Three things eat the bun before the parts do
Skin, kerf and remainder. They come off in that order and only the last one is obvious.
The skin comes off every face, so a half inch of trim removes an inch from each of the three dimensions. On the 24 by 60 by 24 in bun in the form that is 20 cubic feet down to 18.1 — nine per cent gone before a blade touches anything useful. The trim is a field rather than a constant because it depends on what the skin is like on the grade you bought and on whether the finished part can live with it. Some work is happy with skin on one face.
The kerf comes off between every pair of blocks. Eight thousandths does not sound like much until you notice that fourteen blocks in a row need thirteen cuts, and the kerf can be exactly what stops the fourteenth fitting. The page prints the yield with and without the kerf so you can see whether it cost you a block or nothing at all.
The remainder is whatever is left over at the end of each run, and it is set by how you turn the block.
Turning the block
The defaults cut a 22 by 20 by 4 in cushion block out of that bun. Stand the blocks up the way the numbers are written — the 22 in width across the bun, the 20 in length along it, the 4 in thickness stacked up the height — and you get one across, two along and five up: ten blocks. Turn them so the 4 in thickness runs down the 59 in length of the trimmed bun instead, and fourteen fit in that direction alone. Same bun, same blocks, forty per cent more parts.
The reason is remainder. Twenty inches into 59 leaves 19 in doing nothing; four and an eighth inches into 59.125 leaves a quarter of an inch. It is tempting to turn that into a rule — put the smallest block dimension down the longest run of the bun — and the rule is wrong. Swept across a few thousand bun and block combinations, the winning orientation puts the smallest dimension down the bun length only about one time in seven. What wins is whichever of the three dimensions happens to divide that run with the least left over, and which one that is has nothing to do with its size. Orientation changed the yield on roughly seven of every ten combinations tried, so it is worth checking, and the only way to check it is to try all six. That is what the page does, and it prints what the obvious orientation would have given so you can see whether it mattered.
What actually reaches the parts
On the defaults, fourteen blocks is 14.3 cubic feet of finished foam out of a 20 cubic foot bun. Seventy-one per cent. The other twenty-nine per cent is skin, kerf and offcut, and it is worth knowing that number before you compare a quoted price per cubic foot against what a cut part costs you. A bun at three dollars a cubic foot is a bit over four dollars a usable cubic foot by the time it is a block.
The offcut is not necessarily rubbish. Foam scrap has a market, it laminates into thicker stock, and it shreds into fill. None of that is in this calculation, which counts only what comes out as a whole block.
Weight, density and what neither one tells you
Density converts the cube into pounds and that is all it does here. At 1.8 pounds per cubic foot the 22 by 20 by 4 block weighs 1.83 lb and twenty of them weigh 37 lb, which matters for shipping and for whether one person can carry a finished piece.
Density is not firmness, and neither one is a grade. A soft high-density foam and a firm low-density foam both exist and they behave differently over years of sitting. The figure on the form is whatever the supplier spec sheet says for the exact grade you are buying, and this page has no opinion about whether it is the right one for a seat, a mattress, a knee pad or anything else. That question belongs with the supplier and with whatever the finished piece has to satisfy.
Freight is charged on the air
Foam is the classic dimensional weight freight problem: two buns are 40 cubic feet and 72 pounds. Carriers price that on the cube, which is why the form takes a freight rate per cubic foot rather than per pound, and why ordering three buns when you needed two and a bit is more expensive than the foam price alone suggests. If the spare count comes out high, it is worth checking whether a different bun size or a slightly different block dimension drops you a bun.
Questions people ask
How many cushions come out of a foam bun?
For the bun and block in the form — a 24 by 60 by 24 in bun with a half inch of skin off each face, cutting 22 by 20 by 4 in blocks with an eighth inch kerf — fourteen come out, laid with the 4 in thickness running down the length of the bun. The same blocks laid the obvious way round give ten. The orientation is worth more than any other single decision here.
How much of a foam bun is wasted?
On the defaults, about twenty-nine per cent never becomes a block: nine per cent goes with the skin trim and the rest is kerf and the remainder at the ends of each run. That is why a price per cubic foot of bun and a cost per cubic foot of finished part are different numbers, and the page prints both so the gap is visible.
Does the saw kerf really matter on foam?
It matters exactly when it costs you a whole block, and never otherwise. The page prints the yield with and without it. An eighth of an inch across thirteen cuts is 1.6 in, which is nothing against a 59 in run and everything if the last block only had an inch to spare. Measure yours on a scrap rather than guessing, because a band knife and a bandsaw are not close.
What does a piece of foam weigh?
Volume in cubic feet times the density in pounds per cubic foot. A 22 by 20 by 4 in block is 1.02 cubic feet, so at 1.8 lb per cubic foot it weighs 1.83 lb. The density comes off the supplier spec sheet for the grade you are actually buying, and it is the only input the weight depends on.
Is density the same as firmness?
No. Density is mass per unit volume and firmness is how hard the foam pushes back, and foams exist at every combination of the two. This page uses density only to work out weight and freight cube, and it takes no position on what density or firmness suits any particular use. That is a question for the supplier and for whatever requirements the finished piece has to meet.