OSB Calculator
An OSB calculator divides area by 32 sq ft per 4x8 sheet and rounds up: a 24 × 20 ft floor (480 sq ft) needs 15 sheets, or 17 with 10% waste.
OSB calculator formula
An OSB calculator counts how many 4×8 sheets cover an area. The formula is sheets = area in square feet ÷ 32, rounded up to a whole sheet.
Tick the waste box and the calculator multiplies the sheet count by 1.10 before rounding up. The price per sheet then gives the cost. Enter the surface you want to cover, not the footprint of the building, when you have a sloped roof.
Worked examples
Example 1 is a floor deck, 24 ft × 20 ft at $30 per sheet.
- Area: 24 × 20 = 480 sq ft
- 480 ÷ 32 = 15 sheets exactly
- Cost without waste: 15 × $30 = $450
- With 10 percent waste: 15 × 1.10 = 16.5, rounded up to 17 sheets, $510
Example 2 is a roof section with 1,200 sq ft of surface, at $30 per sheet.
- 1,200 ÷ 32 = 37.5, rounded up to 38 sheets, $1,140
- With 10 percent waste: 37.5 × 1.10 = 41.25, rounded up to 42 sheets, $1,260
Half a sheet short is still a whole sheet, and that is why the calculator rounds up before it prices. The first job happens to land on whole sheets, so the waste is a choice. The second job needs a full 38 sheets even with no waste.
Sheets by area
| Area | Without waste | With 10% waste |
|---|---|---|
| 96 sq ft | 3 | 4 |
| 192 sq ft | 6 | 7 |
| 300 sq ft | 10 | 11 |
| 480 sq ft | 15 | 17 |
| 1,200 sq ft | 38 | 42 |
| 2,000 sq ft | 63 | 69 |
For small projects, look at the leftover. A 100 sq ft shed floor needs 4 sheets, because 3 sheets cover only 96 sq ft. The fourth sheet gets cut down to a strip, and the rest is useful for a shelf or a wall patch.
Where OSB is used and common thicknesses
The thickness depends on the span and the use. These are common starting points, and the panel stamp and your local code decide.
| Use | Typical thickness |
|---|---|
| Wall sheathing | 7/16 inch |
| Roof sheathing | 7/16 to 5/8 inch, depending on rafter spacing |
| Subfloor over joists | 23/32 inch (often sold as 3/4), often tongue and groove |
| Shed floor | 3/4 inch, over 16 inch spaced joists |
| Garage or workshop wall lining | 7/16 inch or thicker |
Panels are labeled with a span rating that tells you the maximum joist or rafter spacing. Match the rating to the framing before you buy. If you are unsure, ask your building department or a framer.
Installing it
- Stagger the joints on the floor and the roof, so that four corners never meet at one point.
- Leave about 1/8 inch gap between panels for expansion. Many panel makers print the same spacing on the stamp or the bundle wrap.
- Nail or screw on the pattern the maker and the local code require. A common pattern is closer along the edges and wider in the field, but check the rules in your area.
- Keep the long edge across the joists, not parallel to them, on floors and roofs.
- Seal or cover the sheets quickly. Panel makers sell OSB with a water-resistant coating, but it is not a roof.
How to measure
- For a floor, multiply length by width of the framed area.
- For walls, multiply the length by the height for each wall, then subtract large openings like garage doors.
- For a roof, find the surface area, which is larger than the floor below it. Multiply the footprint by 1.118 for a 6/12 pitch, 1.202 for 8/12, or use the roof area calculator.
- Add hips, valleys, dormers and overhangs to the count, and plan the extra cuts.
Common mistakes
- Using the footprint of the house for the roof. A sloped roof has more surface than the floor.
- Not rounding up. 14.1 sheets is 15 sheets.
- Buying 7/16 for a floor that needs 23/32. Check the span rating first.
- Letting the sheets get rained on repeatedly before the roof is dry-in.
- Forgetting the cost of fasteners, ring-shank nails, and construction adhesive on a floor. Those are separate from sheet cost.
Related calculators
OSB and plywood are priced in the same way, so compare with the plywood calculator. For what goes over a roof deck, use the roofing underlayment calculator, and for the frame below, see the framing calculator.
Frequently asked questions
What size is a standard OSB sheet?
4 ft × 8 ft, which is 32 sq ft per sheet. That is the figure this calculator uses. Longer panels such as 4 × 9 and 4 × 10 exist for walls and some regions, but if you buy those, divide your area by 36 or 40 instead.
Why is OSB usually cheaper than plywood?
OSB is made from strands of fast-growing trees pressed with resin, so it can use smaller logs and wastes less wood than rotary-peeled plywood. Prices move with the market, so check the current price of both at your yard and enter the one you will actually pay.
Does OSB handle moisture as well as plywood?
Both are made to tolerate normal construction weather, but OSB edges can swell if they stay wet, and swollen edges telegraph through finished floors. Keep panels dry, seal cut edges where the maker says to, and do not leave a subfloor open to rain for weeks.
How much waste should I add for OSB?
Ten percent is a common starting point. Use more for a cut-up roof with hips and valleys, or for a layout with many openings. Use less for a simple rectangle that works out to whole sheets, such as 24 × 16 ft.
Is OSB rated the same as plywood for structural sheathing?
Both can carry the same structural rating when they are stamped for the span and use. Look for the grade stamp on the panel and match it to your joist or rafter spacing and your local code. The stamp, not the material name, is what an inspector reads.
Estimates only. Verify quantities with your supplier before purchasing.