Area Calculator · Practical guide
How to Calculate the Area of a Triangle
Area = ½ × base × height. Use the perpendicular height, not the slanted side.
At a glance
The Formula
The area of a triangle is ½ × base × height, written A = ½bh. A triangle with a base of 10 and a height of 6 has an area of ½ × 10 × 6 = 30 square units. The formula works for every triangle — acute, right, or obtuse — as long as the height is measured perpendicular to the base.
Interactive tool
Area Calculator
Enter your own values, then compare the result with the examples below.
Results
Formula Used:
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Worked Examples
| Base | Height | Area |
|---|---|---|
| 10 | 6 | 30 |
| 8 | 5 | 20 |
| 12.5 | 4 | 25 |
| 7 | 9 | 31.5 |
| 3 (right triangle) | 4 | 6 |
Height Means Perpendicular Height
The most common mistake is using a slanted side as the height. The height must form a right angle with the base. In an obtuse triangle, the perpendicular height can even fall outside the triangle — the formula still applies. For a right triangle, the two legs serve as base and height directly, so a 3-4-5 triangle has an area of 6.
Area vs Perimeter
Area uses only the base and height, but the perimeter needs all three sides: P = a + b + c. If a triangle has a base of 10, height of 6, and slanted sides of 8 and 9, the area is still 30 while the perimeter is 10 + 8 + 9 = 27. The calculator above reports both.
Frequently Asked Questions
What if I only know the three sides?
Use Heron's formula: compute s = (a+b+c)/2, then area = the square root of s(s-a)(s-b)(s-c). For sides 8, 9, and 10 that gives about 34.2 square units.
Why is the triangle area half of base times height?
Any triangle is exactly half of a parallelogram with the same base and height, and a parallelogram's area is base times height.
Does the formula work in any unit?
Yes. Enter base and height in the same unit and the area comes out in that unit squared: feet give square feet, meters give square meters.
How do I find the area of a right triangle?
Multiply the two legs and divide by two. A right triangle with legs of 3 and 4 has an area of 6 square units.