Two ways to find the area
With a base and a perpendicular height, the area is half of base times height. The height must meet the base at a right angle, not run along a sloping side.
With all three sides, Heron's formula gives the area: work out half the perimeter, then multiply that by itself minus each side, and take the square root.
Worked example
A triangle with a base of 12 cm and a height of 8 cm has an area of 12 x 8 / 2 = 48 cm², which is 0.0048 m².
A triangle with sides 6 cm, 8 cm and 10 cm has half-perimeter 12, so the area is the square root of 12 x 6 x 4 x 2 = 24 cm². Its perimeter is 24 cm and it is a scalene, right-angled triangle.
Which sides make a triangle
Any two sides must add up to more than the third. Sides of 2, 3 and 6 cannot make a triangle, and the calculator says so instead of returning a nonsense area.
A right angle shows up as the square of the longest side equalling the squares of the other two added together.
Inputs for the triangle calculator
What do you know?
Choose base and height, or all three sides. The boxes below change to match.
Base and height
The base is any side; the height is the perpendicular distance from that side to the opposite corner.
Side a, b and c
The three sides. Any two must add up to more than the third.
Measurement unit
The unit your measurements are in. The area is given in square metres.
CALCULATOR QUESTIONS
Triangle calculator questions
How do I find the area of a triangle?
Multiply the base by the perpendicular height and halve it. A base of 12 cm and height of 8 cm gives 48 cm².
What is Heron's formula?
A way to find area from three sides: with half-perimeter s, the area is the square root of s(s-a)(s-b)(s-c).
Why does the height have to be perpendicular?
The formula measures how far the opposite corner is from the base line. A sloping side measures along the edge instead, which gives a different number.
Can a triangle have sides 2, 3 and 6?
No. Two sides must add up to more than the third, and 2 + 3 is less than 6.