Lye calculator
The weight of sodium or potassium hydroxide your oils actually need. Build your blend, set a superfat and a water discount, and get every figure weighed out — with each oil saponified separately, because a blend has no single saponification value.
INPUTS
Making
Solid bar soap — Caustic soda. Every cold and hot process bar recipe uses this one.
g
35.3 oz of oils
Your oil blend
OilShare
%
%
%
%
The one oil that works alone — 100% olive is Castile. Slow to trace, slow to cure, extremely mild.
Superfat
%
Lye is caustic. Goggles and gloves before the tub is opened, lye into water and never the other way round, and mix somewhere with moving air — the first breath of fumes catches everyone once. Weigh every figure below on a scale; measuring lye by volume is how people get hurt. Keep children and animals out of the room until the batch is in the mould and the tools are washed.
RESULTS
NaOH for this batch
142.9 g
150.4g at 0% superfat −7.5g for 5%
Water
290.1 g
33.0% solution 2.03:1 water to lye 29% of oils
Weigh out
| Oil | Share | Weight |
|---|---|---|
| Olive oil | 40.0% | 400.0 g |
| Coconut oil (76°) | 30.0% | 300.0 g |
| Palm oil | 25.0% | 250.0 g |
| Castor oil | 5.0% | 50.0 g |
Fragrance oil
30.0 g
Superfat left in the bar
50.0 g
Batch weight
1462.9 g
Mould space needed
1540 ml
Run this again for every recipe. Oils saponify at different rates, so the lye figure changes the moment the blend does — a weight carried over from a different recipe is the commonest way a batch goes wrong.
How this is worked out
Every oil has a saponification value: the weight of lye it takes to turn one gram of that oil into soap. Those values differ enough between oils that a blend has no single value of its own — coconut oil needs roughly 36% more lye per gram than olive oil. So the calculator works out each oil on its own and adds the results together, rather than averaging anything. Then it takes the superfat off: a 5% superfat simply means using 5% less lye than the oils could consume, which leaves that share of the oil unsaponified in the finished bar. Water is worked out last, because water changes how the batch behaves but never how much lye it needs.
full lye = Σ (oil weight × saponification value)
lye used = full lye × (1 − superfat)
water = lye × (1 − concentration) ÷ concentration
KOH sap = NaOH sap × 1.4028, then ÷ purity
Questions makers ask
How much lye do I need for 1 kg of oils?
There is no single answer, because it depends entirely on which oils. A standard beginner blend — 40% olive oil, 30% coconut oil (76°), 25% palm oil, 5% castor oil — takes 143 g of sodium hydroxide at a 5% superfat. The same kilo as pure coconut oil would take 174 g, and as pure olive oil 128 g. That spread is why you run every recipe through a calculator rather than reusing a figure.
Why can I not just use an average saponification value?
Because the values are too far apart for an average to be safe. Coconut oil needs about 36% more lye per gram than olive oil does. Average them and a coconut-heavy recipe comes out short of lye — which is merely soft soap — while an olive-heavy one comes out lye-heavy, which burns skin. This calculator saponifies each oil separately and adds the results up.
What superfat should I use?
5% for a normal cold process bar. That leaves 5% of your oils unsaponified, which conditions the bar and, just as importantly, absorbs ordinary measuring error. Below 3% there is no margin left for a scale reading a gram light or for an oil whose real saponification value runs a little high. Above 10% the bar turns soft and goes rancid sooner.
How much water goes with that lye?
At the standard 33% lye concentration, that example batch takes 290 g of water — which is 29% of the oil weight, not the 38% you will see in older recipe books. Water is the one figure soapers quote three incompatible ways, so the calculator shows all three at once and the water discount calculator converts between them.
Can I use this for liquid soap?
Yes — switch the lye to potassium hydroxide. KOH saponification values are the sodium ones multiplied by 1.4028, because a potassium atom is heavier than a sodium one, so liquid soap always needs more lye by weight: 223 g for that same example batch. Flake KOH is also sold at about 90% purity rather than pure, and the calculator divides by the purity you enter.
My percentages do not add up to 100. Is that a problem?
Not for the maths. The calculator treats your numbers as a ratio and gives each oil its share of the total oil weight you entered, so a blend adding to 96% still produces a correct recipe. It is flagged because it is usually a typo rather than a choice, and "Make it 100%" rescales the rows without changing their proportions.
Is it safe to round the lye weight?
Round down, never up, and only to the nearest tenth of a gram. Your superfat is the cushion that makes a small shortfall harmless; there is no equivalent cushion in the other direction. If your scale only reads whole grams, work in a larger batch size so a gram matters less.
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.