40.4, 50 times 0.1811 is 9.1, total 106.6 g. The small difference comes from rounding in the published factors; both are within 1 percent. Apply the superfat, then the purity correction described next.

Water: liquid soap paste is usually made with more water than bar soap because KOH dissolves more readily and the paste is diluted later anyway. Bar soap is commonly made at a lye concentration of 28 to 35 percent (lye as a share of the lye plus water solution). The concentration does not change the lye weight, only how much water is added.

Correcting for 90 percent KOH purity

Most KOH sold to soap makers is about 90 percent pure; the rest is mostly water and potassium carbonate absorbed from the air. Calculators typically assume 90 percent purity for KOH and near 100 percent for NaOH. If a recipe needs 100 g of pure KOH and the flakes are 90 percent, weigh 100 divided by 0.90, which is 111.1 g.

Ignoring the purity turns into an unintended lye discount of about 10 percent. That is why a liquid soap recipe run with a 0 percent superfat on uncorrected KOH still comes out mild, and why the same recipe at a 5 percent superfat may separate or stay cloudy from too much free oil. Set the purity and the superfat separately; do not let one stand in for the other.

Check the supplier's certificate of analysis. If the label says 85 percent, divide by 0.85. Benchlet's lye calculator has a KOH purity input next to the superfat input so the two are not confused.

Dual lye: mixing NaOH and KOH in one recipe

Shaving soap and cream soap use both alkalis. NaOH gives structure so the puck holds its shape; KOH keeps it soft enough to load a brush and produces a dense, stable lather. A common starting ratio is 60 percent KOH and 40 percent NaOH by share of the saponification job, though many formulas run from 50/50 to 80/20 in favor of KOH.

The calculation splits the lye requirement by percentage, and each portion is converted to its own alkali. Using the 10.00 g NaOH equivalent from the earlier example at 60/40: the KOH portion is 14.03 times 0.60, which is 8.42 g of pure KOH; the NaOH portion is 10.00 times 0.40, which is 4.00 g. Then divide the KOH portion by its purity: 8.42 divided by 0.90 is 9.36 g of 90 percent flakes.

Both alkalis can be dissolved in the same water. Add them one at a time to cold water, never water to lye, in a ventilated space with gloves and eye protection. The shaving soap reference suggests a superfat of around 3 percent for these formulas, lower than a bath bar, because free oil dampens the lather a shaving soap exists to produce.

Frequently asked questions

Sources

• The Soap Kitchen: Saponification Values in Soap Making (NaOH and KOH factors, 40/56.1 conversion)
• GloryBee: Soap Making, Saponification and SAP Values (1.403 factor)
• Shaver Soaper: NaOH vs KOH, Ratios and Discounts (60/40 dual lye example)
• RusticWise: Difference Between Sodium Hydroxide and Potassium Hydroxide for Soapmaking