Superfat calculator
Pick a superfat and see what it costs you in lye — or enter the lye weight a recipe states and find out what superfat it was really written at. Both directions of the same sum, with every common setting compared side by side.
INPUTS
What are you working out?
Choose a superfat and see what it does to the lye weight and to the bar.
g
Your oil blend
OilShare
%
%
%
%
100.0% total
Superfat
%
A superfat is not a safety net you can lean on. It absorbs small measuring error — it does not rescue a recipe whose lye weight was wrong to begin with. Weigh everything, and if a stated lye weight comes out at a negative superfat here, do not make that recipe.
RESULTS
NaOH at 5% superfat
142.9 g
150.4g would saponify everything −7.5g held back
Oil left unsaponified in the bar
50.0 g
The same oils at every superfat
| Setting | Superfat | NaOH |
|---|---|---|
| Salt & laundry bars | 2% | 147.4 g |
| Hot process floor | 3% | 145.9 g |
| Cold process standard | 5% | 142.9 g |
| Gentle / facial | 7% | 139.9 g |
| Rich / shampoo bars | 10% | 135.3 g |
| High-superfat specialty | 15% | 127.8 g |
A working superfat. The free oil is a real cushion against measuring error, and at this level it conditions the bar without costing you much lather.
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How this is worked out
Superfat is expressed as a percentage, but it is really a subtraction. The calculator first works out the full lye figure — the weight that would saponify every gram of oil in the blend, with nothing left over — then reduces it by the superfat percentage. A 5% superfat is simply 5% less lye than that full figure, and the oil that lye would have consumed stays in the bar as oil. Running it backwards is the same arithmetic rearranged: divide a stated lye weight by the full lye figure, and what is missing is the superfat the recipe was written at.
full lye = Σ (oil weight × saponification value)
lye used = full lye × (1 − superfat)
superfat = 1 − (stated lye ÷ full lye)
free oil = total oils × superfat
Questions makers ask
What does superfat actually mean?
It is oil the lye never gets to. You deliberately use less lye than your oils could consume, so a share of them stays as oil in the finished bar. On a 1 kg batch at 5%, that is 50 g of oil left free — enough to make the bar feel conditioning rather than stripping, and enough to swallow a small measuring error without leaving live lye in the soap.
How do I know what superfat a recipe uses?
Work backwards from its lye weight, which is what the "read a recipe" mode here does. Enter the recipe's oils exactly as written and then its stated lye weight, and the tool divides that by the full lye figure to give you the superfat. A recipe that comes out near 5% was written at the standard setting. One that comes out negative calls for more lye than its oils can take, and should not be made as written.
How much difference does a few percent make to the lye?
Less than most people expect. On a 1 kg batch of 40% olive oil, 30% coconut oil (76°), 25% palm oil, 5% castor oil, moving from 5% to 8% superfat changes the sodium hydroxide by 4.5 g — from 143 g to 138 g. That is a small weight, which is exactly why it needs a scale that reads tenths of a gram rather than an estimate.
Can I superfat at zero?
You can, and it is how laundry and salt bars are made, but not for anything that touches skin. At 0% the recipe assumes every saponification value is exact, your scale is perfect and your lye has drawn no moisture from the air. None of those are reliably true. 3% is the floor for a body bar and 5% is where most people should sit.
Does a higher superfat make a milder bar?
Up to a point, and then it stops helping. More free oil does mean a more conditioning bar, but it also softens it, cuts the lather and shortens its shelf life, because unsaponified oil is the part that goes rancid. Past about 10% you are making something closer to a lotion bar than a soap.
Which oil ends up as the superfat?
In cold process you do not get to choose. The lye reacts with whatever it meets, and the oils that happen to be slowest to react are what is left over, which is rarely the expensive one you had in mind. Hot process can choose: cook the batch out and stir a weighed oil in at the end, so that oil never meets live lye at all.
What are the usual settings?
The bands this calculator offers are 2% salt & laundry bars, 3% hot process floor, 5% cold process standard, 7% gentle / facial, 10% rich / shampoo bars, 15% high-superfat specialty. They are conventions rather than rules — the useful thing is not the number itself but knowing which one a recipe was written at before you follow it.
Read before your next batch
- How to use a soap calculator What every field means, where the number comes from, and how to tell when a result is wrong before you have mixed anything.
- What is superfat in soap making? The oil left deliberately unsaponified in a bar — what it is for, what each setting changes, and why the number matters more as a safety margin than as a skin treatment.
- Cold process vs hot process soap Same oils, same lye, two very different afternoons — and one difference that decides it for most people.
- Water discounts explained Lye concentration, water-to-lye ratio and per cent of oils are three names for one decision — and the decision has nothing to do with how much lye you need.