Superfatting and Lye Discount: What the Percentage Does to the Bar

A calculator looks up a saponification value for each oil — grams of NaOH (or KOH) needed to saponify 1 gram of that oil. It multiplies by your weights, sums them, and that sum is a 0% superfat lye weight.

A 5% superfat means you use 95% of that lye weight.

Example using round numbers for illustration. Your tool's SAP table is the one that matters; tables differ by a few thousandths.

• 500 g olive + 300 g coconut + 200 g tallow
• Hypothetical combined NaOH at 0%: 142.0 g
• 5% superfat: 142.0 × 0.95 = 134.9 g NaOH
• 8% superfat: 142.0 × 0.92 = 130.6 g NaOH

Same oils. Different leftover fat. Different bar.

If you soap with KOH for liquid soap, the calculator uses KOH SAP values or converts NaOH × 1.403. Do not apply a NaOH gram weight to a KOH batch.

DOS is "dreaded orange spots" — oxidation. Extra unsaturated oil is fuel for it. Superfatting a high-linoleic recipe at 10% is how a pretty swirl spots in eight weeks.

Two methods:

1. 1Lye discount. All oils in the pot from the start. Lye is reduced. The leftover fat is a mix of whatever is in the pot. Simple. Default.
2. 2Superfat at trace. Calculate at 0% (or 2%), mix to trace, then stir in a reserved 5% of a chosen oil — often shea or a fancy liquid oil. That last oil is more likely to remain as that oil.

Method 2 is a preference for people who want the reserved oil's unsaponifiables in the bar. It is not required for a mild soap. Do not do both at full strength unless you meant to make an 10–12% superfat bar.

Some liquid and cream soaps split the alkali between NaOH and KOH. The calculator should ask for a percent split (for example 30% NaOH / 70% KOH by neutralization demand) and a superfat. The discount applies to the combined alkali, not to one of them after the fact. If you hand-calculate, convert everything to one basis first.

• 25–30% coconut body bar: 5–6%.
• 40% coconut "beach" bar: 7–8%, or drop coconut.
• 100% olive: 2–5%. Castile is already mild; extra superfat makes a softer slug.
• Shampoo bar: 3–5% plus a recipe built for that job, not a body-bar recipe with more shea.
• Laundry bar: 0–2%.

Run the number in one calculator. Write the superfat on the mold tag. When the bar feels harsh at week 6, raise superfat 1–2 points next time or cut coconut 5 points. When the bar dissolves in a week, lower superfat and raise hard fats — do not only chase the discount.

40% coconut at 5% superfat can still bite. The fix is often coconut down to 25% at the same 5%, not a jump to 12% superfat. Conversely, a 15% coconut bar at 8% superfat can feel slimy. Change one variable per test loaf. Write it on the tag or you will not remember which lever you pulled.

If two calculators disagree on NaOH by more than 2% on the same oils, pick one and stay there. The disagreement is the SAP table, not your scale. Mixing tables mid-batch is how a 5% superfat becomes an accident.