Three ways to end up with a bar of soap, usually presented as three equivalent hobbies to pick between. They aren't equivalent, and the useful way to see them is this: two of the three are the same craft, and the third is a different one wearing the same word.
What's actually different
| Cold process | Hot process | Melt and pour | |
|---|---|---|---|
| Do you handle lye? | Yes — every batch. | Yes — every batch. | No. The base arrived already saponified. |
| Usable in | 4–6 weeks. | Days. Safe out of the pot; better after two or three weeks. | Hours. |
| Control over the formula | Total. Every oil, the superfat, the water. | Total — the same decisions as cold process. | None. The oils were chosen for you. |
| What it looks like | Smooth, and the only method that does fine swirls and sharp layers. | Rustic. Thick batter fills a mold like mashed potato. | Very clean edges, transparent bases, glitter that shows. |
| Fragrance | Can accelerate, rice or seize. Test everything. | Added after the cook, so nothing to seize — but heat drives off delicate notes. | Behaves. Limited by the base maker’s maximum. |
| Equipment | Scale, stick blender, mold, PPE. | The same, plus a slow cooker. | A knife, a microwave and a mold. |
| Hardest part | Getting the numbers right, and everything the fragrance might do. | Judging the cook, and getting thick batter into a mold neatly. | Sweating in humidity, and additives sinking. |
"Usable in" is when the bar is fit to hand to someone. All three keep improving afterward for the reasons in the cure guide.
Hot process is cold process, cooked
The pair are treated as separate disciplines with separate recipes, and they are not. Both dissolve lye in water, both mix it into oils, both saponify. The only difference is that hot process applies outside heat — a slow cooker, usually — so the reaction runs to completion in an hour or two in the pot instead of over a day or two in the mold.
Our classic bar needs 143.8 g of sodium hydroxide and 292 g of water per kilo of oils at a 33% solution. That is true whether you pour it into a mold or cook it in a slow cooker — same fats, same saponification value, same superfat, same hardness and lather numbers. Every lye calculation on this site applies to hot process unchanged, which is worth saying because the split in how the two are taught implies otherwise. What genuinely differs is the working experience: you lose the swirls and gain a batch that is finished when the lid comes off.
That has one practical consequence worth knowing before you need it. Because hot process adds the fragrance after saponification, there is no reaction left for a difficult fragrance to accelerate — which makes it the answer to a scent that seizes every time in cold process, and the rescue for a batch that has already seized. A slow cooker is a cheap insurance policy for a cold process soapmaker.
Melt and pour is a supply chain, not a method
Melt and pour starts with a base that a factory already made: the saponification happened elsewhere, months ago, to someone else's recipe. You melt it, color and scent it, and pour. That is a real and legitimate craft — it is where the neat layers, the transparent bars and the embedded shapes come from, and it is genuinely safe enough to do with children.
But the decisions this site exists to help with have already been taken. The clearest way to show that is our own tool list:
| Tool | Cold process | Hot process | Melt and pour |
|---|---|---|---|
| Recipe Builder | ✓ | ✓ | — |
| Lye Calculator | ✓ | ✓ | — |
| Mold Volume | ✓ | ✓ | ✓ |
| Batch Resizer | ✓ | ✓ | ~ |
| Lye Concentration Converter | ✓ | ✓ | — |
| Superfat Adjuster | ✓ | ✓ | — |
| Fragrance Load — Soap | ✓ | ✓ | ✓ |
| Liquid Soap Calculator | ~ | ✓ | — |
| Dual-Lye Calculator | ✓ | ✓ | — |
| Masterbatch Lye | ✓ | ✓ | — |
| Essential Oil Blend | ✓ | ✓ | ✓ |
| Citric Acid & Vinegar Lye | ✓ | ✓ | — |
| Soap Cost & Pricing | ✓ | ✓ | ✓ |
| Bar Count & Cut | ✓ | ✓ | ✓ |
Rendered from this site's tool registry, so it stays true as tools are added. ✓ applies · ~ partly · — nothing to calculate.
6 of 14 tools have anything to say about melt and pour, and the ones that do are about molds, money and scent — not about soap. Superfat, water discount, oil selection, lye: all fixed before the base reached you. If what draws you to soapmaking is deciding what goes in the bar, melt and pour will feel like decorating someone else's cake. If what draws you is the decorating, it is the only method that does it properly.
What a bar actually costs
The comparison nobody runs. Melt and pour is cheaper to start — no scale worth the name, no goggles, no stick blender — and that is how it is usually sold. Per bar it is the expensive one, permanently:
| Per 100 g of finished soap | Cold or hot process | Melt and pour |
|---|---|---|
| Oils, or the base | $0.54 | $1.20 |
| Lye | $0.12 | — |
| Fragrance at 3% | $0.19 | $0.24 |
| Ingredients | $0.85 | $1.44 |
Computed by this site's engine from the classic bar at 1,000 g of oils, using the same illustrative bulk prices as the cost calculator's worked example — olive $8.00, coconut $6.00, palm $5.00, castor $12.00, lye $10.00 and fragrance $80.00 per kg, with melt and pour base at $12.00. Cold process is costed against its cured weight (1,247 g from a 1,466 g batch), because that is what you sell; melt and pour loses nothing. Prices vary enormously — put your own into the tool.
$0.85 against $1.44 per 100 g of finished soap on these prices. The gap is not a markup on convenience so much as arithmetic: with cold process you buy the fats, and the water — nearly a third of the batch by weight — costs nothing and evaporates. With melt and pour you buy the finished soap, glycerin, solvents and all, by the kilo, from someone who has already paid for the fats and the labor. Cheap to begin and expensive to scale, which is the opposite of how it is usually pitched to people thinking about selling.
And rebatch, the fourth one
Grate a finished cold process bar, melt it with a splash of liquid, and remold it. Rebatch has no lye handling, no calculation, and the same "you can't change the recipe" limit as melt and pour, because the soap is already soap. It exists for two reasons: to rescue a batch that went wrong — a lye-heavy loaf that needs oil stirred through it, or a wet one that needs water driven off — and to make something rustic on purpose. It is not a starting method, but it is the reason a failed cold process batch is rarely a wasted one.
So which should you do?
- Start with melt and pour if you want soap this weekend, you're making it with children, or the appeal is the design rather than the formulation.
- Start with cold process if you want to choose what's in the bar, you intend to sell eventually, or the chemistry is part of the draw. The first-batch walkthrough is written for exactly this.
- Add hot process when a fragrance defeats you, a batch seizes, or you want to use a bar next week. It is not a separate skill to learn — it is the same recipe with a slow cooker.
- You do not have to choose once. Plenty of makers sell cold process bars and melt and pour novelties from the same table, for the same reason a bakery sells bread and cake.
The short version
- CP and HP are one craft. Same lye, same water, same numbers — 143.8 g and 292 g per kilo either way.
- MP is a bought formula. Only 6 of our 14 tools have anything to say about it.
- MP costs ~70% more per bar and is far cheaper to start.
- Cure is the real time cost: hours, days, or four to six weeks.
- A slow cooker is cheap insurance for anyone doing cold process.