How to nest parts on a 4x8 sheet for CNC cutting
When a CNC job uses sheet goods, the number of sheets is often the biggest material line on the quote. Nesting, meaning how the parts are arranged on each sheet, decides that number. A tighter nest can save a sheet on a small job and several on a big one. Here's how it works and what you can do in your design to help.
The sheet isn't as big as it says
A "4×8" sheet of plywood or MDF is 48 × 96 in. Metric sheets are commonly 1220 × 2440 mm, a little larger. Baltic birch often comes in 5×5 ft (60 × 60 in, or 1525 × 1525 mm) sheets instead. But not all of that area is usable:
- Edge margin. Makers leave a strip around the edge for screws, clamps or because factory edges aren't always straight or square. Half an inch to an inch is common.
- Gap between parts. A router bit removes a channel as wide as its diameter. Every part needs that gap beside it: a 1/4 in bit means 1/4 in between parts. On a plasma table it's the kerf, which is usually narrower.
- Defects. Plywood can have voids, patches or damaged corners that the maker will avoid.
The basic math
For identical rectangular parts in a simple grid:
Parts across = floor((sheet width − 2 × margin + gap) ÷ (part width + gap))
Do the same along the length, then multiply. The "+ gap" on the sheet side accounts for the last part not needing a gap after it.
Example: 12 × 18 in shelf parts on a 4×8 sheet, 1/4 in bit, 1/2 in margin.
- Usable width: 48 − 1 = 47 in. Across: floor(47.25 ÷ 12.25) = 3.
- Usable length: 96 − 1 = 95 in. Along: floor(95.25 ÷ 18.25) = 5.
- 15 parts per sheet.
Rotate the parts 90° and you get floor(47.25 ÷ 18.25) × floor(95.25 ÷ 12.25) = 2 × 7 = 14, so the first way round is better. Forty parts need three sheets either way.
The sheet nesting calculator runs this for you, tries both orientations, and works in inches or millimeters with 4×8, metric and 5×5 sheets.
When a small change saves a sheet
Grid nesting is unforgiving near the edges. In the example, a part 15.5 in wide fits three across (3 × 15.5 + 2 × 0.25 = 47 in, just inside 47 in usable). At 15.75 in wide, only two fit, and you've lost a third of the sheet.
If you have flexibility in your design, check how close your part dimensions are to a break point. Trimming a quarter inch off a non-critical dimension can sometimes add a whole row.
Grain direction
Plywood has a face grain, and for visible parts it matters. Cabinet doors, drawer fronts and tabletops usually need the grain running one way. That stops the maker from rotating parts freely to fit more on a sheet.
If grain direction doesn't matter for some parts (hidden backs, shelves that will be painted, MDF), say so. It gives the maker more freedom to nest tightly.
Mixed parts and true-shape nesting
Real projects rarely have identical rectangles. A bookshelf has sides, shelves, a back and a top. CAM software with a nesting feature (VCarve, Aspire, Fusion and dedicated nesting tools) arranges mixed shapes automatically, rotating and interlocking them to use as much of the sheet as possible. Curved and irregular parts can nest inside each other's cut-outs.
That's why a maker's sheet count may be lower than your hand calculation. It's also why sending a complete cut list in one job usually costs less than ordering parts piecemeal: more shapes give the nesting software more ways to fill gaps.
Leftovers
Every sheet leaves offcuts. Ask whether the maker will include your offcuts with the order (useful for test pieces and small projects), or whether small parts can be cut from offcuts left from a previous sheet, which some makers will reflect in the price.
Tabs, onion skins and small parts
Small parts on a big sheet need holding. Makers use tabs (little bridges left uncut) or an onion skin (a thin layer left at the bottom of the cut, trimmed afterwards). Very small parts may need extra spacing so tabs fit. If you have dozens of tiny parts, mention it; it affects both the nest and the finishing time.
Board feet and cost
Sheet goods are sold by the sheet, but if you're comparing with solid lumber the board-foot figure helps. A board foot is 12 × 12 × 1 in, so a 4×8 sheet of 3/4 in material is 48 × 96 × 0.75 ÷ 144 = 24 board feet nominal.
The quote will usually price material as sheets used, sometimes with a share of the last partly used sheet. Machine time depends on toolpath length, not sheet count, so a tight nest saves material but doesn't save much cutting time.
Measure the real thickness
If parts slot together, the actual sheet thickness matters more than the label. "3/4 in" plywood often measures a little thinner, and metric Baltic birch is 18 mm. Ask the maker to measure the sheet and adjust slot widths before cutting the batch.
What to send
- A cut list: part name, width, length, thickness, quantity, and grain direction where it matters.
- DXF or SVG files for any non-rectangular parts, at 1:1.
- Whether you want offcuts returned.
- Material and grade (e.g. 3/4 in Baltic birch, B/BB face), or "maker's choice."
- Finish, deadline, and pickup or delivery.
Then post the job and let makers nest it and quote. Posting is free, there's no buyer fee, and your payment is held until the parts arrive and you confirm delivery.