Superfat & Lye Discount

How Extra Oil Affects Your Soap

Superfat means using less lye than what's needed to fully saponify all the oils in your recipe. The result is a bar of soap that contains unsaponified fat — free oils that remain in the finished bar to add moisturizing properties and a gentler skin feel. Understanding superfat (also called lye discount) is one of the most important concepts in soap formulation.

What Is Superfat?

Superfat is the percentage of oil in your recipe that is left unreacted after saponification. When you set a 5% superfat, it means 5% of your total oil weight will remain as free oil in the finished soap.

Example: If your recipe has 500g of oils and you set a 5% superfat, then 25g of oil (5% of 500g) will remain as unsaponified fat. The lye is calculated to react with the remaining 475g of oil.

Superfat vs Lye Discount

These terms describe the same thing from different perspectives:

Superfat

Adding extra oil beyond what the lye can saponify. "I'm adding more oil than the lye can handle."

Lye Discount

Using less lye than what's needed to saponify all oils. "I'm using less lye than the oils require."

The math: A 5% superfat = a 5% lye discount = using 95% of the calculated lye.

How Superfat Affects Your Soap

Low Superfat (0–3%)

Harder, longer-lasting bars
Less moisturizing, can feel drying
Better cleaning power
Higher risk of lye-heavy soap if measurements are off

Medium Superfat (4–7%)

Good balance of hardness and moisture
Recommended for most cold process soap
Comfortable safety margin
Good lather and skin feel

High Superfat (8–15%)

Softer bars, more moisturizing
Shorter shelf life (risk of DOS)
Reduced lather, may feel greasy

Choosing Your Level

Bar soap (general): 3–8% — the standard range.
Liquid soap (KOH): 0–3% — prevents cloudiness and separation.
Shaving soap: 5–8% — extra slip for razor glide.
Laundry soap: 0–1% — maximum cleaning, no residue.
Sensitive skin: 5–8% — gentler with more free oil.

Does It Matter Which Oil Is the Superfat?

In cold process, you cannot control which specific oil remains unsaponified. The lye reacts randomly with whatever fatty acids it contacts. In hot process, you can add a luxury oil at the end of the cook to increase the chance of it remaining as the superfat.

Note: The soap calculator accounts for total oil weight. It doesn't matter which oil you consider the "superfat oil" — the math works on the total.

Tip: Use SoapMath to handle superfat

SoapMath automatically calculates your lye amount based on your chosen superfat percentage. Just enter your oil amounts and the calculator handles the rest.

Open SoapMath →