How many bars you will get

Arithmetic that runs in two directions: how many bars from a batch, and what batch to make for an order.

Start from the mold

A loaf mold's internal volume in cubic centimeters, times roughly 0.9 grams per cubic centimeter for wet soap batter, gives the batter weight the mold holds.

Worked: a mold 25 cm long, 8 cm wide, filled to 7 cm deep is 1,400 cm³, which holds about 1,260 g of batter.

Measure the internal dimensions, not the external ones, and measure the fill depth you actually use rather than the mold's full height.

From oil weight to loaf weight

Batter weight is oils plus lye plus water, so oils are roughly 60 to 65 percent of the batter for a common water ratio.

So 1,260 g of batter is about 780 g of oils, with the lye and water making up the rest.

Turn that around for the other direction: oils times about 1.6 gives the batter weight, which tells you whether a batch fits the mold.

From loaf to bars

The loaf loses water during cure, so the cured loaf is lighter than the batter by roughly the water you put in less what stays bound.

Cured loaf weight divided by target bar weight gives bars.

Worked: 1,260 g of batter curing to about 1,030 g, cut at 120 g, gives 8 bars with a little over for the kerf and the ends.

Use your own cure log figure rather than this ratio, because the water ratio you use decides it.

Sellable bars

Subtract the ends, which are usually light and frequently sold as seconds.

Subtract cosmetic rejects, which for a new maker runs higher than expected.

The result is the number that goes into pricing, and it is the honest divisor for material cost per bar.

Track cut bars and sellable bars separately in the batch record, because the gap between them is a process measure worth watching.

References