Why TCL soap calculator uses whole numbers for superfat

A user recently asked if we could allow decimal superfat values like 5.5% or 5.7% instead of whole numbers. It's a fair question, and there's a deliberate reason behind the answer.

Most soap calculators do let you enter decimal superfat values, and I could easily have done the same. I deliberately chose not to. Whole numbers are a conscious decision, because I didn't want to create the false impression that we have precise control over superfatting. We don't - and the decimals would only suggest otherwise.

Key takeaway: Decimal superfat values give a false sense of precision - because the actual superfat in your soap is never as exact as the number suggests.

We don't have precise control over superfat

When you select a 5% superfat, the real figure in your finished soap could land anywhere between roughly 3% and 7%.

The reason comes down to SAP values: the amount of lye each oil needs to saponify. Every soap calculator (The Cosmetics Lab included, and every other one out there) relies on "average" SAP values for each oil. But the actual SAP value of a given batch of olive or coconut oil can differ from the calculator's figure by 2-3% or even more, depending on the source, the crop, and how it was processed.

So the precise-looking number you enter is really an estimate sitting on top of another estimate. A decimal place on top of that doesn't add accuracy; it just hides the uncertainty that's really there.

Superfat is a safety net, not a dial

Many soapmakers think of superfat simply as the thing that makes soap gentler and more conditioning, the extra oils left unsaponified in the bar. That's true enough, but it isn't the main point, and most people aren't aware of what is. The main job of superfatting is to protect you from lye-heavy soap. If the real SAP values of your oils are lower than the ones the calculator assumes, you'd end up with excess lye, unless you've built in enough of a buffer to stay covered either way.

That buffer is exactly what superfat provides. It's why 5% is the most commonly recommended starting point: it's generous enough to absorb the natural variation in your oils and keep your soap safe.

Seen this way, superfat is a margin of safety, not a precision setting. And once you realize that, asking for 4.7% instead of 5% stops making sense. The decimal implies a level of accuracy that simply doesn't exist in real-world soapmaking.

The practical takeaway

With a margin of error this wide, whole numbers for superfat are more than enough in cold process soapmaking.

So here's a simple rule: whenever you come across a decimal superfat value, just round it up to the next whole number (5.2 becomes 6, 5.7 becomes 6, and so on). Rounding up always errs on the side of more superfat and less risk of lye-heavy soap, so you'll be absolutely fine.

Whole numbers keep things honest. And that helps you see what really keeps your soap safe.