Soap Hardness Index Calculator
Calculate soap hardness, cleansing and conditioning scores from your oil percentages.
Covers ten common soap oils and rates the bar against the ideal ranges.
How Soap Hardness Index Is Calculated
Soap hardness is determined by the fatty acid profile of the oils used. Each oil contributes different fatty acids, and the saturated fatty acids (lauric, myristic, palmitic, stearic) are the primary drivers of bar hardness.
The Hardness Formula
Hardness Index = Sum of (Oil_Percentage × Oil_Hardness_Value) / Total percentage
Dividing by the total rather than by a flat 100 matters when your percentages do not add up to 100, which happens constantly while you are still juggling a recipe. The calculator normalises, so a half-finished recipe still gives you the hardness of the blend as far as you have written it.
Each of these three numbers comes from the fatty acid profile of the oil, not from its SAP value. Hardness tracks the saturated fatty acids (lauric, myristic, palmitic, stearic). Cleansing tracks lauric and myristic specifically, the two short-chain acids that strip oil off skin. Conditioning tracks the unsaturated ones (oleic, linoleic, ricinoleic) that stay behind on it.
| Oil | Hardness | Cleansing | Conditioning | Notes |
|---|---|---|---|---|
| Coconut Oil | 79 | 67 | 10 | Very hard, and the only common oil that cleanses hard |
| Palm Oil | 50 | 1 | 49 | Hard, stable lather |
| Olive Oil | 17 | 0 | 82 | Soft, conditioning |
| Castor Oil | 15 | 0 | 90 | Soft, boosts lather |
| Shea Butter | 45 | 0 | 51 | Hard, moisturizing |
| Cocoa Butter | 53 | 0 | 36 | Very hard, luxurious |
| Lard | 43 | 0 | 54 | Hard, creamy lather |
| Tallow | 47 | 0 | 45 | Hard, stable bar |
| Sunflower Oil | 11 | 0 | 86 | Very soft, conditioning |
| Sweet Almond Oil | 15 | 0 | 90 | Soft, mild |
Look down the cleansing column and you will see why coconut oil dominates every discussion of a drying bar. It is doing essentially all of the cleansing in most recipes, so the coconut percentage IS the cleansing number, near enough. Above about 30% coconut, people start describing the bar as squeaky.
Interpreting the Scores
| Hardness Index | Rating | Bar Quality |
|---|---|---|
| 29-54 | Ideal Range | Best balance of hardness and mildness |
| 55-70 | Hard | Long-lasting but may be drying |
| Above 70 | Very Hard | Extremely firm, possibly brittle |
| Below 29 | Soft | May need long cure or salt additive |
Cleansing wants to land between 12 and 22, and conditioning between 44 and 69. Those two pull against each other, because the same oils that raise one lower the other, and a recipe that satisfies both is what people mean by a balanced bar.
Worked Example
A classic soap recipe:
- 30% Coconut Oil: 30 × 79 = 2,370
- 40% Olive Oil: 40 × 17 = 680
- 20% Palm Oil: 20 × 50 = 1,000
- 10% Castor Oil: 10 × 15 = 150
Total: (2,370 + 680 + 1,000 + 150) / 100 = 42.0 hardness
Run the same percentages down the other two columns and you get 20.3 cleansing (almost all of it the coconut, with 0.2 from the palm) and 54.6 conditioning. All three sit inside their ranges, which is why this particular recipe turns up in so many beginner tutorials.
Additional Factors
Sodium lactate (1 teaspoon per 500g oils) can increase hardness without changing the fatty acid profile. Water discount (using less water than standard) also produces harder bars but requires more careful handling during the saponification process. A full cure of 4-6 weeks allows excess water to evaporate, further hardening the bar.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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