Soap Superfat Percentage Effect Calculator
Calculate how superfat percentage affects soap hardness, lather, and skin feel.
Get cleansing, conditioning, and bubble values from superfat lye discount.
Soap Superfat (Lye Discount)
Superfat is the excess oil left after saponification — oils that the lye doesn’t convert to soap. This unsaponified oil makes the bar less harsh and more conditioning, but reduces lather and shelf life.
The math: Superfat % = (1 - actual lye / theoretical lye) × 100
Or: a 5% superfat means using 95% of the calculated lye amount, leaving 5% of oils unsaponified.
Standard superfat ranges:
| Soap Style | Superfat % | Result |
|---|---|---|
| Laundry / kitchen soap | 0-2% | Maximum cleansing, harsh on skin |
| Shampoo bar | 2-4% | Strong cleanse with mild conditioning |
| Standard bar | 5% | Balanced — most common starting point |
| Sensitive / kids | 7-8% | Mild, less lather, conditioning |
| Salt bar | 15-20% | Required — salt destroys lather; high SF compensates |
| Castile (100% olive oil) | 0-5% | Olive oil naturally conditions, low SF needed |
| Goat milk / face soap | 6-8% | Extra moisture for facial use |
Effects of superfat on bar properties:
| Property | Low SF (0-3%) | Standard (5%) | High SF (8-15%) |
|---|---|---|---|
| Hardness | Very hard | Hard | Soft |
| Lather | Strong, voluminous | Good | Reduced |
| Bubble size | Big | Mixed | Small |
| Cleansing | Strong | Standard | Mild |
| Conditioning | Low | Moderate | High |
| Skin feel after | Squeaky / drying | Clean | Moisturized |
| DOS (dreaded orange spots) | Low risk | Standard | Higher risk |
| Shelf life | 1-2 years | 1 year | 6-9 months |
Why salt bars need 15-20% SF: Salt destroys lather. To compensate, salt bar recipes use very high superfat — the unsaponified oils provide moisture and soften the bar enough to lather despite the salt content.
Why castile soap uses low SF: 100% olive oil saponifies into a slow, slick soap with naturally mild characteristics. Adding extra superfat would make it slimy and prone to rancidity (olive oil already has high oleic acid).
The “selecting your superfat oil” choice: Some soap makers reserve a luxury oil (jojoba, shea butter, sweet almond) as the LAST oil added — claiming this oil ends up as the unsaponified superfat. Chemically this is FALSE: saponification is a random process, all oils contribute proportionally to the final superfat. But it doesn’t hurt to add the luxury oil at trace if you prefer.
DOS (dreaded orange spots): DOS are rancidity stains caused by oxidation of unsaturated fats. Higher superfat = more unsaponified unsaturated oils = higher DOS risk. Use:
- Antioxidants (ROE — rosemary extract, vitamin E)
- Saturated oils (coconut, palm, lard) for the SF
- Cool storage, away from light
Trade-offs and recommendations:
- 5% SF is the gold standard for most bath bars
- Above 7%: lather suffers noticeably
- Below 3%: bar becomes harsh
- Salt bars must be 15-20% — exception, not the rule
Math example: For a 32-oz oil recipe with palm/coconut/olive blend:
- 0% SF: needs 4.31 oz NaOH (theoretical)
- 5% SF: 4.31 × 0.95 = 4.10 oz NaOH (5% lye discount)
- 8% SF: 4.31 × 0.92 = 3.97 oz NaOH (8% lye discount)