Drawing & geometry
Floor & wall area calculator
Total rectangular floor zones, wall faces and openings with a labelled plan.
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What a square footage calculator does
A square footage calculator multiplies the length and width of each room or rectangle to give its floor area, then totals the rooms. This one also adds up each room's perimeter and multiplies it by the wall height to give wall area, then deducts the doors and windows you enter to give the net wall area for paint, drywall, wallpaper or paneling.
Each row is one rectangle with a name, length, width and height. Rooms that are not rectangles, such as L-shaped living areas, are entered as two or more non-overlapping rectangles. The results are floor area, the sum of room perimeters and net wall area, with a per-room table and a labelled plan of the first rectangle.
It measures what you enter and nothing more. Measure interior dimensions for flooring and wall finishes. For a gross area for a permit, a listing or a floor area ratio check, measure to the line your jurisdiction's definition uses.
How to use the floor & wall area calculator
- Choose Imperial or Metric. Imperial uses feet and square feet.
- Enter each room or rectangle as a row: a name, its length, its width, the wall height and the total door width to deduct from skirting or trim. Use Add rectangle for more rows and the remove button to delete one.
- For an L-shaped room set Shape to L-shape and enter the leg's length (how far it projects) and width (how much of the room length it shares); the perimeter is worked out with the shared edge removed. Split other irregular rooms into rectangles that do not overlap.
- Or drag on the sketch grid under the table: each rectangle you draw (1 ft or 0.5 m per cell) is added as a row at the first row's wall height.
- Enter the total opening area for all the doors and windows you want deducted from the walls. The field takes one combined figure.
- Read floor area, the sum of room perimeters, net wall area, ceiling area and trim length, with a table of floor, perimeter, gross wall, ceiling and trim per row. Export PDF, CSV or the plan as SVG, or copy the results.
Worked example
Imperial: A 16 × 12 ft room has 192 ft² of floor area and a 56 ft perimeter. At 8 ft height, walls total 448 ft² before openings; the ceiling is 192 ft² and trim runs 53 ft after a 3 ft door.
Metric: A 5 × 4 m room has 20 m² floor area and 18 m perimeter. At 2.4 m height, walls total 43.2 m² before openings.
How to calculate the square footage of a room
Measure the length and width wall to wall, convert any inches to decimal feet (12' 6" is 12.5 ft), and multiply. A 12 × 14 ft bedroom is 168 ft². To convert square feet to square metres, multiply by 0.0929; one square metre is 10.764 ft².
The table gives floor area, perimeter and gross wall area for common room sizes at 8 ft and 9 ft wall heights, before any door or window deductions.
| Room | Floor area | Floor area (m²) | Perimeter | Walls at 8 ft | Walls at 9 ft |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 9.29 | 40 ft | 320 ft² | 360 ft² |
| 10 × 12 ft | 120 ft² | 11.15 | 44 ft | 352 ft² | 396 ft² |
| 12 × 12 ft | 144 ft² | 13.38 | 48 ft | 384 ft² | 432 ft² |
| 12 × 14 ft | 168 ft² | 15.61 | 52 ft | 416 ft² | 468 ft² |
| 14 × 16 ft | 224 ft² | 20.81 | 60 ft | 480 ft² | 540 ft² |
| 16 × 20 ft | 320 ft² | 29.73 | 72 ft | 576 ft² | 648 ft² |
| 20 × 24 ft | 480 ft² | 44.59 | 88 ft | 704 ft² | 792 ft² |
How to measure an L-shaped room
Split the room into rectangles that do not overlap, measure each and add the areas. There is usually more than one way to split an L, and any split gives the same total as long as nothing is counted twice or missed.
Worked example: an L-shaped great room made of a 20 × 12 ft main area and a 10 × 8 ft alcove opening off one of its 20 ft sides. Enter two rows. The floor area is 240 + 80 = 320 ft².
Walls need one correction. The calculator counts every edge of every rectangle, including the imaginary 10 ft line where the alcove meets the main room. The sum of perimeters is 64 + 36 = 100 ft, but the real room perimeter is 80 ft, because that 10 ft line is counted once in each rectangle. At 8 ft high, the gross wall area comes out at 800 ft² against a true 640 ft². To correct it, add the phantom wall area (2 × 10 ft × 8 ft = 160 ft²) to the opening deduction along with your doors and windows.
How to calculate wall area for paint or drywall
Wall area = room perimeter × wall height − doors and windows. Worked example: a 12 × 14 ft bedroom with 8 ft walls has a 52 ft perimeter and 416 ft² of gross wall. Deduct one 3 ft × 6 ft 8 in door (20 ft²) and two 3 × 5 ft windows (15 ft² each), 50 ft² in total, for 366 ft² (34.00 m²) of net wall.
Take that figure to the paint quantity calculator with your coats and coverage rate, or to the drywall sheet calculator. For drywall, consider deducting only large openings, because offcuts around small windows are rarely reusable.
| Opening | Size | Area |
|---|---|---|
| Interior door | 2 ft 6 in × 6 ft 8 in | 16.67 ft² |
| Entry door | 3 ft × 6 ft 8 in | 20 ft² |
| Double door | 6 ft × 6 ft 8 in | 40 ft² |
| Sliding patio door | 8 ft × 6 ft 8 in | 53.33 ft² |
| Window | 3 × 4 ft | 12 ft² |
| Window | 3 × 5 ft | 15 ft² |
Floor area versus gross floor area
Floor area for finishes is measured inside the walls. Gross floor area for zoning, permits or the building code is measured to a line set by the relevant definition, often the outside face of the exterior walls for zoning and the inside perimeter of the exterior walls for the IBC, and it includes corridors, stairs and interior walls. Each authority publishes its own definition.
The calculator does not know which you mean, so measure to the line your definition uses. For a whole-floor gross area, enter the outside dimensions of the floor as rectangles; for a flooring order, enter the room interiors and add waste. See floor area ratio explained for how gross floor area feeds zoning limits.
Who it is for and when to use it
Ordering flooring
Total the floor area of several rooms, then add a waste allowance for the pattern and cuts before ordering.
Painting and wall finishes
Get net wall area for paint, wallpaper, paneling or tile after deducting doors and windows.
Renovation estimates
Build a quick area take-off for a remodel quote from room measurements or a scaled drawing.
Checking plan and listing areas
Recalculate a room or floor area from its dimensions to check a figure on a plan, listing or appraisal.
Method, formulas & assumptions
Floor area = sum(length × width), plus leg length × leg width for an L-shape. Room perimeter = 2(length + width + leg length). Wall area = sum(perimeter × height) − opening area. Ceiling area = floor area. Trim = perimeter − door widths.
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Counting shared walls twice without meaning to. Two adjoining rooms each include the partition between them, which is right for painting both faces but not for counting one wall.
- Leaving phantom edges in an L-shaped room. Split lines between rectangles add walls that do not exist; deduct them.
- Mixing feet and inches. 12' 6" is 12.5 ft, not 12.6 ft.
- Deducting openings from floor area. Doors and windows affect wall area only.
- Overlapping rectangles so part of a room is counted twice, or forgetting closets, bays and alcoves.
Scope & limits
Split composite shapes into non-overlapping rectangles, or use the L-shape with one leg sharing an edge. Perimeters and walls are per room, so shared partitions count on both room faces; remove unwanted faces from your inputs. Sketched rectangles are added as rows at the first row's wall height.
Frequently asked questions
How do I calculate square footage?
Multiply length by width in feet. A 12 × 14 ft room is 168 ft². For several rooms or an irregular room, calculate each rectangle and add them.
How do I calculate the square footage of an L-shaped room?
Set the row's Shape to L-shape and enter the main rectangle plus the leg that projects from it, or split it into two rectangles that do not overlap, calculate each and add them. A 20 × 12 ft area plus a 10 × 8 ft alcove is 240 + 80 = 320 ft² either way; the L-shape row also removes the shared edge from the perimeter.
How do I calculate wall area?
Multiply the room perimeter by the wall height and subtract doors and windows. A 12 × 14 ft room with 8 ft walls has 416 ft² of wall before openings.
How many square feet is a 12 × 12 room?
A 12 × 12 ft room is 144 ft², or 13.38 m². Its perimeter is 48 ft, so at 8 ft high the walls total 384 ft² before openings.
How do I convert square feet to square metres?
Multiply by 0.0929. 200 ft² is 18.58 m². To go from square metres to square feet, multiply by 10.764.
Should I subtract doors and windows from wall area?
Yes for paint and wall finishes. A 3 ft × 6 ft 8 in door is 20 ft² and a 3 × 5 ft window is 15 ft². For drywall, deducting only large openings allows for offcuts.
Does the calculator handle sloped ceilings or gable walls?
No. It treats every wall as a rectangle of the entered height. Work out triangular gable areas separately (base × height ÷ 2) and add them to your total.
Why is the perimeter larger than my room's real perimeter?
Perimeters are summed per rectangle, so any edge shared between two rectangles is counted on both. Deduct shared lengths you do not want counted.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
Further reading
- Floor area ratio (FAR) explained
- Minimum ceiling height requirements
- Renovations and extensions
- All drawing & geometry tools
- All free architecture & construction tools
- Harth resource library
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