Area of a trapezium: in short
Add the two parallel sides, divide by 2 and multiply by the height. A trapezium with parallel sides of 6 m and 3 m and a height of 4 m has an area of (6 + 3) ÷ 2 × 4 = 18 m².
The formula
Area = (a + b) ÷ 2 × h, where a and b are the two parallel sides and h is the height, measured at a right angle between them. It is the average of the parallel sides times the height. In US books the same shape is called a trapezoid.
Examples
| Bottom side | Top side | Height | Area |
|---|---|---|---|
| 4 m | 2 m | 2 m | 6 m² |
| 5 m | 3 m | 3 m | 12 m² |
| 6 m | 3 m | 4 m | 18 m² |
| 8 m | 5 m | 4 m | 26 m² |
| 10 m | 6 m | 5 m | 40 m² |
Right and isosceles trapeziums
In a right trapezium one side makes a right angle with the parallel sides: that side is the height. In an isosceles trapezium the two slanting sides are equal. The formula is the same for all of them.
Trapeziums on a plot or in a house
A plot with one slanting boundary, a terrace that narrows or the wall of a loft are often trapeziums. Measure the two parallel sides and the distance between them, not the slanting side.
Figure 1. One floor of 7.5 × 5 m. The rooms add up to 37.5 m².
About this page · Written by the Squaremetry team. Last checked on 7 October 2026. Found a mistake? Send us a message.