Wood · Sheets
How many sheets of plywood do I need?
Add up the area of the parts and divide by the usable area of one sheet: that is the minimum number of sheets, never the answer. The real number comes from laying the parts out with a saw cut between them and a trim round the sheet, and it is often one more. In the worked example below, 11 parts for a bookcase need at least 3 sheets of 2440 × 1220 mm by area and 4 once they are laid out; moving the back to its own thinner sheet brings the thick plywood down to 3.
From the cut list to the number of sheets
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Write the cut list
Every part as length × width × quantity, in millimetres, with the length along the grain. One line per different part.
Group by thickness: 18 mm parts and 6 mm parts are two different lists.
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Know the usable sheet
A sheet loses a trim on every edge, because factory edges are rarely straight or clean. With 10 mm off each edge, a 2440 × 1220 mm sheet leaves 2420 × 1200 mm to cut from.
A part as wide as the sheet does not come out of it once the edges are trimmed.
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Do the area sum, as a floor
Add up length × width × quantity for every part, with a saw cut on each, and divide by the usable area of a sheet. Round up. Fewer sheets than that is impossible, however the parts are placed.
It is a lower bound, not an estimate.
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Lay it out the way the saw cuts
A panel saw makes straight cuts right across the piece on the table, so the layout has to be cut in strips and then across. Each cut takes the width of the blade, 4 mm here. The optimiser tries many layouts and keeps the one with the fewest sheets.
Grain: parts marked with grain keep their length along the sheet and cannot turn, which can cost a sheet.
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Take thinner parts to their own sheet
Backs and drawer bottoms are usually thinner board. Leaving them in the thick list makes the cut list look worse and wastes good board.
Separate lists, separate sheets, each optimised on its own.
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Keep the offcuts worth keeping
What is left after cutting is waste or stock. The optimiser counts offcuts of at least 300 mm each way as worth keeping, so the next job can start from them.
An offcut with a label is material; one without is rubbish.
Worked example: a bookcase in plywood
The cut list, ready to paste into the optimiser (name, length, width, quantity):
side 1800 400 2
shelf 864 380 5
top 900 400 1
door 1795 447 2
back 1800 900 1
On the optimiser's starting settings: 2440 × 1220 mm sheets, 4 mm saw cut, 10 mm trim.
In the table, the minimum is the area lower bound, and the last column is how much of the usable sheet ends up as parts.
| List | Sheets | Minimum | Used |
|---|---|---|---|
| All of it in one thickness | 4 | 3 | 57.4 % |
| Without the back | 3 | 2 | 57.9 % |
| The back alone, in thinner board | 1 | 1 | 55.8 % |
For the whole list the optimiser reports 6.67 m² of parts in 11.91 m² of sheet.
All in one thickness, the area says 3 and the layout needs 4: the parts are rectangles and every cut runs right across the board, so the area can never be filled completely. Moving the back to a thinner sheet takes the thick plywood down to 3 sheets, plus 1 of the thin board.
The optimiser does not take a list from a link: paste the lines above into its cut list box. Remove the back line for the second result.
Questions
How do I work out how many sheets of plywood I need?
Write the cut list, divide the total area of the parts by the usable area of one sheet to get the minimum, then lay the parts out (by hand or with an optimiser) to get the real number. Add the saw cut between parts and the trim round the sheet, or the count comes out short.
Why does the layout need more sheets than the area says?
Because the parts are rectangles that have to be cut with straight, through cuts, and the strips they leave do not always fit what is left on the list. The area is a floor; the layout is the answer.
Does the grain direction change the number of sheets?
It can. A part marked with grain has to keep its length along the sheet's length, so the optimiser cannot turn it to fill a gap. On a plain board every part can turn.
What saw kerf should I allow?
The thickness of the blade's cut, which depends on the saw. The optimiser starts at 4 mm; set your own.