Order one sheet too few and you lose a morning going back to the merchant. Order two too many on every job and it quietly eats your margin. The answer is a cutting list and a layout, not a guess.
Here's how to work out the sheets properly, and why simply dividing by the sheet's area always comes up short.
Start with a cutting list
List every part: what it is, its length and width, the board it's cut from and how many. Group the parts by board, because each board is its own order: 18 mm MDF for carcasses, 6 mm or 9 mm for backs, a thicker board for worktops, and so on.
Decide your construction before you list the parts. Whether the top and bottom sit between the sides or on top of them changes their length.
Why adding up the area isn't enough
Adding up the parts' area and dividing by a sheet's area (2440 × 1220 is about 2.98 m²) tells you the least you could possibly get away with. Real sheets don't divide that neatly: a part that's 2100 long leaves an offcut that's too short for anything else, and every cut turns a few millimetres into sawdust. In the worked example below, the parts come to barely more than two sheets by area, but the job needs 3 sheets.
Allow for the saw cut and the trims
Every cut loses the width of the blade's cut (the kerf), commonly around 2 to 3.5 mm for track saws and panel saws; check your own blade. Ten cuts across a sheet lose 30 mm or so, enough to stop the last part fitting. It's also worth trimming a few millimetres off a sheet's factory edges if they're damaged or you need a dead-square first cut.
Respect the grain
Plain MDF has no grain, so parts can be turned whichever way fits best. Veneered boards, melamine with a woodgrain and anything with a pattern can't: the grain usually has to run up a door or along a shelf, which limits how the parts fit on the sheet.
Lay the parts out
A simple way that works well on a panel saw or a track saw: rip the sheet into strips the width of your common parts (the carcass depth, the shelf depth), then cross-cut the strips to length. Cut the longest parts first, put parts of the same width together, and keep useful offcuts for plinths, fillers and face frames.
A worked example: a pair of alcove bookcases
Two bookcases, each 900 wide, 2100 high and 330 deep, in 18 mm MDF, with the top and bottom between the sides and four shelves 300 deep. Allowing a 3 mm saw cut, each bookcase needs:
- 2 sides at 2100 × 330
- A top and a bottom at 864 × 330 (900 less two 18 mm sides)
- 4 shelves at 864 × 300
- A sheet rips into three 330 strips (999 mm with the cuts), leaving a narrow strip of about 220 mm. Each 2440 strip gives one 2100 side, with a 337 offcut that's too short for anything else. Four sides take four strips.
- The two spare strips each give two 864 tops or bottoms (1734 mm with the cuts). That's the four. So far: 2 sheets.
- A third sheet rips into four 300 strips, and each gives two 864 shelves: the eight shelves.
Total: 3 sheets of 18 mm MDF, plus 6 mm for the backs. The parts add up to about 6 m², only just over two sheets' worth. Order by area and you'd be a sheet short.
Let the software do the layout
JoinerKit builds the cutting list straight from your drawing and lays the parts out on your sheets with the saw cut, trims and grain allowed for, so you see exactly how many sheets to buy and what they cost. Change a shelf and the layout changes with it.
Questions
What size is a sheet of MDF in the UK?
The standard full sheet is 2440 × 1220 mm (8 × 4 feet). Smaller 1830 × 610 and 1220 × 607 sheets and longer 3050 mm sheets are also sold, depending on the merchant and the thickness.
How much waste should I allow on sheet material?
There's no single figure: it depends on the parts. Rather than adding a percentage to the area, lay the parts out on the sheets, allowing for the saw cut and trims. That shows the real number, which is often more than an area-based estimate.
Does MDF have a grain direction?
Plain MDF doesn't, so parts can be turned to fit. Veneered MDF, melamine-faced boards with a woodgrain and patterned boards do, so their parts need to keep the grain running the right way.