Trapezoid Area Calculator
Calculate the area of a trapezoid (trapezium) from its two parallel bases and height. Formula: Area = 1/2 x (a + b) x h.
How to use this tool
- Enter base 1 (parallel side a), base 2 (parallel side b) and height (perpendicular) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your area and the full breakdown beneath it.
Formula
Area = 1/2 x (a + b) x h
How it works
Add the two parallel bases, multiply by the perpendicular height, then divide by 2. When both bases are equal the shape is a rectangle.
Worked example
Trapezoid with bases 6 m and 4 m, height 5 m
- Area = 1/2 x (6 + 4) x 5
- Area = 1/2 x 10 x 5 = 25 m^2
25 m^2
Common mistakes to avoid
- Adding only one base instead of both parallel bases before multiplying by h/2.
- Using the slant leg length as the height rather than the perpendicular distance between the two parallel sides.
- Forgetting to divide by 2, computing (a+b) x h instead of 1/2 x (a+b) x h.
Key terms
- What is a trapezoid?
- A quadrilateral with exactly one pair of parallel sides (called bases).
- Trapezoid vs trapezium?
- In the US, trapezoid means one pair of parallel sides; in the UK, the same shape is called a trapezium.
Frequently asked questions
- How do I identify which sides are the parallel bases?
- The two bases are the sides that run parallel to each other. For a standard trapezoid drawn flat, they are the top and bottom horizontal sides.
- Can I use this formula if one base has length zero?
- Yes. When one base is zero the shape is a triangle and the formula reduces to 1/2 x base x height.
- I have the four side lengths but not the height. Can I still find the area?
- Not directly. You would need to derive the height from the side lengths using trigonometry or the Pythagorean theorem first.