Triangle area calculator
Enter the base and the height dropped onto that base, and the calculator works out the area, showing the numbers substituted into the formula.
A triangle with base 8 and height 5 has an area of 20.00.
Formula
a is the length of the base and h the height dropped onto it. The division by two comes from a triangle being half a rectangle.
How we got there
The height must be perpendicular to the base. Using the length of a slanted side instead will overstate the answer.
How to work out the area of a triangle by hand
- Multiply the base by the height
8 × 5 = 40. That is the area of a rectangle with the same dimensions.
- Halve the result
40 ÷ 2 = 20. The triangle covers exactly half that rectangle.
- Make sure the height is perpendicular
Height is measured at a right angle to the base, not along the side of the triangle.
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Frequently asked questions
Multiply the base by the height and divide by two. For a base of 8 and height of 5: (8 × 5) ÷ 2 = 20.
The one that drops onto the base you entered, at a right angle. Each base has its own height.
Yes. The height may fall outside the triangle, but the formula is unchanged.
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