Why shave soap uses both lyes
Sodium soaps are hard and slow to dissolve; potassium soaps are soft and lather fast. A shaving soap wants both: enough sodium that the puck holds its shape in the bowl, enough potassium that a wet brush raises a dense lather in thirty seconds. So shave-soap makers saponify part of the oils with NaOH and part with KOH (typically 50–70% of the job done by KOH), on a recipe that is half stearic acid for the dense, stable foam. Cream soaps and shaving creams push further toward KOH, to 75–90%, and stay spoonable.
In this calculator, 60/40 KOH/NaOH means KOH saponifies 60% of the oils and NaOH the other 40%. The calculator works it out as: full KOH for the whole batch × 60% ÷ purity, plus full NaOH for the whole batch × 40%. Because KOH is a heavier molecule (56.1 vs 40) and is only 90% pure, the weights come out quite different from 60/40: on the shave recipe at 500 g, a 40% NaOH share is 28.8 g NaOH and 67.3 g KOH, which is 30% NaOH by weight. A calculator that splits by weight instead would give a noticeably different soap. This one splits by saponification and prints the weight ratio too. Check the original recipe’s convention before copying a split: a ratio of weighed flakes is not the same input.
The shave recipe at every split
| NaOH share | KOH share | NaOH | KOH (90%) | Total lye | NaOH by weight | Result |
|---|---|---|---|---|---|---|
| 0% | 100% | 0.0 g | 112.1 g | 112.1 g | 0% | Soft cream / croap |
| 20% | 80% | 14.4 g | 89.7 g | 104.1 g | 14% | Cream soap, shaving cream |
| 30% | 70% | 21.6 g | 78.5 g | 100.1 g | 22% | Soft puck, the common shave range |
| 40% | 60% | 28.8 g | 67.3 g | 96.1 g | 30% | Soft puck, the common shave range |
| 50% | 50% | 36.0 g | 56.1 g | 92.1 g | 39% | Firm puck, still lathers fast |
| 70% | 30% | 50.4 g | 33.6 g | 84.0 g | 60% | Hard puck, slower to load |
| 100% | 0% | 72.0 g | 0.0 g | 72.0 g | 100% | Hard bar soap, poor shave lather |
Stearic acid 50 / coconut 25 / shea 15 / castor 10 at 500 g, 3% superfat, 33% lye concentration, KOH at 90% purity. Total lye grows as KOH's share grows because KOH weighs more per mole and is 90% pure.
What the recipe looks like
Shave soap is the one recipe where the quality bars are supposed to look wrong. The example above reads hardness 76, cleansing 17, creamy 68, bubbly 26. A bath-bar calculator would flag the conditioning at 20 as far too low. For a lather that stands up on a brush that is the point: stearic and palmitic acids make the dense, slow-draining foam, coconut adds the fast bubbles that build it, castor holds it together, and the butter changes the fatty-acid balance. These are comparative indices, not skin or shaving-performance measurements. The oil chart explains the profiles behind them; a trial batch is still needed to judge the lather.
A high-stearic recipe can become thick very quickly. Use a complete hot-process method designed for that formula; this tool calculates ingredient weights, not cooking temperatures or a completion test. The trace guide distinguishes thickening from reaction progress, and the process guide explains the cook. Clarity is not proof of safe residual alkalinity.
Cream soap
Cream soap (and its shaving-cream cousin) is the same idea with less stearic acid and far less sodium, whipped with extra water and glycerin after the cook into a paste you scoop. An olive-softened example at a 20% NaOH share needs 14.1 g NaOH and 88.0 g KOH per 500 g of oils at 3% superfat. The liquid soap calculator handles the all-KOH end of the spectrum.
Superfat, purity and the usual cautions
Dual-lye recipes run low superfats (2–5%) because excess oil dulls a shave lather. KOH purity defaults to 90% as sold; NaOH to 100%. The combined solution remains caustic; follow the lye safety checklist: goggles, gloves, and a pot with room to foam. Check any recipe against a second calculator before the lye goes in; SoapCalc handles dual-lye recipes too, and this page's arithmetic is checked against SoapCalc's formulas and SAP values.