Soap mold calculator

Your mold's volume, the oils it takes, and the bars it makes once they have cured — or the reverse, if you know how many bars you need. The water that leaves over the cure is taken off rather than quietly ignored.

INPUTS
Which way round?
Fill this mould and see how many bars come out of it once they have cured.
g
3.5 oz — the label weight, after the cure.
Your mould
Mould
10 × 3.5 × 3.5 in. Every volume here is worked out from those dimensions rather than quoted — moulds sold under the same name vary, so measure yours if it matters.
Leave headroom. A mould filled to the brim has nowhere to go if the batch gels and rises, and a loaf that overflows is a caustic mess to clean up. Pour to about an inch below the top of a wooden mould, and never calculate to a depth you cannot actually reach.
RESULTS
Bars at 100 g
16
2007 ml of mould 1303.6g of oils
Where the weight goes
StageWeightPer bar
Oils1303.6 g81.5 g
Lye and water564.3 g35.3 g
Batter in the mould1907.0 g119.2 g
Water lost drying−228.8 g12%
Cured soap1678.2 g100 g
Mould space needed
2007 ml
Mould capacity
2007 ml
A 100 g bar starts life as about 119.2g of batter. The difference is water, and it leaves over the 28-day cure — which is why a batch weighed on the day it is poured always looks like it will make more bars than it does.

How this is worked out

Yield is a chain, and the link people drop is the cure. A mould has a volume; filled with batter at about 0.95 g per millilitre, that gives a batter weight. Working backwards from batter to oils needs one more piece of information — how much batter a gram of oil actually becomes — and that depends on your water discount, so the calculator measures it from a probe batch of your own recipe rather than assuming a figure. Then the cure: roughly 12% of the batch leaves as water over the following weeks, so cured weight is batter less that, and the bar count is cured weight divided by your finished bar weight. Run the whole chain in reverse and it answers the more useful question — how much oil to buy for the bars you actually need.

mould ml = length × width × pour depth (× 16.3871 if in inches)
batter = mould ml × 0.95
oils = batter ÷ ((oils + lye + water + fragrance) ÷ oils)
cured = batter × (1 − 12%)
bars = cured ÷ finished bar weight

Questions makers ask

How many bars of soap does one batch make?
It depends on the mould, not the recipe. A 10 in silicone loaf holds about 2007 ml, which takes roughly 1304 g of oils and makes about 1907 g of batter. After the cure that is 1678 g of soap — 16 bars at 100 g, not the 19 the wet weight suggests.
Why does the cure change the bar count?
Because about 12% of a batch is water that leaves over the 28-day cure. A 100 g bar starts life as roughly 119 g of batter in the mould. Count bars off the wet weight and you will consistently be one or two short.
How much oil do I need for 12 bars?
About 932 g of oils, for 12 bars at 100 g. Work back through the chain: 1200 g of cured soap needs 1364 g of batter, and that batter comes from roughly 932 g of oil once lye, water and fragrance are added — about 1435 ml of mould.
How do I work out my soap mold volume?
Multiply the internal length, width and the depth you actually pour to. In inches, multiply the result by 16.3871 to get millilitres. Use the pour depth rather than the full height of the mould — a loaf filled to the brim has nowhere to go if the batch rises.
Does my recipe change how many bars I get?
Through the water, yes. A heavily discounted batch makes less batter per gram of oil than a full-water one, so the same oil weight fills less of the mould. The oils themselves barely matter — it is the water you chose that moves the yield.
What is a normal bar weight?
Between about 90 and 130 g for a handmade bar, with 100 g and 4 oz (113 g) the two most common labelled weights. Cut thicker bars and you get fewer of them from the same loaf, which is worth knowing before you price them.
Should I fill the soap mold to the top?
No. Leave headroom — a batch that gels can rise noticeably, and a loaf overflowing with caustic batter is a genuinely unpleasant thing to clean up. Calculate to a pour depth you are comfortable with rather than to the mould’s full height.

Read before your next batch

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