NaOH vs KOH: Which Lye Do You Need for Soap Making?
If you're new to soap making, the lye question comes up fast. There are two types, and using the wrong one will ruin your batch. Here's the breakdown.
Quick Answer
NaOH (sodium hydroxide) makes solid bar soap. KOH (potassium hydroxide) makes liquid soap. You cannot swap them. Each has a different saponification value and produces a completely different product.
| Feature | NaOH (Sodium Hydroxide) | KOH (Potassium Hydroxide) |
|---|---|---|
| Makes | Solid bar soap | Liquid soap / soft soap |
| Common name | Caustic soda, lye | Caustic potash, potash lye |
| SAP values | Lower | Higher (about 1.4× NaOH) |
| Solubility | Less soluble | More soluble |
| Typical use | Cold process, hot process bars | Liquid soap, shaving soap |
| Safety | Equally dangerous | Equally dangerous |
| Can you swap them | No | No |
NaOH (Sodium Hydroxide)
- ✓Making cold process bar soap
- ✓Making hot process bar soap
- ✓Making solid shampoo bars
- ✓Any recipe that says "lye" for bar soap
KOH (Potassium Hydroxide)
- ✓Making liquid castile soap
- ✓Making shaving soap
- ✓Making soft/cream soaps
- ✓Any recipe that specifically says "potassium hydroxide"
Frequently Asked Questions
Can I use NaOH to make liquid soap?+
No. NaOH produced solid soap. For liquid soap, you need KOH. If you try to make liquid soap with NaOH, you'll get a solid, crumbly mess that won't dissolve properly.
Are NaOH and KOH equally dangerous?+
Yes. Both are highly caustic and can cause severe chemical burns. Always wear gloves, eye protection, and long sleeves. Work in a ventilated area. Keep vinegar nearby for skin contact (but use water for eyes).
How do I know which one my recipe needs?+
If the recipe makes bar soap, it uses NaOH. If it makes liquid soap, it uses KOH. The recipe should specify. If it just says "lye," it almost always means NaOH.