Stearic acid: SAP value and fatty acids
Stearic acid has a saponification value of 203 mg KOH/g. One gram needs 0.145 g of sodium hydroxide (NaOH) for bar soap or 0.203 g of potassium hydroxide (KOH) for liquid soap to turn fully into soap, before any superfat.
Key numbers
- SAP, NaOH
- 0.144715 g per g oil
- SAP, KOH
- 0.203 g per g oil
- Saponification value
- 203 mg KOH/g
- INCI name
- Stearic Acid
Example: 100 g of Stearic acid with no superfat needs 14.5 g of 100% NaOH, or 20.3 g of 100% KOH. A 5% superfat uses 5% less.
In soap
Its main fatty acids in our cited profile are palmitic (54%) and stearic (45%). The eight fatty acids used in soap-quality scores make up 99.8% of the profile.
On its own it scores 100 for hardness, 1 for cleansing and 0 for conditioning, with a saturated : unsaturated split of 100 : 0.
Stearic acid is a free fatty acid, not a fat. It reacts with lye straight away and can make batter thicken fast. Its lye need depends on the grade: common 'triple pressed' stearic (about half palmitic) needs about 208–211, while 90%+ stearic needs about 197–199. Check the grade and use your supplier's COA if you can. Published sources for this oil differ by more than 3%, so checking your supplier's certificate of analysis matters more than usual.
Fatty-acid profile
| Fatty acid | % of fatty acids | Counts toward |
|---|---|---|
| lauric | 0% | hardness, cleansing, bubbly lather |
| myristic | 0.8% | hardness, cleansing, bubbly lather |
| palmitic | 54% | hardness, creamy lather |
| stearic | 45% | hardness, creamy lather |
| ricinoleic | 0% | conditioning, bubbly and creamy lather |
| oleic | 0% | conditioning |
| linoleic | 0% | conditioning |
| linolenic | 0% | conditioning |
Basis: Supplier spec midpoints for 'Stearic Acid 50 NF' (formerly 'triple pressed'): C14 ≤1.5 (recorded 0.75), C16 52–56, C18 42–48; consistent with JP18 type 'stearic acid 50'.
Soap-quality scores (this oil alone)
| Hardness | 100 |
|---|---|
| Cleansing | 1 |
| Conditioning | 0 |
| Bubbly lather | 1 |
| Creamy lather | 99 |
Each score adds up certain fatty acids, the way Soap Ledger scores a whole recipe. They are for comparing recipes; they don't predict how a bar will feel, and they say nothing about safety or skin.
Sources
SAP value: 203 mg KOH/g (Midpoint of JP18 Stearic Acid acid-value spec (harmonized pharmacopoeial monograph)), from The Japanese Pharmacopoeia, 18th ed. (English), Ministry of Health, Labour and Welfare (Japan). Printed value: Acid value 194 – 212 mg KOH/g.
Published values we checked:
- Acid value 194 – 212 mg KOH/g — Ministry of Health, Labour and Welfare (Japan)
- 208 – 213 mg KOH/g — Acme-Hardesty (supplier)
- 208 - 213 mg KOH/g — From Nature With Love (soap/cosmetic supplier)
- NaOH 0.1411 / KOH 0.1987 g alkali per g oil — Certified Lye
- 56105.6 / 284.48 = 197.2 (pure stearic); 56105.6 / 256.42 = 218.8 (pure palmitic); 50:50 by weight = 208.0 mg KOH/g — Ministry of Health, Labour and Welfare (Japan)
Fatty acids: Stearic Acid 50 NF Powder – Specification, Acme-Hardesty (supplier). Also checked: Ministry of Health, Labour and Welfare (Japan) (Stearic Acid monograph, composition table).
NaOH and KOH values are converted from mg KOH/g with the molar masses 39.997 and 56.106 and rounded to 6 decimals, exactly as in the Soap Ledger app. Real oils vary by origin, grade and lot: check your supplier's certificate of analysis. Verify every weight before you make this, and follow lye safety. Not a safety or compliance certification.
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