To calculate square footage, multiply the length by the width in feet. A 12 ft by 10 ft room is 120 sq ft. For irregular rooms, split the plan into non-overlapping rectangles, calculate each, and add. For circles, triangles, and trapezoids, the square footage formula changes but the conversion from inches to feet does not.
Use the square footage calculator for rectangles, circles, triangles, L-shapes, and a multi-room tally. Every formula below is worked through with an example, including the feet-to-inches conversion that most estimates get wrong.
What Square Footage Measures — and What It Does Not
Square footage is a two-dimensional area. One square foot is a square measuring one foot on every side, or 144 square inches. It is not a length, a perimeter, or a volume. A room listed at 200 sq ft has 200 sq ft of floor surface inside its walls.
Materials are priced three different ways, and only one of them uses square footage. Flooring is priced per square foot of area. Skirting and trim are priced per linear foot of perimeter. Concrete and gravel are priced per cubic yard of volume. A single square-footage number serves only the first of those three.
| Measurement | Unit | Used for | Formula |
|---|---|---|---|
| Area | Square feet (sq ft) | Flooring, paint, roofing, sod | Length × Width |
| Perimeter | Linear feet (lf) | Skirting, trim, fencing, edging | 2 × (Length + Width) |
| Volume | Cubic feet or cubic yards | Concrete, gravel, mulch, soil | Area × Depth (in feet) |
If the job needs material by volume, the square foot calculator gives the area; the depth conversion happens after.
What Is the Square Footage Formula for Each Shape?
| Shape | Formula | Worked example | Result |
|---|---|---|---|
| Rectangle | Length × Width | 12 ft × 10 ft | 120 sq ft |
| Square | Side × Side | 15 ft × 15 ft | 225 sq ft |
| Circle | π × radius² | Radius 5 ft | 78.54 sq ft |
| Triangle | 0.5 × Base × Height | Base 16 ft, height 10 ft | 80 sq ft |
| Trapezoid | 0.5 × (a + b) × height | a = 10, b = 6, h = 8 | 64 sq ft |
| L-shape | Main rectangle + wing rectangle | 20 × 10 + 10 × 8 | 280 sq ft |
How Do You Measure a Room for Square Footage?
- Sketch the room first. A hand drawing, even on a phone, gives every dimension a place to live and exposes alcoves you would otherwise walk past.
- Measure wall to wall, not along the skirting. Run the tape level and straight. A tape that follows a curved baseboard reads long.
- Measure width perpendicular to length. If the two measurements are not at right angles, the rectangle formula does not apply — the room needs decomposition.
- Record feet and inches separately. Write "12 ft 6 in," not "12.6." Inches are twelfths of a foot, not decimal tenths.
- Measure at floor level and at waist height. Walls bow. If the two readings differ by more than an inch, use the smaller dimension and note the bow — ordering to the larger dimension wastes material.
How Do You Convert Feet and Inches to Square Feet?
Writing 12 ft 6 in as 12.6 ft is the single most common square footage error. There are 12 inches in a foot, so 6 inches is 0.5 ft, not 0.6 ft.
A room measuring 12 ft 6 in by 10 ft 3 in becomes 12.5 ft by 10.25 ft, giving 128.125 sq ft, rounded to 128.13 sq ft. Writing 12.6 × 10.3 gives 129.78 sq ft, an overestimate of more than 1.5 sq ft before any material is bought.
The square feet calculator accepts feet and inches as separate fields, which removes the conversion step entirely.
How Do You Calculate Square Footage of an Irregular Room?
An L-shaped room is not a special formula. It is two rectangles that share a corner. Cut the plan into rectangles that do not overlap, calculate each, and add.
For a shape where the two rectangles do overlap — a bay window recess, or a room with a projecting chimney breast — subtract the overlapping area once. The square footage calculator asks for the two rectangles separately and adjusts for overlap automatically.
Circular and triangular extensions
A semicircular bay is half of πr². A triangular gable end is half base × height, with the height measured perpendicular to the base, not along the sloping edge. For a trapezoidal room, 0.5 × (a + b) × height, where a and b are the two parallel sides.
How Do You Subtract Doors and Windows from Wall Area?
For floor area, openings do not exist — the floor runs to the wall. For wall area, every door and window is a deduction. Measure the opening, calculate its area, and subtract it from the wall area once per opening.
| Opening | Typical size | Area to subtract |
|---|---|---|
| Standard interior door | 0.8 m × 2.0 m (2 ft 8 in × 6 ft 8 in) | ~1.6 m² / ~17.2 sq ft |
| Standard window | 1.2 m × 1.2 m (4 ft × 4 ft) | ~1.44 m² / ~15.5 sq ft |
| Patio door | 1.8 m × 2.1 m (6 ft × 7 ft) | ~3.78 m² / ~40.7 sq ft |
For paint, subtract openings only for the first coat. On a second coat over a previously painted surface, the cutting-in around frames still consumes paint — many painters deduct half the opening area rather than the full area to allow for that edge work.
How Do You Turn Square Footage into Material Quantity?
Every material has a coverage rate and a waste factor. The square footage is the same number; what changes is the divisor and the buffer.
| Material | Sold by | Typical coverage | Waste buffer |
|---|---|---|---|
| Laminate flooring | Box | 20–30 sq ft per box | 10% |
| Hardwood flooring | Box | 18–24 sq ft per box | 10% |
| Ceramic tile | Box | 8–15 sq ft per box | 10–15% |
| Interior paint | Gallon | 350–400 sq ft per coat | 5% |
| Exterior paint | Gallon | 250–350 sq ft per coat | 10% |
| Roofing shingles | Bundle or square | 100 sq ft per square | 10% |
| Concrete | Cubic yard | Area × thickness | 5–10% |
Waste rises with pattern. A straight plank floor in a rectangular room wastes 5–8% on cuts. A diagonal plank floor in the same room wastes 12–15%. Running-bond tile wastes more than a straight grid. Add the buffer before ordering, not after the last box is opened.
Worked example: flooring for the L-shaped room
The L-shaped room above measures 280 sq ft. Laminate at 24 sq ft per box, with 10% waste:
280 × 1.10 = 308 sq ft → 308 ÷ 24 = 12.83 boxes → order 13 boxes.
Rounding down to 12 boxes leaves the job 20 sq ft short — about one box, which is exactly the box the pattern cuts consume.
The Mistakes That Cost Money
- Mixing units in one formula. Length in feet and width in inches produces a wrong answer every time. Convert every dimension to feet before multiplying.
- Writing inches as decimals. 6 inches is 0.5 ft, not 0.6 ft. The error is small per dimension and large after multiplication.
- Forgetting to subtract openings on wall area. A door and two windows can account for 45–50 sq ft on a single wall — enough to change a paint order by a gallon.
- Ordering to the exact area. No material installs with zero waste. The buffer is part of the quantity, not an optional extra.
- Measuring a bowed wall at its widest point. Use the smaller dimension and note the bow; the wider reading overstates the area and the order.
- Confusing floor area with footprint. A two-storey house has two floor areas measured separately and added. The footprint is the ground-floor outline only.