How to measure for sheathing
A floor is the easy one: length by width. A wall is length by height, and if you have large openings, take them off by area. Small ones are not worth deducting, because the cutouts rarely make a usable piece.
A roof needs more care. Start with the building's footprint and add the overhangs on every side, then allow for pitch, as the sloped surface is bigger than its plan. A 6 in 12 roof has about 12 percent more surface than its footprint. This assumes a plain gable or shed roof. Hips and valleys cut a lot of angles, and that waste has no published figure.
The count does not choose your panel. Thickness and span rating have to match your framing. For roofs, R803.2.2 of the 2021 IRC points to its own table or APA's guide. For example, APA lists a 24/16 panel, 7/16 inch thick, for spans up to 24 inches with or without edge support. Read the panel stamp.
A worked example
Here is a 40 by 30 foot roof at 6 in 12 with no overhang, in 4 by 8 foot sheets. These lines come from the same formula the calculator above runs.
40 by 30 ft roof at 6 in 12
- Footprint: 40 × 30 = 1200 sq ft
- With pitch: 1200 × 1.118 = 1342 sq ft
- With 10% extra: 1476 sq ft
- Sheets: 1476 ÷ 32 = 46.12, so 47
For a roof, the roofing calculator uses the same area for shingles, and the rafter length calculator sizes the boards underneath. Walls start with the stud calculator.
Sheets for common jobs
Roofs here use 6 in 12 and a 1 foot overhang. Use these to check your own number, as a real job has openings and cuts that a plain rectangle does not.
| Job | Area | 4 × 8 sheets, with 10% |
|---|---|---|
| 10 × 10 ft floor | 100 sq ft | 4 |
| 12 × 16 ft shed floor | 192 sq ft | 7 |
| 12 × 20 ft shed floor | 240 sq ft | 9 |
| 12 × 16 ft shed roof | 282 sq ft | 10 |
| 24 × 24 ft garage roof | 756 sq ft | 26 |
| 40 ft wall, 8 ft high | 320 sq ft | 11 |
Mistakes that make a sheathing job harder
- Roof by footprint alone. Pitch and overhangs add real area, and you pay for that in sheets.
- A thickness by habit. Span rating must match your rafter or joist spacing. APA notes that high wind areas may need more nailing too.
- No gap at joints. APA recommends 1/8 inch at all edges and ends, unless the panel maker says otherwise.
- Clips where you do not need them, or none where you do. APA's rule is one clip per span when required, and two for spans of 48 inches or more.
- Leaving panels out in the rain. Exposure 1 panels are for short exposure during building, so cover them when you can.
Questions
How many sheets of plywood for a 10x10 floor?
Four 4 by 8 foot sheets for a 10 by 10 floor, as 100 square feet plus 10 percent is 110, and 110 divided by 32 is 3.44. Cut waste on a small floor is often higher, since the sheet size rarely lines up.
How many sheets of plywood do I need for a 12x16 shed?
Seven 4 by 8 sheets for the floor, at 192 square feet plus 10 percent. A roof at 6 in 12 with a 1 foot overhang takes 10 more, and walls about 16 more at 8 feet high, before windows and doors.
How many sheets of plywood for a 24x24 garage?
About 26 sheets of 4 by 8 for a 24 by 24 foot roof footprint at 6 in 12 with a 1 foot overhang, including 10 percent. Walls at 8 feet high add about 27 more before you take off doors and windows.
What size sheathing do I need?
Match the span rating to your framing, because this calculator only counts sheets. APA's table lists 7/16 inch 24/16 panels for up to 24 inches on center, for example. Check the panel stamp and the span table for your rafter spacing and your local code.
Do I leave gaps between sheets?
Yes, APA recommends 1/8 inch at all edge and end joints unless the panel maker says otherwise. Panels swell with moisture, and without a gap they can buckle.
How many sheets for a hip roof?
More than this says, as hips and valleys cut lots of angles and waste more. No primary source puts a number on it, so raise the extra in the further options, and allow for it. A roofer can take off a real plan for you.
This plywood count is a planning estimate and it does not choose a panel. Thickness and span rating have to match your joists or rafters, so check the panel stamp and the span tables, and hips and valleys waste more than any simple percentage. Confirm the sizes on hand with your supplier and read the label before you nail anything down.
Sources and review
- Roof construction, Engineered Wood Construction Guide excerpt, APA, hosted by Georgia-Pacific, 2019.
- 2021 residential code, chapter 8, as adopted in Colorado, UpCodes, 2021.
- Plywood calculator, Inch Calculator, no date shown, read November 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Sheets
Sheets = Area × (1 + Extra ÷ 100) ÷ Sheet size, rounded up
- Extra
- Percent added for cuts, usually 10
- Sheet size
- 32 square feet for a 4 by 8
Worked through: 800 ÷ 32 = 25 sheets, before any extra
Roof area
Area = (Length + 2 × Overhang) × (Width + 2 × Overhang) × √(1 + (Pitch ÷ 12)²)
- √(1 + (Pitch ÷ 12)²)
- The pitch multiplier
Worked through: 1200 × 1.118 = 1342 sq ft
The constants, and where they come from
SUGGESTED_WASTE_PERCENT
Modelled from published dataExtra to add for waste, in percent
Applies to: Simple rectangular floors, walls and gable roofs. Hips and valleys waste more
Source: Plywood calculator, Inch Calculator, no date on the page, read 2026-11.
A competitor's advice, as APA's guide gives no sheet count or waste method and no primary source was found. No source puts a number on how much more a hip roof wastes.
STANDARD_SHEET_SQ_FT
DefinitionalSquare feet in a 4 by 8 foot sheet
Applies to: A 4 by 8 foot sheet
True by definition, so there is nothing to cite. The derivation is below.
4 × 8 = 32 square feet. Longer sheets come in the other options.
INCHES_OF_RUN
DefinitionalThe run a pitch is stated against, in inches
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
A pitch of 6 in 12 means 6 inches of rise over 12 inches of run, by the way the trade writes it.
What the labels mean
- Modelled from published data:
- derived rather than measured, calibrated against the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the plywood calculator for sheathing
- 1
Choose a floor, a wall or a roof
- 2
Enter the sizes, and a roof's pitch and overhang
- 3
Choose your sheet size
- 4
Take off any big openings in the further section
- 5
Read the sheets, then check the panel stamp against your framing
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- A plain rectangle for a floor or a wall, and a rectangular footprint at one pitch for a roof
- The waste figure comes from a competitor's advice, since no primary source gives one
- Openings come off by area, so tiny offcuts are not counted as saved
- A roof's pitch and overhangs are included. Hips and valleys are not
Pro tips
- Stagger the joints from one course to the next
- Lay out a sketch of the sheets first, as a layout can save a lot of waste
- Keep panels off the ground and covered until they go up