Area is the surface inside a shape — for a rectangle it is simply length × width, which covers most rooms, gardens, and floors.
Area is the amount of surface inside a shape, and for a rectangle it is simply length times width. That single formula covers most rooms, gardens, and floors you will ever measure. Everything else is either a different formula for a different shape, or an irregular space you break into rectangles and add together. This guide gives you the formulas for every common shape with worked examples, shows how to handle L-shaped rooms and circular patios, and covers converting between square metres and square feet without getting the factor wrong. The CalcRange Area Calculator handles the arithmetic once you know which shape you are dealing with.
What Area Is
Area measures two-dimensional space, which is why the units are always squared. A square metre is a square measuring one metre on each side, and any shape covering the same amount of surface has an area of one square metre regardless of its outline.
The squaring is what trips people up when converting. Doubling the length of a room’s sides quadruples its area, not doubles it, because both dimensions grew.
Formulas for Every Common Shape
| Shape | Formula | What the letters mean |
|---|---|---|
| Square | s² | s = side length |
| Rectangle | l × w | l = length, w = width |
| Triangle | ½ × b × h | b = base, h = perpendicular height |
| Circle | π × r² | r = radius, π ≈ 3.1416 |
| Parallelogram | b × h | h = perpendicular height, not the slanted side |
| Trapezoid | ½ × (a + b) × h | a and b = the two parallel sides |
| Ellipse | π × a × b | a and b = the two semi-axes |
Two of these carry a trap worth flagging. For triangles and parallelograms, the height must be measured perpendicular to the base, straight up at a right angle, not along the sloping edge. Using the slant length inflates the answer.
And circles use the radius, which is half the diameter. Measuring across a circular patio gives you the diameter, so halve it before squaring.
Worked Examples
Rectangular room
A room 5.5 m by 4.2 m: 5.5 × 4.2 = 23.1 m²
Triangular garden bed
Base 6 m, perpendicular height 4 m: ½ × 6 × 4 = 12 m²
Circular patio
Measured 6 m across, so the radius is 3 m: π × 3² = 3.1416 × 9 = 28.27 m²
Trapezoidal plot
Parallel sides of 5 m and 8 m, 4 m apart: ½ × (5 + 8) × 4 = ½ × 13 × 4 = 26 m²
Calculate an Area
Pick your shape, enter the dimensions, and the CalcRange Area Calculator returns the area. For three-dimensional spaces, the volume calculator takes over, and the unit converter switches between measurement systems.
Irregular and L-Shaped Spaces
Real rooms are rarely tidy rectangles, and there is no formula for an L-shape. There is a method instead, and it works for almost any irregular outline.
Splitting
Divide the space into rectangles, calculate each, and add. An L-shaped room splits naturally into two rectangles along the inside corner. Say the first is 5 m × 4 m = 20 m² and the second is 3 m × 2 m = 6 m², giving 26 m² in total.
Subtracting
Sometimes it is quicker to treat the space as a full rectangle and remove what is missing. A room that would be 8 m × 6 m if not for a 3 m × 2 m notch in one corner is 48 − 6 = 42 m².
Both routes reach the same answer, so use whichever needs fewer measurements. Subtraction is usually faster when the missing piece is small.
Genuinely curved outlines
For a garden with a wandering boundary, approximate it with a series of rectangles and triangles that follow the shape, then add them. It is an estimate rather than an exact figure, but the errors tend to cancel out and it is accurate enough for ordering turf or paving.
Square Metres and Square Feet
This is the conversion most often done wrong, because people reach for the linear factor.
Square metres to square feet: multiply by 10.764
Square feet to square metres: multiply by 0.0929
One metre is 3.281 feet, but one square metre is 3.281² = 10.764 square feet. Multiplying an area by 3.281 rather than 10.764 understates it by more than two thirds.
The room from earlier: 23.1 m² × 10.764 = 248.6 square feet.
For larger plots, an acre is 4,047 m² or 43,560 square feet, and a hectare is 10,000 m², which is about 2.47 acres.
Where You Actually Need This
Flooring, tiles, and turf are all sold by area, and all three need a waste allowance on top of the calculation. Around 10% is standard for straight-laid flooring, rising to 15% for diagonal or herringbone patterns where offcuts are less reusable.
Paint uses wall area rather than floor area, which is the perimeter times the height, and our guide on how much paint you need covers that calculation and the openings you should subtract.
Concrete needs area first, then depth to reach a volume, and the concrete quantity guide takes it from there.
Property listings quote area as a measure of size, though what gets counted varies by country. Some measures include internal walls, balconies, or shared space, so comparing two listings from different markets on floor area alone can mislead.
Common Mistakes
- Mixing units. Convert everything to metres or everything to feet before multiplying, never one of each.
- Using the linear conversion factor on an area. It is 10.764, not 3.281.
- Using diameter instead of radius in the circle formula, which produces four times the correct answer.
- Measuring the slanted side of a triangle or parallelogram instead of the perpendicular height.
- Forgetting the waste allowance when ordering flooring or tiles.
- Trusting a floor plan over a tape measure. Plans show nominal dimensions and buildings deviate from them.
Frequently Asked Questions
How do I calculate the area of a room?
Multiply length by width for a rectangular room, with both in the same unit. A room 5.5 m by 4.2 m has an area of 23.1 m². For L-shaped rooms, split the space into rectangles, work each out, and add them together.
How do I convert square metres to square feet?
Multiply by 10.764. This is the square of the linear conversion factor, because both dimensions are being converted. Using 3.281 by mistake is the single most common error in area conversion.
What is the formula for the area of a circle?
Pi times the radius squared, with pi approximately 3.1416. The radius is half the diameter, so a circle measuring 6 m across has a 3 m radius and an area of 28.27 m². Squaring the diameter instead gives four times too much.
How do I find the area of an irregular shape?
Break it into rectangles and triangles, calculate each, and add them. Alternatively, treat it as a larger regular shape and subtract the missing pieces. For curved boundaries, approximating with a series of simple shapes gives a good working estimate.
Why does the height have to be perpendicular?
Because the formula measures how far the shape extends away from its base at a right angle. Using a sloping side overstates that distance and inflates the area, and the error grows the more the shape leans.
How much extra flooring should I order?
About 10% above the calculated area for straight-laid flooring, and closer to 15% for diagonal or herringbone layouts where cuts produce less reusable offcut. Keeping a small surplus is also worth it for future repairs, since batches vary.
Is area the same as square footage?
Square footage is simply area expressed in square feet. The concept is identical; only the unit differs. Property markets in the US, UK, and much of South Asia commonly quote square feet, while most of Europe uses square metres.
The Bottom Line
Length times width handles most of what you will measure, and the rest is a short table of formulas for triangles, circles, and trapezoids. When a space refuses to be one shape, split it into rectangles and add, or take a bigger rectangle and subtract the gap. Watch two things above all: measure perpendicular height rather than a sloping edge, and convert areas with 10.764 rather than the linear factor. Then add 10% before ordering anything sold by the square metre. Run your dimensions through the area calculator and measure twice, since a tape is cheaper than a second delivery.
Last reviewed: August 2026. Evergreen content — recommended editorial review: every 12 months.