Hot process soap calculator
The same lye as cold process, sized for a batch that has to survive a cook — plus the stages to watch for, and the superfat, fragrance and liquid weights to stir in once the pot comes off the heat.
INPUTS
g
A slow cooker wants headroom — the batter puffs up alarmingly at the mashed-potato stage. Fill it about half full at the start.
Your oil blend
OilShare
%
%
%
%
100.0% total
Hot process is lye plus heat, and a slow cooker of cooking soap will burn far worse than cold batter. Long sleeves, goggles and gloves stay on for the whole cook, not just the mixing. Zap-test before you add anything at the end — if it stings the tongue it is not done, and no amount of extra cooking time on a chart overrides that test.
RESULTS
Sodium hydroxide
142.9 g
calculated at 5% superfat 5% in the finished bar
Water
367.4 g
28% solution 510.2g of lye solution
Into the pot
| 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 |
Stirred in once the cook is done and the pot is under 80°C (176°F)
| Addition | Rate | Weight |
|---|---|---|
| Fragrance oil | 3% | 30.0 g |
| Extra liquid | 4% | 40.0 g |
The cook, 45–90 minutes on low
| Stage | From trace | Looks like |
|---|---|---|
| Trace to first bubbles | 10–20 min | The batter thickens and the edges start to go translucent. |
| Mashed-potato stage | 20–35 min | It puffs up and looks curdled. Stir it down and keep going — this is normal. |
| Full gel | 35–60 min | Glossy, translucent, folds like vaseline. Zap-test before you add anything. |
Into the mould
1536.1 g
Mould space needed
1617 ml
Usable after about 7 days and properly good at 21 — the saponification finished in the pot, so the wait is only for water to leave the bar. That is the real reason to cook a batch rather than the 28-day cold process cure. The timings above tell you when to start checking; the zap test tells you when it is done.
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Soap Making Safety Checklist
How this is worked out
Hot process is cold process with the cure brought forward. You saponify the same oils with the same weight of lye, then cook the batter until the reaction has finished in the pot instead of finishing slowly in the mould. Two things follow from that. The batch has to start wetter, because the cook evaporates water and stiff batter will not mould — 28% lye concentration rather than the 33% you would use cold. And anything you want to survive intact goes in after the cook rather than at trace: fragrance, because heat drives it off, and your superfat oil if you want to choose which oil it is. Cook at 3%, then stir the difference in at the end.
lye = Σ (oil weight × sap) × (1 − superfat)
water = lye × (1 − 28%) ÷ 28%
after-cook superfat oil = oils × (target superfat − cook superfat)
fragrance = oils × fragrance %, stirred in under 80°C
Questions makers ask
Is the lye different for hot process?
No. Same oils, same saponification values, same superfat arithmetic — a hot process recipe and a cold process one made from the same blend call for exactly the same weight of sodium hydroxide. What changes is the water, when the fragrance goes in, and how long you wait to use the bar.
Why does hot process need more water?
Because the cook drives water off, and batter that has lost water will not spoon into a mould. On a 1 kg batch, 28% lye concentration gives 367 g of water against 290 g at the 33% cold process default — about 77 g more to spend on the cook.
How long does hot process soap take to cook?
45 to 90 minutes on low in a slow cooker, in three recognisable stages: it thickens and the edges go translucent, it puffs into something that looks like curdled mashed potato, and then it settles into a glossy translucent mass that folds like vaseline at around 35–60 minutes. Those timings tell you when to start looking. The zap test tells you when it is done.
What is the zap test?
Touch a cooled scrap of the batter to the tip of your tongue. Finished soap tastes of soap. Unfinished soap gives you a sharp electric sting, and if you feel it the cook is not over — keep going and test again. It is the only check that actually measures what you care about, which is whether there is free lye left.
When do I add the fragrance?
After the cook, once the pot has dropped below about 80°C (176°F). Add it at trace, the way you would in cold process, and the heat of the cook drives most of it straight back out again. The same goes for anything you added for its skin benefit rather than its colour.
Can I choose which oil ends up as the superfat?
Yes, and it is the one real advantage hot process has. Cook the batch at 3% and stir a weighed oil into the finished batter — on a 1 kg batch that is 20 g to reach 5% overall. That oil never meets live lye, so it goes into the bar intact. Cold process cannot do this: the lye reacts with whatever it meets, and the leftovers are whichever oils were slowest.
How long does hot process soap need to cure?
It is usable in about 7 days and properly good at 21, against 28 days for cold process. The saponification already finished in the pot, so the wait is only for water to leave the bar — and it is still worth waiting, because a bar that has dried out lasts far longer in the shower.
Why does my hot process batter look lumpy and rustic?
Because it is cooked soap being pushed into a mould rather than liquid being poured into one, and no amount of technique makes it pour like cold process. Extra liquid stirred in at the end — yogurt, milk, or sodium lactate solution — loosens it enough to settle. A degree of rustic texture is what hot process looks like.
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.