Soap recipe resizer

Take a recipe written for someone else’s mould and restate it at the size you want. Enter it as printed, pick a target — an oil weight, your mould, or a bar count — and every oil, the lye and the water come back resized.

INPUTS
How is your recipe written?
Type it in exactly as printed. The first thing this tool does is convert those weights into shares of the blend, which is what makes the recipe portable.
The recipe you have
OilWeight
g
g
g
g
454.0g of oils
The size you want
Scale it to
g
Recipe settings
%
%
% of oils
The lye below is recalculated from your oils, not scaled from the original recipe. That is deliberate — a scaled lye weight is only correct while the blend is untouched. Never carry a lye figure across from the recipe you started with.
RESULTS
Total oils, resized
1000.0 g
2.20× the original from 454.0g
Weigh out
OilShareWeight
Olive oil40.0%400.0 g
Coconut oil (76°)30.0%300.0 g
Palm oil25.0%250.0 g
Castor oil5.0%50.0 g
Sodium hydroxide
142.9 g
Water
290.1 g
Fragrance oil
30.0 g
Mould space needed
1540 ml
Batch weight
1462.9 g
Bars at 100 g
12
Percentages are what travel between batches, not weights. Once a recipe is written as shares of the blend it fits any mould you own — which is why every figure here starts by converting your weights back into shares.

How this is worked out

Resizing looks like multiplication and mostly is, but the order matters. First the source recipe is converted into shares of the blend — each oil divided by the total oil weight — because that is the form that travels. Then the new batch size is resolved: either you state an oil weight directly, or the calculator works one out from a mould's volume or from the bars you want, using a probe batch to learn how much batter a gram of oil actually becomes for your particular water discount. Finally each share is applied to the new total, and the lye is recalculated from those oils rather than scaled alongside them. Scaling a lye figure gives the right answer only while the blend is unchanged; recalculating gives the right answer always.

share = oil weight ÷ total oil weight
new weight = share × new total oil weight
batter/oil = (oils + lye + water + fragrance) ÷ oils
oils for a mould = mould ml × 0.95 ÷ batter/oil
lye = recalculated from the new oils, never scaled

Questions makers ask

How do I resize a soap recipe?
Convert the oil weights into percentages of the blend, then apply those percentages to your new total oil weight. A recipe sized for a 10 in silicone loaf uses about 1304 g of oils; the same recipe in a 12 in wooden loaf needs about 1564 g, a factor of 1.20.
Can I just multiply the lye by the same factor?
In this one case the arithmetic happens to agree — 186.2 g scaled by 1.20 lands on 223.5 g. But it only agrees while the blend is untouched, and it silently stops agreeing the moment you swap an oil. This calculator recalculates the lye from your oils every time, which is right in both cases.
How much oil does my mould take?
Work out its internal volume and divide by roughly 1.46 — a gram of oil becomes about that much batter once lye, water and fragrance are in. A 10 × 3.5 × 3.5 inch loaf holds about 2007 ml, which is roughly 1304 g of oils.
Why does the batter ratio depend on my recipe?
Because water is a choice. A batch at a heavy water discount makes noticeably less batter per gram of oil than a full-water one, so the same mould takes a different amount of oil. That is why this tool asks for your concentration rather than assuming one.
Should I scale the fragrance too?
Yes, and it happens automatically here because fragrance is quoted as a percentage of the oils. If your source recipe gave fragrance as a fixed weight, convert it to a percentage of that recipe’s oils first, then enter that.
My recipe is in ounces. Can I still use this?
Enter the ounce figures in the weight fields — the conversion to percentages does not care what unit you used, as long as you use the same one for every oil. Switch the results panel to ounces and you get the resized recipe back in ounces too.
Why store recipes as percentages at all?
Because a percentage recipe fits any mould you will ever own, and a weight recipe fits exactly one. It is also how nearly every published recipe and every soaping forum writes them, which makes them comparable at a glance.

Read before your next batch

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