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Calculators

Melt and pour

No lye to handle: the soap is already made. What is left is arithmetic that goes wrong in small ways, so this works out how much base to melt for the mould you have, with the fragrance counted as a share of the finished soap, the way IFRA counts it.

How much
Fill it with water and weigh the water: a gram is a millilitre. For a mould with several cavities, all of them together.
Most glycerin bases weigh 1.03 to 1.10 g per mL. Weigh one filled cavity of your own mould to know yours.
What goes in

Each as a percentage of the finished soap.

What stays on the jug and the spoon. 5 % is usual for a small pour.

fill in the mould or the bars you want

Calculated at soapformulator.com, every value sourced. Check the figures against your own scale.

The formula, and where it comes from 
finished  = mould volume × base density     (or bars × grams per bar)
fragrance = finished × fragrance %            # % of the finished soap, as IFRA counts it
base      = finished − fragrance − additions
melt      = base × (1 + extra %)

rebatch:
liquid          = grated soap × liquid %
added superfat  = stirred-in oil ÷ (soap × oil share) × 100
  • IFRA Standards express every limit as a percentage of the finished product; a supplier Certificate of Conformity states the maximum for category 9 (rinse-off soap) the same way.
  • Glycerin melt-and-pour bases weigh a little more than water, about 1.03–1.10 g/mL depending on the base. The default of 1.06 g/mL is the middle of that. — Weigh a filled cavity of your own mould once and use that: it replaces this and the cavity volume in one go.
  • Working convention for melt-and-pour: fragrance around 1–3 % of the soap, added oils and butters no more than about 1 tablespoon per pound (≈ 3 %), and dry additions such as clay or exfoliants a few percent. Past these a base sweats, softens, loses lather or does not set. — A habit of the craft rather than a measured threshold; the base maker’s own data sheet comes first.
  • Working convention for rebatching: liquid of about 10–25 % of the grated soap’s weight, the less the fresher the soap. Too little and it scorches and will not come together; too much and it stays soft for weeks. — A habit of the craft rather than a measured threshold.
  • Oil stirred into finished soap is not saponified: there is no free lye left to react with it, so every gram adds to the bar’s superfat, as a percentage of the oils in the original recipe.

Melt and pour

Melt the base gently, in short bursts in the microwave or in a double boiler, and keep it under about 70 °C: hotter and it scorches, turns yellow and loses the water that keeps it clear. Stir the fragrance and additions in as it cools, spritz the surface with alcohol to break the bubbles, and pour. Wrap the bars once they are hard: the glycerin in the base draws water from the air, and that is the sweat on an unwrapped bar.

Rebatch

Grate the soap, add the liquid, and heat it gently with a lid on until it turns to a thick, translucent mash. Take it off the heat, stir in the oil and the fragrance, and press it into the mould. It will not pour smooth like fresh soap. Give it a few days to firm up before you unmould it and a couple of weeks before you use it.

Common questions

Why is the fragrance a percentage of the finished soap?
Because that is how IFRA states its limits and how your supplier’s certificate reads: the most of the fragrance allowed in the finished product. A percentage of the base gives a little more for the same number, which is why the page shows both.
Why do my melt-and-pour bars sweat?
The glycerin in the base pulls water from the air and it beads on the surface. It is harmless. Wrap the bars in cling film as soon as they are hard, store them somewhere dry, and keep the fragrance and added oils within what the base can hold.
Does rebatching change the superfat?
Yes, if you stir in oil. There is no lye left in cured soap to turn it into soap, so every gram stays as free oil and adds to the superfat the recipe already had. The page says how much.

Goes well with

All tools
Fragrance & allergen check IFRA load by product type and EU/Canada allergen declarations from supplier sheets
Label maker A soap or cosmetic label with every particular the EU, Great Britain, the US, Canada or Australia asks for, checked against the rule

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