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Generator Hut

Plywood Calculator

Sheets of plywood or OSB for a floor, wall or roof deck.

Your measurements

Covers cuts at the perimeter and around openings.

Sheets required

10sheets

Buy 10 sheets

Area to cover20 ft × 14 ft
280 sq ft
With waste allowance+10%
308 sq ft
Sheets requiredat 32 sq ft each
10
Total supplied40 sq ft spare
320 sq ft

Sheets are laid in a staggered pattern with joints falling on framing members, which wastes more than a plain area calculation suggests. Ten per cent is a minimum, and a room whose dimensions are not multiples of four feet will use more.

Subfloor sheathing needs a gap of about an eighth of an inch between sheets to allow for expansion. Sheets butted tight will buckle when the humidity changes.

Thickness is determined by the span between joists and the load above, not by area. This calculates how many sheets, not which sheets.

Enter the area and the sheet size and it returns the number of sheets plus what the rounding leaves spare. Sheets are laid staggered with joints landing on framing, which wastes more than a plain area division suggests.

Why use this tool?

Three sheet sizes

4x8, 4x10 and 4x12. Longer sheets mean fewer joints, which matters for both strength and finish.

Spare shown

How much surplus the rounding leaves, so you know whether you have a margin for a bad cut.

Realistic waste default

Ten per cent, because staggered joints on framing centres waste more than a plain area calculation implies.

Floors, walls and roofs

The same arithmetic covers subfloor, wall sheathing and roof decking.

How this plywood calculator works

Area times the waste allowance, divided by the sheet area, rounded up. A 4x8 sheet covers 32 square feet, a 4x10 covers 40 and a 4x12 covers 48.

The waste allowance matters more here than the arithmetic suggests, because sheets cannot simply be butted in a grid. Joints have to land on framing members and are staggered between rows for strength, so a room whose dimensions are not multiples of four feet generates offcuts at every row.

Thickness is not part of this calculation. It is determined by the span between joists or rafters and the load above, which is a structural question — this tells you how many sheets, not which sheets.

How to use it

  1. Step 1: Measure the area

    Length and width of the floor, wall or roof plane. Calculate each plane separately for a roof.

  2. Step 2: Choose the sheet size

    Longer sheets mean fewer joints but are heavier and harder to manoeuvre. Check you can get them into the space.

  3. Step 3: Set the waste allowance

    Ten per cent minimum. More if the dimensions are awkward or there are many openings to cut around.

  4. Step 4: Leave expansion gaps

    When fitting, leave about an eighth of an inch between sheets. Butted tight, they buckle when humidity changes.

Example usage

A 20 by 14 subfloor
280 square feet plus 10% is 308. In 4x8 sheets that is 10 sheets supplying 320 square feet, leaving 40 spare.
The same floor in longer sheets
In 4x12 sheets it is 7 sheets supplying 336 square feet. Fewer joints across the floor, and joints are where a subfloor squeaks.
Roof sheathing
Calculate each roof plane separately using its actual sloped dimensions, not the footprint. Use the roofing calculator to get the sloped area first.

Frequently asked questions

How many sheets of plywood do I need?

Divide the area by the sheet coverage — 32 square feet for a 4x8, 40 for a 4x10, 48 for a 4x12 — then add about 10 per cent for waste and round up.

How much waste should I allow?

Ten per cent as a minimum. Sheets are laid staggered with joints on framing centres, so a room whose dimensions are not multiples of four feet generates an offcut at every row.

What thickness of plywood do I need?

That depends on the span between joists or rafters and the load above, not on the area. Span tables give the answer, and the requirement differs between subfloor, wall sheathing and roof decking.

Should I leave gaps between sheets?

Yes, about an eighth of an inch. Sheet goods expand and contract with humidity, and panels butted tight will buckle at the joints — which on a subfloor becomes a permanent ridge under the flooring.

Can I use OSB instead of plywood?

For most sheathing and subfloor applications yes, and the sheet sizes and this calculation are identical. OSB handles repeated wetting less well, so plywood is often preferred where exposure during construction is likely.

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