What Is Superfat in Soap?

If you read about handmade soap, you will see the word superfat a lot. It describes how much oil is left in the bar that did not react with lye.

The simple definition

When soap is made, each oil needs a specific amount of lye to turn fully into soap. Soap makers deliberately use slightly less lye than that. The extra oil that stays unreacted is the superfat.

• A 0% superfat uses exactly the amount of lye the oils need.
• A 5% superfat leaves about 5% of the oils free in the bar.
• Reducing the lye this way is sometimes called a lye discount.

Why soap makers superfat

• It adds a safety margin, so no excess lye is left in the bar.
• The free oils make the soap feel milder and less drying.
• It allows for small variations between batches of oil.

Typical percentages

Bramble Berry notes that makers usually choose between 1% and 20%, with 5% a common choice. Very high-coconut recipes such as salt bars often use a much higher superfat, around 20%, to offset coconut oil's strong cleansing.

Can a bar have too much superfat?

Yes. More free oil can make a bar softer, reduce lather and make it go rancid sooner. It is a balance, which is why recipes and lye calculators matter.

Superfat vs lye discount: is there a difference?

The two terms are often used interchangeably, but some soap makers draw a line between them. A lye discount describes using less lye than the oils need. Superfat can also describe adding extra oil, sometimes a special oil such as shea butter, at the end of the process. In both cases the result is the same: a percentage of free oil left in the bar.

How to choose a superfat for your recipe

• Everyday body bar: around 5% is a common starting point.
• Very cleansing oils, such as high-coconut or salt bars: makers often go much higher to soften the cleansing effect.
• Laundry or cleaning soap: some makers use 0% to 1% for maximum cleaning power. These bars are not meant for skin.
• Liquid soap: many makers use a very low superfat, because free oil can make liquid soap cloudy.

Superfat and shelf life

Free oils are the part of a bar most likely to oxidize over time. Oxidized oil can cause orange spots and an off smell, sometimes called 'dreaded orange spots' by soap makers. A sensible superfat, fresh oils, and storing bars somewhere cool and dry all help a bar last.

Do lye calculators handle superfat?

Yes. Most online lye calculators ask you to choose a superfat percentage and then work out the lye amount automatically. That is the safest way to set it, rather than adjusting lye amounts by hand.

A quick worked example

Imagine a recipe whose oils would need exactly 100 g of lye to fully saponify. At a 5% superfat, the calculator would recommend about 95 g of lye. The 5 g you leave out means roughly 5% of the oils stay unreacted in the finished bar. (This is only an illustration; always use a lye calculator with your real oil weights.)

What this means when buying soap

Most brands do not list their superfat, but a soap made with an accurate recipe should feel gentle and not harsh. If a handmade bar feels very drying, it may have a low superfat or a high proportion of coconut oil.

Learn how the reaction works in what is saponification.