Castor oil in soap making
Published by The Soap Brain Team
Castor oil is a conditioning soap-making oil rich in ricinoleic acid (90%). A gram of it turns to soap with about 0.128 g of NaOH (lye). It makes a mild, gentle bar and pairs well with a harder, cleansing oil. Most soapers use it at 5–10% of their oils.
Fatty-acid profile
Castor oil is a gentle, conditioning oil, mostly unsaturated fatty acids that leave a mild, moisturising bar with a softer, lower lather. It pairs naturally with a harder, more cleansing oil to firm the bar up and add bubbles.
Castor oil is about 2% saturated fat and 98% unsaturated — that unsaturated majority is what makes it conditioning and slower to trace, but softer on its own.
| Fatty acid | Share | What it does in soap |
|---|---|---|
| Ricinoleic acid | 90% | an unusual hydroxy fatty acid, effectively unique to castor oil, that boosts and stabilises a creamy, bubbly lather |
| Oleic acid | 4% | a monounsaturated fatty acid that makes a gentle, conditioning, moisturising bar with a slick, lower lather and a slower trace |
| Linoleic acid | 4% | a polyunsaturated fatty acid that is very conditioning but oxidises readily — a driver of DOS (dreaded orange spots) at high levels |
| Palmitic acid | 1% | a saturated fatty acid that builds a hard, long-lasting bar with a stable, creamy lather |
| Stearic acid | 1% | a saturated fatty acid that adds hardness and a thick, stable lather; a large share can speed up trace |
Castor oil in the bar
On its own Castor oil throws a big, quick, bubbly lather — the kind of foam most people associate with a cleansing bar. Balance it with conditioning oils so the lather stays generous without drying the skin.
Castor oil traces at a fairly typical pace; how fast the whole batch moves will depend mostly on the other oils, your temperatures and any fragrance you add.
In a blend Castor oil is the conditioning, skin-feel component and can often make up the bulk of the oils. Add a firmer, more cleansing oil — coconut, palm or a hard butter — to bring hardness and bubbles the finished bar would otherwise miss.
Closest substitutes for Castor oil
Out of Castor oil? These oils behave most like it in a bar — ranked by how close their hardness, cleansing and conditioning profile and lye (SAP) requirement are. The numbers are predicted properties for a 100% single-oil bar, not a safety guide; always recalculate the lye when you swap an oil.
| Oil | Hardness | Cleansing | Conditioning | SAP (NaOH) |
|---|---|---|---|---|
| Castor oil (this oil) | 2 | 0 | 98 | 0.128 |
| Canola oil (low-erucic) | 6 | 0 | 94 | 0.133 |
| Apricot kernel oil | 5 | 0 | 90 | 0.135 |
| Kukui nut oil | 6 | 0 | 90 | 0.135 |
| Flaxseed / linseed oil | 5 | 0 | 89 | 0.135 |
Using Castor oil in a recipe
One gram of Castor oil needs about 0.128 g of NaOH (sodium hydroxide) to turn fully to soap, within a documented range of 0.125–0.132 g/g across sources. The calculator below uses this value; always confirm the lye weight before you mix.
Its iodine value is about 86 — a moderate value, a good all-round balance of hardness and conditioning. Iodine value is only a rough guide, not a hard rule, but it gives you a feel for how a bar built around this oil will wear.
Most soapers use Castor oil at roughly 5–10% of their oils.
Maker's note: Boosts and stabilises lather. Use a small amount — it turns sticky and gummy if overused (over ~15–20%).
Calculate lye for Castor oil
The calculator below is pre-loaded with Castor oil. Enter your weights, add other oils, and it works out the exact NaOH (lye) weight, water and quality numbers. Always weigh lye, oils and water — never measure by volume, wear gloves and eye protection, and add lye to water (never the reverse).
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Where these numbers come from
Every figure on this page is backed by at least two independent references, listed below — so you can check our work instead of taking our word for it.
- FAO/WHO JECFA — Castor oil monograph (INS 1503) — SAP 176–185 mg KOH/g, iodine 83–88
- SoapCalc oil list — SAP, iodine
- Wikipedia — Ricinoleic acid — fatty-acid profile
SAP data last updated · 51 oils covered.
Castor oil soap FAQ
- Can you make soap with 100% Castor oil?
- It is not recommended. Castor oil shows its best in a blend, usually up to about 10% of the oils. On its own the bar would be unbalanced — too soft or low-lathering for everyday use.
- What superfat should I use with Castor oil?
- A 5% superfat is a safe, common starting point for recipes using Castor oil; adjust to taste once you know how the finished bar feels. Never drop to 0% or below without a deliberate reason — the calculator will ask you to confirm it.
- Does Castor oil speed up or slow down trace?
- Castor oil traces at a fairly typical pace; how fast the whole batch moves will depend mostly on the other oils, your temperatures and any fragrance you add.