Calculate how many 4×8 sheets of plywood or OSB you need for a subfloor or sheathing job. Enter the area to cover and a waste allowance.
Usage Tip
Add more waste for diagonal layouts, many obstructions, or complex room shapes.
Each 4×8 sheet covers 32 square feet
A waste allowance covers cuts, offcuts, and mistakes so you buy enough.
The result is rounded up to whole sheets.
For 4×12 sheets, divide the sheet count by 1.5.
How Much Subfloor Plywood Do I Need?
Measure the floor area, add a little waste for cuts around the perimeter and openings, divide by 32 sq ft per sheet, and round up. Then match the thickness to your joist spacing and intended floor finish.
Sheet Coverage
A standard 4×8 sheet of plywood or OSB covers 32 square feet. Quick reference (before waste):
| Floor area | 4×8 sheets |
|---|---|
| 100 sq ft | 4 |
| 250 sq ft | 8 |
| 500 sq ft | 16 |
| 1000 sq ft | 32 |
Add roughly 10% for cuts and waste, and buy a spare sheet for mistakes and future repairs.
Subfloor Thickness Guide
| Use | Common thickness |
|---|---|
| Basic subfloor (16 in joists) | 5/8 in |
| Stiffer floor | 3/4 in |
| Tile prep | 3/4 in + underlayment/backer |
| Repair patch | Match existing |
Thicker is stiffer. For tile especially, deflection is the enemy – a 3/4 in subfloor plus the right underlayment prevents cracked grout and tile.
Plywood vs OSB
| Plywood | OSB | |
|---|---|---|
| Cost | Higher | Lower (about 20-40% less) |
| Weight | Lighter | Heavier |
| Moisture | Dries faster, recovers | Swells at edges, slow to recover |
| Fastener hold | Slightly better | Good |
Both are code-approved subfloor materials. OSB saves money and is common in new construction; plywood is preferred where moisture is a risk or for tile. Use the tongue-and-groove version for subfloor either way.
Joist Spacing
Joist spacing sets the minimum thickness, because the subfloor spans between joists:
| Joist spacing | Minimum subfloor |
|---|---|
| 16 in on center | 5/8 in (3/4 in better) |
| 19.2 in on center | 3/4 in |
| 24 in on center | 3/4 in T&G (7/8-1 in better) |
Run sheets perpendicular to the joists, stagger the end joints, and leave a 1/8 in gap between sheets for expansion. Always confirm against local code and the finished-floor manufacturer.
Tile, Laminate & Vinyl Subfloor Prep
Tile: needs a stiff base – typically 3/4 in subfloor plus cement backer board or an uncoupling membrane; total deflection must be low or grout cracks. Laminate and vinyl plank: are more forgiving and float over a flat subfloor, but flatness still matters – level high and low spots first. For all of them, a solid, squeak-free, flat subfloor is the foundation that makes the finished floor last.
Fasteners & Adhesive
- Screws: use subfloor/deck screws, spaced about 6 in along sheet edges and 8-12 in in the field. Plan roughly 45-50 screws per 4×8 sheet.
- Adhesive: run a bead of construction (subfloor) adhesive along every joist before laying the sheet – it stops squeaks. A 28 oz tube covers roughly 90 linear ft of joist.
- Glue and screw is the standard for a quiet floor; nails alone tend to pop and squeak over time.
Subfloor Repair
For a patch, match the existing thickness exactly so the surface stays flush, and support the edges of the patch on joists or added blocking. Cut back damaged material to the centre of the nearest joists, treat any moisture source first, and glue and screw the patch in. Soft, spongy, or water-stained areas should be replaced, not covered.
Common Measuring Mistakes
- Forgetting waste – perimeter cuts and openings always create offcuts; add about 10%.
- Wrong thickness for the span – too thin over wide joists flexes and squeaks.
- No expansion gap – tight sheets buckle; leave 1/8 in.
- Skipping adhesive – the difference between a silent floor and a squeaky one.
- Ignoring subfloor condition – laying new floor over a soft or uneven base.
Frequently Asked Questions
How many sheets of plywood for a subfloor?
Floor area divided by 32 sq ft per 4×8 sheet, plus about 10% waste, rounded up. A 500 sq ft room needs about 16-18 sheets.
How thick should subfloor plywood be?
5/8 in over 16 in joists at minimum, 3/4 in for a stiffer floor or tile, and 3/4 in T&G or thicker over 24 in joists.
Is OSB or plywood better for subfloor?
Both are code-approved. OSB costs less; plywood handles moisture better and is preferred under tile.
Should I glue and screw the subfloor?
Yes – adhesive on the joists plus screws is the standard for a squeak-free floor.
How much waste should I add?
About 10% for cuts, plus a spare sheet. Complex layouts with many openings may need 15%.
Does subfloor thickness affect tile?
Very much – tile needs a stiff, low-deflection base. Use 3/4 in subfloor plus backer board or membrane.
Related Flooring Calculators
Note: sheet, thickness, fastener, and adhesive figures are planning estimates and vary by product, span rating, layout, and finished floor. Subfloor thickness and span requirements are governed by local building code and the finished-floor and joist manufacturer requirements – confirm before buying, check the condition of any existing subfloor, and follow all codes. General DIY guidance, not professional structural advice.