Add each floor, wall, roof or ceiling with its size (and doors, windows or roof pitch), choose your sheet size, and get the number of plywood or OSB sheets to buy with a waste allowance and the cost.
Surface 1
ft
ft
Surface 2
ft
For several walls, add their lengths: a 12 × 16 room has 56 ft of wall
ft
sq ft
Their area: a 3 × 6⅔ ft door is 20 sq ft
%
About 10% for floors and simple roofs, 15–20% with many cuts
$
Sheets to buy
21
Cost
$945.00
Area to cover
596 sq ft
With 10% waste
655.6 sq ft
One sheet covers
32 sq ft
Laid on a plain grid, no offcuts reused
20 sheets
655.6 sq ft ÷ 32 sq ft per sheet = 20.49 → 21 sheets.
Sheets for each surface
Surface
sq ft
Sheets
1. Floor
192
6.6
2. Wall
404
13.9
Plywood thickness and nailing depend on joist or rafter spacing and your local building code; check them before you buy.
How to use the plywood calculator
Choose the sheet size you can buy and the unit you measured in.
Add a surface for each floor, wall run, roof or ceiling. For walls, add up their lengths and take off doors and windows; for roofs, enter the pitch.
Set the waste allowance and, if you like, the price per sheet.
Worked example
A 12 × 16 ft room gets a plywood floor (192 sq ft) and sheathed walls 8 ft high. The walls run 2 × (12 + 16) = 56 ft, so they're 448 sq ft, less a door (20 sq ft) and two 3 × 4 ft windows (24 sq ft): 404 sq ft. Together that's 596 sq ft; with 10% waste, 655.6 sq ft ÷ 32 = 20.49, so buy 21 sheets of 4 × 8.
Formulas
Sheets = area × (1 + waste %) ÷ sheet area, rounded up
Wall area = length × height − doors and windows
Roof area = footprint × √(1 + (rise ÷ 12)²), or footprint ÷ cos(pitch angle)
The count works for OSB, drywall, cement board or any sheet material of the same size. Everything runs in your browser.
Frequently asked questions
How many sheets of plywood do I need?
Divide the area to cover by the area of one sheet, add a waste allowance and round up. A 4 × 8 ft sheet covers 32 sq ft, so a 15 × 17 ft floor (255 sq ft) with 10% waste needs 255 × 1.1 ÷ 32 = 8.77, which is 9 sheets.
How do I work out plywood for a roof?
Measure the roof's footprint, including overhangs, and multiply by the pitch factor √(1 + (rise ÷ 12)²). A 6-in-12 roof has a factor of 1.118, so a 2,000 sq ft footprint has 2,236 sq ft of roof; with 10% waste that's 77 sheets of 4 × 8. The calculator takes the pitch as rise in 12 or in degrees.
Should I subtract doors and windows from walls?
Yes, take off their area: a 3 × 6⅔ ft door is 20 sq ft and a 3 × 4 ft window 12 sq ft. The pieces cut out of a sheet for an opening are often too small to reuse, which is one reason to keep the waste allowance.
What size are plywood sheets?
In the US and Canada the standard sheet is 4 × 8 ft (48 × 96 inches), with 4 × 9 and 4 × 10 ft for taller walls. India and the UK sell 2440 × 1220 mm sheets (8 × 4 ft), and India also has 7 × 4, 6 × 4 and 8 × 3 ft. Australia and New Zealand use 2400 × 1200 mm, slightly smaller.
What does "laid on a plain grid" mean?
It's the number of full sheets if each surface were covered in a simple grid of whole sheets and no offcut were used anywhere else. It's a safe upper limit for that layout. Careful cutting and reusing offcuts gets close to the area-based count.
