How to Find the Area of an L-Shaped Room or an Irregular Yard
Most rooms and gardens are not neat rectangles. They have alcoves, bay windows, angled walls and curved beds. Yet every flooring, paint and landscaping estimate starts with an area, so it helps to have a reliable way to measure the awkward ones.
The approach is always the same: break the plan into shapes you know, find each area, then add the pieces or subtract the gaps. You need only four formulas, a tape measure and a sketch.
The four formulas you need
| Shape | Area formula | Measurements |
|---|---|---|
| Rectangle | length × width | Two sides |
| Triangle | ½ × base × height | Base and the perpendicular height |
| Trapezoid | ½ × (a + b) × height | Two parallel sides and the distance between them |
| Circle | π × radius² | Radius (half the width across) |
The height of a triangle or trapezoid is measured at a right angle to the base, not along the slanted side. If a triangle is made of three long sides that you cannot easily square up, measure a perpendicular height with a straight edge and a level, or check your figures with the area of a triangle calculator.
An L-shaped room
Draw a rough plan and mark every wall length. Then there are two equivalent ways to split an L.
By adding: a main area of 16 ft by 12 ft plus a wing of 8 ft by 6 ft. That is 16 × 12 = 192 ft², plus 8 × 6 = 48 ft², for a total of 240 ft².
By subtracting: imagine the whole shape as a bounding rectangle of 16 ft by 18 ft, which is 288 ft², and remove the missing corner of 8 ft by 6 ft (48 ft²). Again 288 − 48 = 240 ft².
Doing the sum both ways is a good check on your measurements: if the two answers disagree, one of the wall lengths is probably wrong. To convert the result, 240 ft² is about 22.3 m², found by dividing by 10.764. The area calculator and the square footage calculator are handy for checking each piece before you add them up.
A yard with angled and curved edges
Suppose a lawn area has two parallel sides of 20 ft and 14 ft, set 10 ft apart. It is a trapezoid, with area ½ × (20 + 14) × 10 = 170 ft². A round flower bed 8 ft across sits inside it. The radius is 4 ft, so the bed is 3.1416 × 4² = 50.3 ft². The lawn that is left, for seed or turf, is 170 − 50.3 = 119.7 ft².
The area of a circle calculator is useful for a quick check on rounded features. For a bed that is a half circle, find the full circle and halve it.
When the boundary wiggles
A winding border has no exact formula. Use strips instead. Lay a straight line down the long axis, measure the width of the shape at regular intervals, and apply the trapezoid rule: add half of the first and last widths to all the widths in between, then multiply by the spacing.
area ≈ spacing × (first ÷ 2 + middle widths + last ÷ 2)More measurements give a closer answer.
Worked example: a bed 20 ft long with widths of 6, 8, 7, 9 and 5 ft measured every 5 ft. Half of the ends is 3 + 2.5 = 5.5, and the middle widths add up to 8 + 7 + 9 = 24. The total is 29.5, and 5 × 29.5 = 147.5 ft². Using narrower strips improves accuracy; the answer is always an estimate for a curved edge.
Avoiding common mistakes
- Use one unit for every measurement. Convert inches to decimal feet before multiplying.
- Check that corners really are square. Measure 3 ft along one wall and 4 ft along the other; if the corner is square, the diagonal between those marks is 5 ft.
- Subtract fixed features only when they affect the job, for example a tub that will not be tiled underneath.
- Measure to the nearest inch or centimetre and be consistent. A 1 in error on a 20 ft wall is 1 ÷ 240, about 0.4%, which is small next to a normal waste allowance.
- Add your waste allowance after you have the net area, not before.
Once you have the area, you can move on to quantities. See our guides to flooring and to tiles for the next steps.
- Area Calculator: Choose a shape, enter the measurements you know and get its area in the unit you prefer. Triangles can be solved from sides, angles, or a hypotenuse and a leg.
- Area of a Triangle Calculator: Choose what you know about the triangle, enter the measurements and get the area in the unit you prefer. The guide lists the formula behind each case and works through an example for every one of them.
- Area of a Circle Calculator: Enter the radius and pick a unit to get the area of the circle in square units. The guide covers the formula, how to start from a diameter or circumference, and a quick lookup table.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
More articles
Cubic Feet, Cubic Yards and Cubic Metres: Converting Job Volumes
Bags are labelled in cubic feet, trucks deliver cubic yards or cubic metres, and depth is in inches or millimetres. These are the conversions that tie them together.
How Many Bricks or Blocks Do You Need for a Wall?
A wall needs a fixed number of bricks or blocks per unit of area once you include the mortar joint. Work that number out once and the rest is multiplication.
How Much Concrete Do You Need? Slabs, Footings and Post Holes
Concrete is bought by volume, and every shape reduces to length times width times depth. Here is how to get the number, turn it into bags, and avoid ordering short.