MetraNestIdentify the two parallel bases
On your sketch, identify the two sides that are parallel. Call them A and B and measure each in the same unit. Swapping the bases does not change the area. Equal lengths give the same area calculation as a rectangle with that base and height.
Measure the perpendicular distance
The height is the distance between the lines of the bases, measured at a right angle. A sloping edge may be longer and must not be entered in its place. If the two sides are not parallel, divide the area into known shapes or take more measurements.
Use the area without inventing side lengths
The result can be a starting area for buying a covering or for another calculation. It includes no cutting allowance. The diagram centres the bases to illustrate the formula; trapezoids with the same area may have different offsets and perimeters.
Area = (base A + base B) × perpendicular height ÷ 2.
Two bases of 6 m and 4 m
With a perpendicular height of 3 m, the average of the bases is (6 + 4) ÷ 2 = 5 m. The area is therefore 5 × 3 = 15 m². The length of a sloping side is not used in this example.
Details to check
- Use a single unit for all three lengths.
- Check that the bases are parallel on your sketch.
- Add any allowance in the following purchasing calculation.
Frequently asked questions
Can I swap bases A and B?
Yes. Their sum and average remain the same, so the area does not change.
Why is the perimeter not shown?
The two bases and the height do not determine the two sloping sides. You would need to measure those sides or know the offset between the bases.
A method you can check · Content review :
Each tool shows its formulas and assumptions. Worked examples let you repeat the calculation and understand how rounding works.