Three materials, three ways of buying
A cushion is foam, a batting wrap and a cover, and each is bought in a different unit. Foam comes as a rectangular sheet of a fixed thickness, so what you actually pay for is the sheet, and the useful question is how many cushions you can nest into one. Batting comes off a roll of fixed width, so what you pay for is length. Cover fabric is the same, except that the pieces are shaped and the boxing strip runs the other way from the panels.
The volume figure people quote for foam is the board foot: 144 cubic inches, which is a piece one foot square and one inch thick. A 22 by 20 by 4 inch cushion is 1,760 cubic inches, or 12.2 board feet. It is a useful figure for comparing prices between suppliers who quote per board foot, and completely useless for working out how many sheets to buy, which depends on nesting. Both are shown above for that reason.
Why the batting wrap is there at all
A bare foam core in a fitted cover looks like a block. The wrap of bonded polyester batting, often called a dacron wrap in the trade, does three jobs: it rounds the hard top edges so the cushion reads as upholstery rather than as a foam block, it fills the cover so the fabric is under slight tension and does not wrinkle, and it lets the cover slide on without gripping the foam, which is otherwise like pushing a rubber block into a pillowcase.
A full wrap covers the top, bottom and all four sides, and the two ends are butted or lightly glued. Some makers wrap only the top and bottom faces and leave the boxing bare, which uses less material and gives a squarer edge. Neither is wrong. The wrap thickness also affects the cover fit: an ounce or two of batting adds enough bulk that a cover cut exactly to the foam dimensions ends up tight, which is generally what you want, but it is worth knowing that it is deliberate rather than an error in the cut list.
The cover cut list
A boxed cushion cover is two panels and a boxing strip. Each panel is the cushion face plus a seam allowance on all four sides. The boxing is a long strip the height of the foam plus two seam allowances, run around the perimeter with a little overlap at the join. If it has a zipper, the back section of the boxing is normally made as a separate two-part panel that is wider than the plain boxing so the zipper tape has somewhere to live, which is why the calculator adds a second boxing run when the zipper box is ticked.
Upholstery seam allowances tend to run half an inch rather than the quarter inch used in quilting, because the fabric is heavier, the seams take load, and welt cord is often sewn into them. That is a shop convention rather than a rule, and if you work at three eighths, change the field. The one thing not to do is mix conventions between the panels and the boxing on the same cushion.
What a volume calculation cannot tell you
Foam is specified by density and by firmness, and they are separate properties. Density is how much material is in a given volume and it predicts how long the foam lasts; firmness, usually given as an indentation load figure, is how hard it feels when you sit on it. A low density foam can be made to feel firm and will still collapse in a year of daily use. A high density foam can be soft and still recover for a decade. Seat cushions, back cushions and mattress use all call for different combinations, and none of them fall out of the cubic inches.
So use this page for how much to buy and where it comes from on the sheet, and take the specification itself from a supplier who will tell you both numbers for the foam they are selling. If they will only quote a thickness and a price, you cannot compare it with anything.
Health note, once
Cutting foam releases fine particulate, and the contact and spray adhesives used to bond foam and batting release solvent vapour that is heavier than air and collects in a closed room. Do the cutting and the gluing outside, or with real cross-ventilation and not just an open door. Read the adhesive label; that is where the specific handling requirements for the product you bought are written, and they differ between water-based and solvent-based formulations.
Once the foam is sized, the cover pieces are just rectangles, and if you want to check the yardage against a different fabric width the fabric yardage calculator will lay them out for you. Cut sizes with seam allowances and ease come from the seam allowance and cut size calculator. If the cushion edge takes welt cord cut on the bias, the bias binding calculator works out the square you need.
Questions people ask
How thick should upholstery foam be for a seat?
Common practice for a dining or bench seat is 2 to 3 inches, and for a sofa or lounge seat 4 to 6 inches, but thickness alone does not determine whether it is comfortable or durable. Density does. A 5 inch cushion in a low density foam bottoms out faster than a 3 inch cushion in a high density one. Ask the supplier for the density and the firmness rating of the specific foam, and if they cannot provide either, treat the price as uninformative rather than as a bargain.
How do I calculate board feet of foam?
Multiply width by depth by thickness in inches and divide by 144. A 24 by 24 by 4 inch cushion is 2,304 cubic inches, which is 16 board feet. Suppliers who quote per board foot are pricing the material; suppliers who sell sheets are pricing the sheet whether you use all of it or not. The two only line up if your cushions nest into the sheet without much offcut, which is what the sheet count above is checking.
Do I need batting over the foam?
For anything that should look like furniture rather than like a foam block, yes. The wrap softens the top edge, fills out the cover so the fabric sits smooth, and makes it physically possible to slide the cover on. You can skip it on a hard-edged outdoor cushion or a tight modern look where the square edge is the point. If you do skip it, cut the cover panels tighter, because a cover sized to include a wrap will wrinkle over bare foam.
How much fabric for a bench cushion cover?
It depends on the fabric width and whether the pattern has to match, which is what the calculator handles, but as an illustration a single 22 by 20 by 4 inch cushion with a zippered boxing takes roughly a yard and a quarter of 54 inch goods with normal slack. Two of them do not take twice that, because the panels start sharing rows across the width. Always calculate the whole set at once rather than calculating one and multiplying.
Can I cut foam with a kitchen knife?
You can, and the result is usually a wavy edge that shows through the cover. Foam is cut cleanly by a long thin blade moving fast: an electric carving knife is the common home shop answer, a bandsaw with a fine blade works if you can support the block, and a serrated foam saw is the purpose-made tool. Mark the line on both faces, keep the blade vertical, and let the blade do the cutting rather than compressing the foam ahead of it. Do it where the dust can disperse.