Subfloor Calculator
A 20 by 24 ft garage floor (480 sq ft) needs 17 sheets of 4x8 subfloor with 10% waste. Enter your room length and width, the sheet size you plan to buy, and a waste percentage to get the number of subfloor sheets to order. The calculator divides your floor area by the coverage of one sheet and rounds up so you have full sheets for every section.
Quick answer
Sheet count is the floor area divided by the coverage of one sheet, rounded up to whole sheets.
What this tells you
- •Sheet count is the floor area divided by the coverage of one sheet, rounded up to whole sheets.
- •Most subfloor comes in 4 by 8 ft sheets that each cover 32 square feet.
- •A waste allowance covers cuts around walls, openings, and damaged sheets.
How to Use
- 1Enter the room length in feet.
- 2Enter the room width in feet.
- 3Set the sheet width and height (4 by 8 ft is the standard size).
- 4Enter a waste percentage (10% is a good starting point for most rooms).
- 5Click Calculate to see the number of subfloor sheets you need.
How It Works
Formula
Floor Area = Room Length x Room Width
Sheet Coverage = Sheet Width x Sheet Height
Sheets = ceil(Floor Area x (1 + Waste / 100) / Sheet Coverage)The floor area is the room length times the width. One sheet covers its width times its height, which is 32 square feet for a standard 4 by 8 sheet. Divide the floor area by the sheet coverage, add the waste allowance, and round up so every part of the floor gets a full sheet.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
20 by 24 ft garage with 4x8 sheets
Floor area = 20 x 24 = 480 sq ft. Sheet coverage = 4 x 8 = 32 sq ft. Sheets = ceil(480 x 1.1 / 32) = ceil(16.5) = 17 sheets.
10 by 10 ft room with 4x8 sheets
Floor area = 10 x 10 = 100 sq ft. Sheets = ceil(100 x 1.1 / 32) = ceil(3.44) = 4 sheets.
Subfloor Sheets by Room Size
Sheet counts for common room sizes using standard 4 by 8 ft sheets with 10% waste added.
| Room Size | Floor Area (sq ft) | 4x8 Sheets |
|---|---|---|
| 10x10 | 100 | 4 |
| 12x12 | 144 | 5 |
| 16x20 | 320 | 11 |
| 20x24 | 480 | 17 |
| 24x30 | 720 | 25 |
Counts assume full 4 by 8 ft sheets and a 10% waste allowance. Cut-heavy layouts may need a few extra sheets.
Plywood vs OSB Subfloor
Plywood and OSB (oriented strand board) are the two common subfloor materials, and both come in 4 by 8 ft sheets. Plywood resists moisture a little better and dries faster if it gets wet, while OSB usually costs less and comes in larger sheet options for fewer seams.
Tongue-and-groove edges on either material lock sheets together and reduce squeaks and movement at the joints. For most homes the choice comes down to budget and local availability, since both meet code when installed over properly spaced joists.
Whichever material you pick, the sheet count is the same because both use the standard 4 by 8 ft footprint. Plan your layout so seams land on joists and stagger the rows for a stronger floor.
Common mistakes
- Forgetting to add waste for cuts around walls, cabinets, and openings
- Measuring the room in inches instead of feet, which throws off the area
- Assuming a sheet covers more than its actual 32 square feet for a 4 by 8 panel
- Ordering exactly the calculated count with no spare sheet for mistakes or damage