Area Calculator
Calculate the area of a circle, square, rectangle or triangle. Pick a shape and enter its dimensions.
Dimension 1 = radius (circle) / side (square) / length (rectangle) / base (triangle). Dimension 2 = width (rectangle) / height (triangle).
What Is an Area Calculator?
An area calculator works out the amount of flat surface a two-dimensional shape covers, measured in square units such as square metres, square feet or square centimetres. This tool supports the four shapes you meet most often in school, construction and everyday DIY: the circle, the square, the rectangle and the triangle. For the straight-sided shapes it also reports the perimeter, the total distance around the outside edge.
Whether you are buying tiles for a floor, paint for a wall, turf for a garden or simply checking maths homework, knowing the area and perimeter saves money and avoids waste. Enter the measurements for your chosen shape and the calculator returns both values at once, using the standard geometry formulas explained below.
How It Works: The Formulas
Each shape uses a simple, well-established formula. Make sure every measurement is in the same unit before you calculate, otherwise the result will be wrong.
- Circle: area = pi × r², where r is the radius and pi is about 3.14159. The circumference (the perimeter of a circle) is 2 × pi × r.
- Square: area = side². The perimeter is 4 × side.
- Rectangle: area = length × width. The perimeter is 2 × (length + width).
- Triangle: area = 0.5 × base × height. The height must be the perpendicular distance from the base to the opposite point.
Area is always squared because you multiply two lengths together, while perimeter stays in plain linear units because you only add lengths.
Worked Example
Suppose you have a rectangular room that is 5 metres long and 4 metres wide and you want both its area and its perimeter.
Area = length × width = 5 × 4 = 20 square units. So the floor covers 20 square metres, which tells you how much flooring to buy.
Perimeter = 2 × (length + width) = 2 × (5 + 4) = 2 × 9 = 18 units. So the skirting board or fencing around the edge needs to be 18 metres long. With those two figures you can price up flooring by the square metre and edging by the running metre in one go.
Tips for Accurate Measurements
Accurate inputs matter more than the calculation itself. A few practical habits keep your results reliable.
- Keep units consistent. Do not mix centimetres with metres. Convert everything to one unit first.
- Use the radius, not the diameter, for circles. The radius is half the diameter. If you measured across the full width, divide by two before entering it.
- Measure perpendicular height for triangles. The height is the straight-up distance from the base, not the length of a slanted side.
- Add a margin for real projects. When buying materials, order around 10 percent extra to allow for cutting, breakage and offcuts.
For odd-shaped rooms, split the space into rectangles and triangles, find each area separately, then add them together to get the total.
Frequently Asked Questions
Use area = pi × radius². Multiply the radius by itself, then multiply by pi (about 3.14159). For a circle with a radius of 3 units, the area is 3.14159 × 9 = about 28.27 square units. Remember to use the radius, which is half the diameter.
Area is the surface a shape covers and is measured in square units, while perimeter is the distance around the outside edge and is measured in plain linear units. A rectangle 5 by 4 has an area of 20 square units and a perimeter of 18 units.
Use area = 0.5 × base × height. The height must be the perpendicular distance from the base to the opposite point, not the length of a sloping side. For a base of 6 and a height of 4, the area is 0.5 × 6 × 4 = 12 square units.
The calculator is unit-neutral, so the answer comes out in whatever unit you put in. If you enter metres, the area is in square metres and the perimeter is in metres. Just make sure every measurement uses the same unit before you calculate.
Split the irregular shape into simpler shapes such as rectangles and triangles. Calculate the area of each piece on its own, then add all the areas together to get the total. This method works for L-shaped rooms and most floor plans.
Area is found by multiplying two lengths together, for example length times width, so the unit is also multiplied by itself and becomes squared. That is why area is written as square metres or square feet, while perimeter, which only adds lengths, stays in plain units.