What is Superfatting in Soapmaking?

If you are just starting out with cold process soapmaking, you have probably already run across the term "superfat" (or surgraissage in French).

You might have seen recipes calling for a 5%, 7%, 8%, or 10% superfat. But what does that percentage actually mean? Does a higher superfat automatically make a soap more moisturizing? And how does it relate to the amount of lye you use?

Let's break it down simply, step by step.

What Is Superfatting?

Superfatting means voluntarily reducing the amount of caustic soda (lye) used in a recipe, leaving a portion of the oils and butters theoretically unsaponified.

During saponification, oils and butters react with sodium hydroxide (NaOH) to form soap and glycerin. When you choose a specific superfat percentage, you are telling your lye calculator to use slightly less lye than the theoretical amount needed to saponify 100% of the fats.

For example:

• 0% superfat: The exact theoretical amount of lye needed to saponify 100% of the fats is used.

• 5% superfat: The lye amount is reduced to theoretically saponify about 95% of the fats.

• 8% superfat: The lye amount is reduced to theoretically saponify about 92% of the fats.

• 10% superfat: The lye amount is reduced to theoretically saponify about 90% of the fats.

The key word here is "theoretically."

You shouldn't assume that an 8% superfat soap contains exactly 8% of a specific oil sitting around completely intact. Chemical reality is much more complex!

Why Use a Superfat?

One of the main reasons is to formulate a soap that is less stripping and more comfortable for the skin.

Soap is naturally designed to wash away oils and grease from the skin's surface—that is precisely how it cleans. Superfatting slightly reduces the amount of free lye available during the reaction, which helps create a gentler wash. It can also influence how the soap feels on your skin afterward.

However, do not view superfatting as simply "pouring a precise percentage of moisturizing oil" directly into the finished soap.

An 8% superfat does not necessarily mean 8% of your finished bar is made of free, unsaponified oil. The actual composition depends on:

• The specific oils used and their fatty acid profiles

• The amount of lye

• The amount of water

• Manufacturing conditions

• The curing time

How Does Superfat Relate to Lye?

This is one of the most critical concepts for beginners. Superfatting directly changes the amount of caustic soda in your recipe.

Every oil has a saponification value (known as the SAP value), which tells you roughly how much lye is needed to turn that specific oil into soap.

• If a calculator tells you a certain amount of lye is needed for a 0% superfat...

• If you switch to a 5% superfat, the calculator reduces the lye.

• If you bump it to 8%, the lye is reduced even more.

• At 10%, it drops further still.

This is why you should never guess or tweak lye amounts randomly. Whenever you change your superfat, the lye must be recalculated.

What Happens with a Very Low Superfat?

A low superfat means more lye is available to react with the fats.

• The upside: The soap will have a stronger cleansing power and leave a very " squeaky clean" feeling.

• The downside: For people with naturally dry skin or those who wash frequently, a low-superfat soap can feel harsh or uncomfortable.

That doesn't mean a low superfat is inherently bad—some specialty soaps are intentionally formulated this way. The key is understanding why you are choosing that level.

What About a Very High Superfat?

It’s easy to fall into the trap of thinking: "The higher the superfat, the better and more moisturizing the soap." But it's not that simple.

A very high superfat can make a soap feel rich, but it can also kill the lather, reduce cleaning power, and—in certain formulations—increase the risk of rancidity. This can lead to what soapmakers call "Orange Spots", those little orange spots caused by oxidized oils.

More superfat does not automatically mean a better bar of soap. Balance is everything!

What Superfat Percentage Should Beginners Choose?

There is no single "perfect" percentage for every soap. For everyday body bars, many soapmakers stick to a sweet spot between 5% and 8%.

Here is a general guide:

Note: These are general guidelines, not absolute laws. The ideal superfat depends on your oil blend, how the soap will be used, and the properties you want.

Does Superfat Make Soap Moisturizing?

This is a very common misconception among beginners.

Soap is a rinse-off cleansing product, not a leave-on body lotion. A higher superfat can make the wash less stripping, but it does not turn a bar of soap into a moisturizer.

Your overall recipe formulation matters far more than the superfat number alone. For instance, two soaps can both have an 8% superfat yet feel entirely different on the skin depending on their ratios of coconut oil, olive oil, shea butter, castor oil, and other fats. Superfat is just one piece of the puzzle!

Superfat vs. Lye Concentration: Don't Get Them Confused

These two percentages pop up on every soap calculator and are frequently mixed up, even though they mean completely different things:

• Superfat: Determines the reduction of lye relative to the theoretical amount needed to saponify the fats.

• Lye Concentration: Determines the proportion of caustic soda relative to water in your lye solution (a 30% lye concentration means 30% lye and 70% water).

Changing your lye concentration mainly changes your water amount. Changing your superfat changes your lye amount.

A Simple Way to Remember It

Think of superfatting simply as reducing the lye:

• Higher superfat = Less lye

• Lower superfat = More lye

And remember: Superfat does not control your water amount! Water is calculated separately based on your lye concentration or water-to-lye ratio.

In Summary

Superfatting is a foundational concept in cold process soapmaking. It allows you to formulate a soap with less lye than the theoretical maximum.

For everyday body bars, a moderate 5% to 8% superfat is a great starting point. Once you master this concept, you'll have a much firmer grasp on lye calculations, oil selection, cleansing power, lather, and overall recipe balance.

Above all else, remember this golden rule: Never blindly copy the lye amount from someone else's recipe. If you change the oils, batch size, superfat, or any other parameter, always run it through a reliable lye calculator first!