How many sheets of plywood do I need?

The quick method, and why it is low

A standard sheet at 2800 by 2070 mm is 5.796 m². If your parts add up to 22 m², the quick method says just under four sheets, so you buy four. Four is almost never enough, and the reason is that a sheet holds less than its own area.

Four things eat the difference:

• Saw kerf. Every cut between two parts turns a strip of the panel into sawdust. Three or four millimetres per cut sounds like nothing until you count the cuts on a full sheet.
• Edge trim. Suppliers deliver sheets with a damaged or out of square edge. If you trim ten millimetres off each side of a 2800 by 2070 panel you have given away 1.7 % of it before placing a single part.
• Grain lock. A part that must run with the grain cannot be turned to fill an awkward gap. Every locked part removes an option from the optimizer.
• Geometry. Rectangles do not tile perfectly. A 700 mm part on a 2800 mm panel looks like it fits four times across, and once the kerf and the trim are honest about themselves it fits three.

A real plan, opened up

The plan on the home page is a small carcass: two sides at 1950 by 600, a top and a bottom at 700 by 600, three shelves at 900 by 600 and three fronts at 596 by 220. Ten parts, one sheet of 2800 by 2070 mm, a 3.2 mm blade and 10 mm of edge trim.

The parts add up to 5 193 360 mm², which is 89.6 % of the panel. Here is where the other 10.4 % went:

Five figures, adding to exactly 100 %. Sawgrid computes them in whole tenths of a millimetre and refuses to show a plan whose books do not close, because an optimizer that quietly loses area is an optimizer that quietly loses you a sheet.

The one part of this you can do in your head

If you want a figure before you open anything, take the parts area, divide by the sheet area, and add somewhere between fifteen and thirty five per cent for kerf, trim and layout, then round up. That gets you close enough to know whether you are ordering three sheets or seven.

What it will not do is tell you which of those two you actually need on a tight job. The difference between 5.4 sheets by area and the real answer of six or seven is decided entirely by how the rectangles happen to fall, and there is no shortcut for that except to place them.

What Sawgrid does with the same input

You give it the parts, the sheet size you can buy, your blade width and your edge trim. It runs 108 deterministic strategies, keeps the best result, and reports the panel count, a drawing per panel, the cut sequence and the balance above. Every requested copy is either placed or named as unplaced with a reason. Nothing is dropped in silence, so the count you take to the merchant is the count the job needs.