Evening primrose oil: SAP value and fatty acids
Evening primrose oil has a saponification value of 189.3 mg KOH/g. One gram needs 0.135 g of sodium hydroxide (NaOH) for bar soap or 0.189 g of potassium hydroxide (KOH) for liquid soap to turn fully into soap, before any superfat.
Key numbers
- SAP, NaOH
- 0.134949 g per g oil
- SAP, KOH
- 0.1893 g per g oil
- Saponification value
- 189.3 mg KOH/g
- Iodine value
- 155
- INS
- 34
- INCI name
- Oenothera Biennis Oil
Example: 100 g of Evening primrose oil with no superfat needs 13.5 g of 100% NaOH, or 18.9 g of 100% KOH. A 5% superfat uses 5% less.
In soap
Its main fatty acids in our cited profile are linoleic (70.8%) and linolenic (10.7%). The eight fatty acids used in soap-quality scores make up 99.4% of the profile.
On its own it scores 8 for hardness, 0 for cleansing and 92 for conditioning, with a saturated : unsaturated split of 8 : 92.
Fatty-acid profile
| Fatty acid | % of fatty acids | Counts toward |
|---|---|---|
| lauric | 0.2% | hardness, cleansing, bubbly lather |
| myristic | 0.1% | hardness, cleansing, bubbly lather |
| palmitic | 6.2% | hardness, creamy lather |
| stearic | 1% | hardness, creamy lather |
| ricinoleic | 0% | conditioning, bubbly and creamy lather |
| oleic | 10.4% | conditioning |
| linoleic | 70.8% | conditioning |
| linolenic | 10.7% | conditioning |
Basis: Peer-reviewed GC of evening primrose seed oil (Pharmaceuticals 2026, Table 3), mean of 2023 and 2024 harvests; linolenic = GLA + ALA.
Soap-quality scores (this oil alone)
| Hardness | 8 |
|---|---|
| Cleansing | 0 |
| Conditioning | 92 |
| Bubbly lather | 0 |
| Creamy lather | 7 |
| Iodine | 155 |
| INS | 34 |
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: 189.3 mg KOH/g (Median of sources), from Dash B. et al. (2026) Cold pressed oils: a systematic review on physicochemical properties, nutritional composition and pharmacological activities. Frontiers in Nutrition. doi:10.3389/fnut.2026.1747029, Frontiers (open access, peer-reviewed review). Printed value: 187.35 mg KOH/g.
Published values we checked:
- 180 - 195 mg KOH/g — From Nature With Love (soap/cosmetic supplier)
- 192 mg KOH/g — The Soapery (UK soap supplier)
- NaOH 0.1362 / KOH 0.1918 g alkali per g oil — Certified Lye
- 187.35 mg KOH/g — Frontiers (open access, peer-reviewed review)
Fatty acids and iodine: Phytochemical Analysis of Lipid Composition and Biological Activities in the Seeds of Evening Primrose from Bulgaria. Pharmaceuticals 19(2):225 (2026), MDPI. Also checked: Ministry of Health, Labour and Welfare (Japan) (PDF p. 1837).
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.
Last updated .