The mould holds batter, and you weigh oil
The loaf mould in the form is 10 inches by 3.5 inside, poured to 2.5 deep. That is 87.5 cubic inches, and at 16 grams per cubic inch it holds 1,400 grams of batter. The number you actually need is the oil weight, and it is not 1,400.
On the recipe in the form — alkali 14 percent of the oils, water 28.4, fragrance 3, everything else 2 — one gram of oil arrives in the mould as 1.474 grams of batter. So the mould wants 949.8 grams of oils, which turns up with 133 grams of alkali, 269.7 of water, 28.5 of fragrance and 19 of additions. Weigh out 1,400 grams of oil for that mould and you will be looking for somewhere to put another 664 grams of batter.
The density is the input nobody has
Everything above rests on grams of batter per cubic inch, and it is the one figure that has to be measured rather than looked up. Water is 16.39 grams per cubic inch. Most soap batter sits a little under that, and batter with air beaten into it sits well under. Fill the mould with water and weigh it to get the volume honestly, then pour one batch you have already weighed and see what the mould took. After that the number is yours and the oil weight is real rather than approximate.
Bars, and the ends
A 10 inch loaf sliced at 1 inch with a quarter inch trimmed off each end gives 9 bars from 9.5 inches of usable run, with half an inch left over that is not thick enough to be a tenth. Each bar is 140 grams the day it is cut, or just under 5 ounces, and 95 grams of that is oil — the rest is water, alkali and additions.
The arithmetic that surprises people is the waste line. The two end trims and the short piece come to 140 grams, exactly one bar in ten, and that is at a quarter inch a side. Trim half an inch instead and the loss doubles. It is still soap and most makers use it, but it is not a bar and it should not be in the count you price against.
Slab moulds and turning the bars round
A slab is a grid rather than a run, and the orientation matters more than it looks. The page tries the bars both ways round and reports whichever fits more, because a slab that takes four rows one way often takes five the other, and nobody works that out standing over the mould.
What this cannot tell you
It does not know your recipe. The percentages in the form come from your own lye calculation and your own decisions, and the page multiplies whatever you put there. It has no view on what the bar should weigh, on how thick to slice, or on whether anything about the result is suitable for anything. Cosmetic labelling and product rules vary by jurisdiction and are not on this page. And the alkali involved is caustic, which is named here and given no procedure.
Questions people ask
How much oil does my soap mould take?
Volume divided by the batter multiplier. The 10 by 3.5 by 2.5 inch loaf in the form is 87.5 cubic inches and holds 1,400 grams of batter, and on a recipe where alkali, water, fragrance and additions come to 47.4 percent of the oils, that is 949.8 grams of oils. The gap between the two numbers is the whole point of the page: water and alkali take up room in the mould.
How many bars does a loaf mould make?
Usable length divided by slice thickness, rounded down. Ten inches with a quarter inch off each end leaves 9.5, which is 9 bars at an inch with half an inch left over. The bar weight follows from its share of the loaf rather than from any assumption: 1,400 grams over 10 inches is 140 grams an inch.
What weight is soap batter per cubic inch?
Yours is yours and it is worth measuring. Water is 16.39 grams per cubic inch and soap batter is usually a little lighter, but a batter with air whipped into it can be much lighter and the difference goes straight into the oil weight. Pour one batch you have weighed, see what the mould took, divide.
Why does the page ask for the alkali as a percent rather than working it out?
Because the alkali depends on the saponification values for your oils and on your superfat, neither of which belongs in a mould calculator. Work them out on the lye and water calculator, divide the alkali by the oil weight, and put that percentage here. Keeping the two apart means neither page has to guess at the other.
Does the bar weight change after it is cut?
It falls, because most of the water leaves. On the recipe in the form the water is 269.7 grams out of 1,400, which is 19.3 percent of everything poured. How much of that has gone by the time you wrap the bar depends on your room and your cure, which is a measurement rather than a constant, so this page reports the cut weight and hands the cure to the water and bar weight calculator.
My mould is a strange shape. Can I still use this?
Yes — measure its volume by filling it with water and weighing that, and put the figure in with the unit you measured in. You lose the slicing geometry, since a measured volume carries no dimensions to cut along, but you keep the oil weight and you can still get a bar count by entering the bar weight you are aiming at.