Pick a shape, enter its dimensions in whatever unit you measured in, and get the area in that unit squared plus the same figure converted to the other measuring system. Most people arrive here on the way to buying flooring, paint or turf, so the conversion is done for you.
Key terms
- Area — the amount of surface inside a shape, always measured in squared units.
- Perpendicular height — the straight-up distance from the base at a right angle, not the length of a slanted side.
- Radius — from the centre of a circle to its edge; the diameter is twice that, all the way across.
How it works
Six shapes
rectangle = w × l · triangle = b × h ÷ 2 · circle = πr² · trapezoid = (a + b) ÷ 2 × h · parallelogram = b × h · ellipse = πab
The trapezoid formula is the general case: it averages the two parallel sides, and a rectangle is simply the version where they are equal.
Area grows with the square of the sides
Make a room 20% wider and 20% longer and you need 44% more flooring, not 40%. Double every dimension and the area quadruples. This catches people out constantly when scaling a previous job “up a bit” — the estimate always comes in short, and it comes in short by more the bigger the change.
The arithmetic is simple once you see it: 1.2 × 1.2 = 1.44. Any percentage applied to both dimensions gets multiplied by itself.
Height means perpendicular height
In a triangle, base × height ÷ 2 only works if the height is measured straight up from the base at a right angle. Use the length of a slanted side instead and the answer comes out too large, because the slanted side is always longer than the perpendicular. The two are only the same thing in a right-angled triangle, where one side is the height.
Radius or diameter — a factor-of-four difference
A tape measure laid across a round table gives you the diameter. The circle formula wants the radius, which is half of it. Feed the diameter in as a radius and the area comes out four times too big, because the radius is squared. That is why this calculator asks which one you measured rather than assuming.
How to use this calculator
- Choose the shape and the unit you measured in.
- Enter the dimensions. Only the fields that shape needs are shown.
- Read the area in your unit, and the conversion if you are buying in the other system.
- For flooring or paint, add a waste allowance on top — 10% is a common rule for straight rooms, more for diagonal patterns.
Frequently asked questions
How do I calculate the area of a room that is not rectangular?
Split it into rectangles and triangles, work out each one, and add them up. An L-shaped room is two rectangles. This is faster and far less error-prone than looking for a single formula.
How many square feet is a square metre?
About 10.764. It is the square of 3.28084 feet per metre — another place the squaring catches people, since the length conversion and the area conversion are different numbers.
What is the difference between area and perimeter?
Area is the surface inside, measured in squared units, and tells you how much flooring to buy. Perimeter is the distance around the edge, measured in plain units, and tells you how much skirting board to buy.
How do I find the area of a circle from its circumference?
Divide the circumference by 2π to get the radius, then use πr². A circumference of 18.85 gives a radius of 3 and an area of 28.27.
Why does my flooring supplier quote more than this figure?
Because cutting produces waste and most people add 5–10% for offcuts, plus extra for diagonal or patterned layouts. This calculator gives the true area; the allowance is a separate decision.
