Superfatting: What It Is and How to Choose Your Percentage

What is Superfatting?

Superfatting means intentionally using less lye than needed to saponify all the oils in your recipe. The "extra" unsaponified oil remains in the finished bar, providing additional moisturizing and conditioning benefits.

For example, at 5% superfat, 5% of the total oils in your recipe remain as free oils in the bar rather than being converted to soap.

Why Superfat?

There are two main reasons to superfat:

1. Safety margin — Even with a precise scale, small measurement errors can occur. A superfat of at least 3% ensures no free lye remains in the cured bar, even if your measurements are slightly off.

2. Skin feel — Free oils left in the bar make it feel more conditioning and less stripping on skin. Higher superfat = more emollient feel.

How Much Superfat?

The sweet spot for most soap makers is 5% — enough for a safety margin and a pleasant skin feel without sacrificing bar hardness or shelf life.

Superfat and Shelf Life

Higher superfat means more free oils in the bar. Free oils can go rancid over time, especially unsaturated ones. If you're superfatting above 8%:

• Avoid oils high in linoleic/linolenic acid in your recipe
• Add an antioxidant like rosemary oleoresin extract (ROE) at 0.1% of total oils
• Cure in a cool, dark place
• Use bars within 6–12 months

Superfat at Trace vs. Lye Discount

There are two schools of thought:

Lye Discount (Most Common)

Simply reduce the amount of lye by the superfat percentage. This is what SoapIndex and most calculators do. The downside: you don't control which oil remains unsaponified, because lye reacts with all oils somewhat randomly.

Superfat at Trace

Use full lye for a 0% superfat recipe, let saponification begin, then add a specific "superfat oil" at trace. The theory is that this oil is more likely to remain unsaponified. In practice, some soapers debate whether this truly works in cold process since saponification continues for hours after pouring.

Recommendation: Use the lye discount method. It's simpler, widely validated, and achieves the same practical result.

Superfat by Soap Type

• Bar soap (NaOH): 3–8% is standard
• Liquid soap (KOH): 0–3% is typical (higher superfat can make liquid soap cloudy)
• Dual lye (NaOH + KOH): 3–5% works well for cream/paste soaps
• Hot process: Can superfat slightly higher since saponification completes in the pot

Using the Calculator

In the SoapIndex calculator, adjust the superfat slider in Batch Settings. The lye amount updates in real time. Watch the quality metrics — higher superfat increases conditioning but may push other metrics outside ideal ranges.

The assessment flags will warn you if your superfat is unusually high (>10%), reminding you of the shelf life implications.