Soap Hardness Index Calculator
Calculate soap hardness index, cleansing, and conditioning scores from oil percentages using SAP values.
Returns hardness out of 100 and longevity prediction.
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) / 100
Each oil has a known hardness contribution value based on its saponification profile:
| Oil | Hardness Value | Notes |
|---|---|---|
| Coconut Oil | 79 | Very hard, high cleansing |
| Palm Oil | 50 | Hard, stable lather |
| Olive Oil | 17 | Soft, conditioning |
| Castor Oil | 15 | Soft, boosts lather |
| Shea Butter | 45 | Hard, moisturizing |
| Cocoa Butter | 53 | Very hard, luxurious |
| Lard | 43 | Hard, creamy lather |
| Tallow | 47 | Hard, stable bar |
| Sunflower Oil | 11 | Very soft, conditioning |
| Sweet Almond Oil | 15 | Soft, mild |
Interpreting the Index
| 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 |
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
This falls in the ideal range (29-54), producing a hard yet gentle bar.
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.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.