How many sheets do I need?
Dividing part area by sheet area gives a number that is always too low or exactly right, never too high. It tells you the least you could buy, not what the parts need once they are laid out with a saw blade between them.
The quick estimate
Add up the area of every part and divide by the area of one sheet. Three 600 mm base cabinets in 18 mm board are 21 parts and 7,804,128 mm² of board. One 2440 × 1220 mm sheet is 2,976,800 mm². The parts are 2.62 sheets of area, so the estimate rounds up to 3 sheets.
That number assumes the parts could be poured onto the board like liquid. Real parts are rectangles. They cannot overlap, cannot cross the edge of the sheet, and lose a blade width at every cut.
| Part | Width (mm) | Height (mm) | Qty |
|---|---|---|---|
| Side | 560 | 720 | 6 |
| Bottom | 564 | 560 | 3 |
| Top | 564 | 560 | 3 |
| Shelf | 564 | 540 | 3 |
| Back | 600 | 720 | 3 |
| Door | 596 | 716 | 3 |
What the layout needs
With a 3 mm kerf, 10 mm of trim on every edge and rotation allowed, CutGrid’s layout needs 4 sheets. 34.5% of the board you buy is left as offcuts, trim and sawdust. The largest offcut is 1853 × 1200 mm.
The gap between estimate and layout comes from shape, not area. The strips left between large parts are often too narrow for the next part, and the trim takes 10 mm from every edge before anything is placed.
Layout produced by CutGrid’s optimizer for this example. Coloured blocks are parts, dashed areas are offcuts, and the outer band is trim.
- Side
- Bottom
- Top
- Shelf
- Back
- Door
The floor no layout can beat
CutGrid also computes a lower bound for every job: the fewest sheets any layout could possibly use, from the area inside the trim and from parts too large to share a sheet. For this list the bound is 3 sheets.
The plan is above the bound. That does not prove a better layout exists, because the bound only counts area and area is optimistic. It does tell you the most a harder search could save.
When the trim makes the difference
Run the same list with trim at 0, for board with clean, square factory edges, and the layout needs 3 sheets at 12.6% waste. Ten millimetres per edge looks small on one sheet, but it shrinks every sheet you open and can decide whether the last row of parts fits.
Only set trim to 0 when the edges really are square and undamaged. A chipped edge on a door costs more than a sheet of board.
Layout produced by CutGrid’s optimizer for this example. Coloured blocks are parts, dashed areas are offcuts, and the outer band is trim.
- Side
- Bottom
- Top
- Shelf
- Back
- Door
Common mistakes
Ordering the rounded area estimate
The estimate is a floor. On this list it says 3 and the layout needs 4. Order from a layout, or add a sheet to the estimate on any job with tall or wide parts.
Forgetting the blade
Every cut turns about 3 mm of board into dust. Twenty cuts across a sheet are 60 mm, enough to push the last part off the edge.
Counting boxes instead of parts
A drawing shows a cabinet. The cut list has two sides, a top, a bottom, a shelf, a back and a door, each with its own size. Derive every size before you estimate.
Mixing thicknesses in one estimate
An 18 mm carcass and a 6 mm back come from different sheets. Estimate each material on its own; CutGrid lays out each material on its own stock.
Questions
- How many cabinets can I get from one sheet of plywood?
- It depends on the cabinet. Three of the 600 mm base cabinets on this page, full back included, are 2.62 sheets of area and needed 4 sheets in the layout.
- Should I buy an extra sheet?
- If all you have is the area estimate, yes. If you have a layout, buy what it shows and keep the largest offcut for the next job. On this list that offcut is 1853 × 1200 mm.
- Does a bigger sheet help?
- Often. The same three cabinets need fewer large sheets; the sheet sizes guide compares 2440 × 1220, 2800 × 2070 and 3660 × 1830 mm board.
- Is the lower bound the answer?
- No. It is the least any layout could use. When CutGrid’s plan equals it, the plan is proven minimal and the app says so. When it does not, the true minimum is somewhere between the bound and the plan.