How to use the Area Calculator (Rectangle, Triangle, Circle, Trapezoid)
- Choose the shape.
- Enter the sides, height or radius that the shape needs.
- Read the area and the formula used.
- Convert to another area unit if you need to, with the area converter.
Formula used
Rectangle l × w. Triangle ½ × base × height. Parallelogram base × height. Trapezoid ½ × (a + b) × height. Circle π × r².
Worked example
Here is the Area Calculator (Rectangle, Triangle, Circle, Trapezoid) worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| Shape | Rectangle |
| Side 1 (length, base, or first parallel side) | 8 |
| Side 2 (width, or second parallel side of a trapezoid) | 5 |
| Height (triangle, parallelogram, trapezoid) | 4 |
| Radius (circle) | 3 |
| Result | |
| Area | 40 |
| Formula used | length × width |
Formulas for the five shapes
A rectangle is length × width: 8 × 5 = 40. A triangle is ½ × base × height: with base 8 and height 4 that is 16. A parallelogram is base × height, 32 for the same numbers.
A trapezoid has two parallel sides a and b: area = ½ × (a + b) × height. With 8, 5 and a height of 4 it is 26. A circle is πr², so a radius of 3 gives 28.27.
Which measurements to use
For a triangle, parallelogram or trapezoid the height is the perpendicular distance between the base and the opposite side or corner, not the length of a slanted side. Measuring along the slant is a common mistake that overstates the area.
For a circle, use the radius, the distance from the centre to the edge. If you have the diameter, halve it. The circle calculator on this site works from any one of radius, diameter, circumference or area.
Units of area
Area units are squared lengths: 1 square metre is 10,000 square centimetres and about 10.764 square feet. An acre is 43,560 square feet, about 4,047 square metres, and a hectare is 10,000 square metres or about 2.47 acres.
Do not convert area by the length factor alone. Going from metres to centimetres multiplies lengths by 100 but areas by 100 × 100 = 10,000.
Irregular shapes and practical uses
For an L-shaped room or an unusual plot, split it into rectangles and triangles, find each area and add them. Subtract cut-outs, such as a window from a wall. This is also how you estimate flooring, paint, turf or tiles.
Add a margin for waste, often 5 to 10% for flooring and tiles, and check the coverage on the product. This calculator gives the geometric area, not the quantity of material to buy.
Frequently asked questions
How do I find the area of a rectangle?
Multiply the length by the width. A rectangle 8 by 5 has an area of 40.
What is the area formula for a triangle?
Half the base times the perpendicular height.
How do I find the area of a trapezoid?
Add the two parallel sides, multiply by the height and halve the result.
What unit is area in?
Square units of your length unit, such as m² or ft².
Do I enter the slanted side for a parallelogram?
No. Use the base and the perpendicular height, not the slanted side.
How do I find the area of an irregular shape?
Break it into simple shapes, find each area and add them up.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.