Subfloor Calculator — How Many Panels Do I Need?
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Subfloor Calculator
Find out how many subfloor panels you need. Free, no sign-up.
- Step 1 — Measure: measure the length and width of your room, or type the area if you already know it.
- Step 2 — Choose: pick your panel size and how much extra to allow for cuts.
- Step 3 — See the answer: the number of panels to buy appears on the right.
If you get stuck, press "Fill in an example" below and change the numbers to yours.
Step 1 — How big is the room?
How to measure: stand at one wall and measure to the opposite wall — that is the length. Then measure the other two walls — that is the width. Write feet and inches separately. Example: if the tape shows 12 feet 6 inches, type 12 in Feet and 6 in Inches.
Step 2 — What panel are you buying?
Use the size printed on the product you are actually buying. Do not guess — panels come in different sizes.
Save this project
Projects are saved in this browser only. No account, and nothing is sent to a server.
Your result
Enter a room size to see your result.
Totals are calculated before rounding. Displayed values are rounded to 2 decimal places.
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What this tool counts — and what it does not count
This tool counts subfloor SHEETS/PANELS — plywood or OSB panels laid over joists. It does not count finish flooring boxes and it does not count underlayment. Thickness is not calculated here: thickness is chosen by joist spacing and the product guide. As general information only, 16-inch joist spacing usually uses 3/4-inch panels, but you must check your product guide and local code. This page gives no structural, joist-span or load advice. If your floor feels bouncy, sags, or you are changing joists, talk to a qualified builder or engineer before you buy.
How the calculation works
Net area = length × width (feet + inches ÷ 12), or the area you type directly. Order area = net area + waste allowance. Panel area = panel length × panel width. Panels = order area ÷ panel area, rounded up to the next whole panel, because you cannot buy part of a panel. Two kinds of extra are shown separately: the waste allowance you chose for cuts, and the extra that comes from rounding up to whole panels.
Worked examples
You can check these by hand before trusting the tool with your own room.
| Project | Net area | Waste | Order area | Panels |
|---|---|---|---|---|
| Room 12 ft × 15 ft, panel 4 ft × 8 ft, straight | 180.00 sq ft | 10% (+18.00) | 198.00 sq ft | 7 panels (224.00 sq ft) |
| Room 10 ft × 12 ft, panel 4 ft × 8 ft, straight | 120.00 sq ft | 10% (+12.00) | 132.00 sq ft | 5 panels (160.00 sq ft) |
| Area 100 sq ft, custom panel 4 ft × 4 ft, no waste | 100.00 sq ft | 0% (+0.00) | 100.00 sq ft | 7 panels (112.00 sq ft) |
Panel size guide
A 4 ft × 8 ft panel covers 32 sq ft and is the standard size most suppliers stock. A 4 ft × 4 ft panel covers 16 sq ft and is easier to carry through a doorway or up stairs, but you will need roughly twice as many panels and there will be more joints. Whatever size you pick, use the size on the product you are buying — a "4 × 8" label that actually covers less because of a tongue-and-groove edge must be entered as Custom with its true covered area.
Common mistakes
- Counting finish flooring boxes instead of subfloor panels: they are different products with different coverage.
- Entering inches as 12 or more: inches must be 0–11. Convert 12 inches to 1 foot first.
- Forgetting waste, or forgetting that panels round up: both extras are shown on separate lines here.
- Measuring an L-shaped room as one rectangle: split it into two rectangles, add the areas, and use the direct area input.
Frequently asked questions
What is a subfloor?
A subfloor is the flat panel layer fixed to the floor joists. It is the base that the finish floor is installed on. This calculator counts subfloor panels, not finish flooring.
What is the difference between a subfloor, underlayment and finish floor?
The subfloor is the panel layer over the joists. Underlayment is a thinner layer sometimes placed on top of the subfloor to smooth it or add cushioning. The finish floor is the visible surface you walk on. This tool counts only subfloor panels.
What subfloor panel size should I pick?
The most common panel is 4 ft by 8 ft (32 sq ft). A 4 ft by 4 ft panel (16 sq ft) is easier to carry in a small space. Use the panel size printed on the product you are actually buying, and use Custom if it is a different size.
Why do I need to add waste for subfloor panels?
Panels must be cut at walls, around openings and at the ends of rows, and some cut pieces cannot be reused. A straight layout usually starts at 10% extra. The calculator also rounds up to whole panels, which is a second, separate extra.
What thickness of subfloor do I need?
This calculator does not choose a thickness. Thickness depends on joist spacing and the product guide. As general information only, 16-inch joist spacing usually uses 3/4-inch panels, but you must check your product guide and local code before buying. Do not treat this page as a structural recommendation.
What does this calculator not do?
It does not give structural, joist-span or load advice, it does not count finish flooring boxes, and it does not count underlayment. For finish flooring boxes use the Flooring Box Calculator, and for a full project price use the Flooring Cost Breakdown Calculator.
Last updated: 28 October 2026. Waste percentages are starting points; your product guide overrides them.