Castor oil: SAP value and fatty acids
Castor oil has a saponification value of 181.5 mg KOH/g. One gram needs 0.129 g of sodium hydroxide (NaOH) for bar soap or 0.182 g of potassium hydroxide (KOH) for liquid soap to turn fully into soap, before any superfat.
Key numbers
- SAP, NaOH
- 0.129388 g per g oil
- SAP, KOH
- 0.1815 g per g oil
- Saponification value
- 181.5 mg KOH/g
- Iodine value
- 85
- INS
- 97
- INCI name
- Ricinus Communis Seed Oil
Example: 100 g of Castor oil with no superfat needs 12.9 g of 100% NaOH, or 18.2 g of 100% KOH. A 5% superfat uses 5% less.
In soap
Its main fatty acids in our cited profile are ricinoleic (84.8%) and linoleic (5.9%). The eight fatty acids used in soap-quality scores make up 100% of the profile.
On its own it scores 3 for hardness, 0 for cleansing and 97 for conditioning, with a saturated : unsaturated split of 3 : 97.
Fatty-acid profile
| Fatty acid | % of fatty acids | Counts toward |
|---|---|---|
| lauric | 0% | hardness, cleansing, bubbly lather |
| myristic | 0% | hardness, cleansing, bubbly lather |
| palmitic | 1.5% | hardness, creamy lather |
| stearic | 1.7% | hardness, creamy lather |
| ricinoleic | 84.8% | conditioning, bubbly and creamy lather |
| oleic | 5.7% | conditioning |
| linoleic | 5.9% | conditioning |
| linolenic | 0.4% | conditioning |
Basis: Peer-reviewed GC of castor oil (Materials 2025, Table 2); scaled to 100% from 100%.
Soap-quality scores (this oil alone)
| Hardness | 3 |
|---|---|
| Cleansing | 0 |
| Conditioning | 97 |
| Bubbly lather | 85 |
| Creamy lather | 88 |
| Iodine | 85 |
| INS | 97 |
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: 181.5 mg KOH/g (JP18 pharmacopoeia range midpoint (no Codex standard for castor oil)), from The Japanese Pharmacopoeia, 18th ed. (English), Crude Drugs and Related Drugs monographs, Ministry of Health, Labour and Welfare (Japan). Printed value: 176–187 mg KOH/g.
Published values we checked:
- 176–187 mg KOH/g — Ministry of Health, Labour and Welfare (Japan)
- 176–185 mg KOH/g — From Nature With Love (soap/cosmetic supplier)
- 182 mg KOH/g — The Soapery (UK soap supplier)
- NaOH 0.1286 / KOH 0.1811 g alkali per g oil — Certified Lye
Fatty acids and iodine: Characterization of Vegetable Oils for Direct Use in Polyurethane-Based Adhesives: Physicochemical and Compatibility Assessment. Materials 18(5):918 (2025), MDPI. Also checked: Ministry of Health, Labour and Welfare (Japan) (PDF p. 2247).
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 .