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Soap quality numbers, explained
What hardness, cleansing, conditioning and the rest actually mean, and which fatty acids drive each one. Treat every range here as a guideline you can knowingly ignore, not a pass/fail test.
Which fatty acid does what
| Fatty acid | Hardness | Cleansing | Bubbly | Creamy | Conditioning |
|---|
| Lauric | ✓ | ✓ | ✓ | | |
| Myristic | ✓ | ✓ | ✓ | | |
| Palmitic | ✓ | | | ✓ | |
| Stearic | ✓ | | | ✓ | |
| Ricinoleic | | | ✓ | ✓ | ✓ |
| Oleic | | | | | ✓ |
| Linoleic | | | | | ✓ |
| Linolenic | | | | | ✓ |
The pattern: saturated fats (lauric, myristic, palmitic, stearic) drive hardness and lather. Unsaturated fats (oleic, linoleic, linolenic) drive conditioning. Ricinoleic - castor oil's signature acid - is the one that does a bit of everything at once.
The seven numbers
- Hardness (29-54) - how firm the bar is and how long it lasts. Too low and it dents and wears down fast.
- Cleansing (12-22) - how strongly it strips oil from skin. Coconut and palm kernel oil push this up; too high and it gets drying. Recipes often cap coconut around 30% for this reason.
- Conditioning (44-69) - how moisturising the bar feels, from oils left on the skin after rinsing. Olive oil is the classic conditioning oil.
- Bubbly lather (14-46) - big, foamy lather. Low bubbly usually means low lauric/ricinoleic - a splash of castor oil (5-8%) is the usual fix.
- Creamy lather (16-48) - dense, stable lather rather than foam. Bubbly and creamy tend to trade off against each other; most recipes land somewhere between the two.
- Iodine (41-70) - how unsaturated the oil blend is. Lower means harder; much above 70 and the bar softens, and well above that, shelf life starts to suffer.
- INS (136-165) - a combined hardness indicator built from iodine and SAP. 160 gets cited as "ideal" a lot, but 100% olive oil soap sits around 105 and is still excellent soap - treat INS as one data point, not a verdict.
These are guidelines, not rules. 100% olive oil (Castile) soap fails half of these ranges and is one of the most-loved soaps there is. If a recipe sits outside a range on purpose and you like how it performs, that's the number that's wrong for your recipe - not your recipe.
Want to see these calculated live against your own oils? Open the calculator, or check how to use it if you're just getting started.
