Square footage is length times width — for a rectangle. Every other shape has its own one-line formula, and any awkward room is just a handful of simple shapes added together. Get those two ideas and you can measure anything, from a bedroom to a curved flower bed.
Rectangles and squares
Multiply length by width. A room 12 × 10 ft is 120 sq ft (a 3.7 × 3.0 m room is about 11.1 m²). A square is the same thing with equal sides — side × side. That is the whole calculation for the great majority of rooms and slabs.
Every shape’s formula
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Circle | π × radius² |
| Triangle | ½ × base × height |
| Trapezoid | ½ × (a + b) × height |
| Sector (pie slice) | (angle ÷ 360) × π × radius² |
| Ring / annulus | π × (R² − r²) |
Area Calculator
Pick a shape, type your measurements in feet, inches, metres or millimetres, and read the square footage — with a labelled diagram.
Odd or L-shaped rooms
Split the space into simple shapes — rectangles and triangles — work out each, and add them up. Most rooms break into two or three rectangles; a bay or a cut corner is usually a triangle or a sector. In the calculator, do each piece and press Add to total: it keeps a running total, lets you set a quantity for identical pieces, and adds an optional waste allowance, so you get the combined square footage as one figure.
Circles and rings
A circle is π × radius². Measure across the middle (diameter) or from centre to edge (radius) — the calculator takes either. A circle 10 ft across (5 ft radius) is about 78.5 sq ft; a 3 m diameter is about 7.1 m². A ring (annulus) is the area between an outer and inner circle — a circular path, pipe wall or hollow column — found by subtracting the inner disc from the outer.
Triangles
½ × base × height, where the height is measured at a right angle to the base — not along a sloping side. A 6 ft base and 4 ft height is 12 sq ft. If you only have the three side lengths, switch to Three sides and the calculator uses Heron’s formula instead.
From square footage to a materials order
Area is usually the first step, not the last. Once you have the square footage, carry it into the tile, paint, paving, plaster, turf or gravel calculators to turn it straight into a materials count — how many tiles, gallons of paint or cubic yards of gravel the area needs — and then a quote. The whole job, including the running total, round-trips in the page URL, so “Copy link” reproduces your estimate.
Working in metric?
The formulas are identical — only the units change. Enter metres or millimetres instead of feet and inches and the result comes back in m²; flip the units switch to see it either way. A 4 × 3 m room is 12 m², which is about 129 sq ft — the same physical floor, two ways of saying it.