Understanding 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 rather than being converted into 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 in the bar. The lye is calculated to react with the remaining 475g of oil.
Superfat vs Lye Discount
Superfat and lye discount are two ways of describing the same thing from different perspectives:
Superfat = adding extra oil beyond what the lye can saponify. You think of it as "I'm adding more oil than the lye can handle."
Lye discount = using less lye than what's needed to saponify all oils. You think of it as "I'm using less lye than the oils require."
The math: A 5% superfat = a 5% lye discount = using 95% of the calculated lye. If full saponification requires 100g of NaOH, a 5% lye discount means you use 95g instead.
How Superfat Affects Your Soap
Low Superfat (0–3%)
- Harder, longer-lasting bars
- Less moisturizing, can feel drying on skin
- Better cleaning power
- Higher risk of lye-heavy soap if measurements are slightly off
Medium Superfat (4–7%)
- Good balance between hardness and moisturizing
- Recommended range for most cold process soap
- Comfortable safety margin against lye-heaviness
- Good lather and skin feel
High Superfat (8–15%)
- Softer bars, more moisturizing
- Shorter shelf life due to excess free oils
- Increased risk of DOS (dreaded orange spots) from rancid oils
- Reduced lather, may feel greasy
Choosing Your Superfat Level
Does It Matter Which Oil Is the Superfat?
In cold process soap making, you cannot control which specific oil remains unsaponified. All oils are mixed together before adding the lye, and the lye reacts with whatever fatty acids it contacts. The unsaponified portion will be a random mix of all the oils in your recipe.
In hot process soap making, some soapmakers add a luxury oil (like avocado or argan oil) at the very end of the cook, after saponification is largely complete. This oil is more likely to remain as the superfat, though it's still not guaranteed.
Note: The soap calculator accounts for total oil weight when calculating superfat. It doesn't matter which oil you consider the "superfat oil" — the math works on the total.
How the Soap Calculator Handles Superfat
When you set a superfat percentage in the Soap Math calculator, the tool reduces the calculated lye amount accordingly. Here's what happens behind the scenes:
- Step 1: The calculator determines the SAP value for each oil and computes the total lye needed for 100% saponification.
- Step 2: It multiplies the total lye by (1 − superfat%), reducing the lye amount.
- Step 3: The water amount is calculated based on the reduced lye amount (using your chosen water ratio).
- Step 4: The result is a recipe with slightly less lye than needed, leaving free oil in the finished bar.
Tips for Working with Superfat
- Always use a digital scale accurate to at least 1g (0.1g is better) — small lye errors matter more at low superfat levels.
- If you're new to soap making, start with 5% superfat — it's a safe, forgiving starting point for most recipes.
- Higher superfat shortens shelf life. If your soap will sit for months before use, keep superfat moderate (3-5%) and add an antioxidant like ROE (rosemary oleoresin extract).
- Oils high in linoleic and linolenic acids (like grapeseed, hemp) go rancid faster at high superfat. Stick to 5% or lower when using these oils heavily.
- Always run your recipe through a lye calculator before making soap — never rely on manual math alone.