Why dividing part area by sheet area underbuys plywood

Dividing total part area by sheet area gives the fewest sheets the job could ever use, and the real count usually sits above it. The arithmetic treats board as if it could pour into every corner. Parts are rigid rectangles. They leave strips and offcuts too narrow for any remaining part, and the blade and edge trim take board the sum never counts. If that figure decides how many plywood sheets you order, the last cabinets have no board to come from.

Area division gives a floor, not an order quantity

Picture a sheet with three carcass sides already placed. A 300 mm strip is left along one edge, and every part still on the list is wider than 300 mm in both directions. The strip counts as usable area in the division, but nothing fits in it. Most sheets in a job end with some version of that strip. The division adds those strips together into a sheet that does not exist, and the count comes out low.

The distance between the area figure and the real count depends on the part mix. A job of many small, similar parts packs tight, and plywood yield lands close to the area figure. A job with a few large panels leaves bigger strips that only small parts can fill. If the list has no small parts, those strips become offcuts. Look at the largest parts on your list before trusting any estimate built from square metres.

On a panel saw, each cut runs straight across the piece. That rule means an L-shaped space left between parts cannot be shared freely; each cut decides which side the remaining board belongs to. Standard (Shelf) and Advanced (Guillotine) placement keep that rule, so their layouts are ones the saw can follow. The price of a sawable layout is some board the area figure assumes is free.

Kerf and trim eat board the area estimate never sees

In CutGrid, kerf is the gap left between parts for the blade, set once for the whole job. Every cut turns that width of plywood into dust. A sheet cut into twenty parts has many cut lines across it, and each line removes a strip as long as the cut and as wide as the blade.

Trim is the margin taken off the stock edge before parts are placed. Factory edges on plywood are often chipped, out of square, or dented from handling, so many shops take a few millimetres off each side. That margin comes off every sheet, and it shrinks the usable sheet before the first part goes down. The area division uses the nominal sheet size, so it counts board you will trim off and discard.

The two losses stack. A part that would fit a sheet edge to edge can fail once trim narrows the sheet and kerf separates it from its neighbour. This is where an estimate tips over by a whole sheet: the last row of parts misses by a few millimetres and moves to a new board. To check your job, set the real blade width and your usual trim, then compare the planned sheet count with the area figure.

A lower bound proves a minimum, not a better layout

A lower bound is the fewest sheets any layout could use for the job. Area division is the simplest version of one. Read the planned sheet count against it. When the two numbers match, no layout can use fewer sheets, so there is no tighter plan to wait for.

When the plan sits above the bound, the gap tells you less. The layout is a fast placement, not an exhaustive search, so a better one might exist. The bound might also be too low, because it cannot see every strip and kerf loss the real cut forces. A plan one sheet above the bound is not proof that you overbought.

Use the bound when someone quotes a smaller number. If a supplier or colleague says the job fits in fewer sheets, ask whether their figure sits at or above the lower bound. Below the bound, it is wrong. Between the bound and the plan, it might be right, and their layout with the same kerf and trim will show it. Sizes, kerf, and trim in the plan are applied exactly and parts never overlap, so the plan is a real cut.

Deciding how many plywood sheets to order from the layout

Before the planned count becomes an order, check the constraints the area figure ignored. Wood parts with a grain direction are not rotated, so a cabinet door that must run vertically cannot turn sideways to fill a strip. Turning rotation off for the job, or marking grain on the visible parts, can add board. CutGrid keeps marked defect areas on a sheet clear of parts, which moves pieces and can push the count up again.

Offcuts work the other way. Leftover pieces from earlier jobs can be kept as stock and entered in the next layout. If a few shelves fit on last month's offcut, the new order shrinks. Measure those offcuts before entering them, since a piece logged at the wrong size gives a plan that fails at the saw. This is the point to settle how many plywood sheets the job needs: after grain, defects, and offcuts are in.

Enter the sheet price and the waste is shown as money. That figure helps decide whether to buy one spare sheet for a re-cut or a hidden void in the veneer. Weigh the spare against the cost of stopping work to fetch one. If the plan already matches the lower bound, the spare is the only margin left. The guide on how many sheets a job needs runs a cabinet example from part list to plan.