Triangle Area Calculator
Find the area of any triangle from a base and its perpendicular height — area = ½ × base × height — in whatever units you measure in.
One simple formula
Half the base times the perpendicular height gives the area of every triangle, whatever its shape.
Use the right height
The height must be measured at a right angle to the base — not along a slanted side, or the area comes out too large.
What is the area of a triangle?
The space inside the three sides
The area of a triangle is the amount of flat space enclosed by its three sides. For any triangle you only need two measurements: a base (any one side) and the perpendicular height to that base. The area is half the product of the two — area = ½ × base × height. The result is in the square of whatever unit you measure in, so metres give square metres and inches give square inches.
A triangle is exactly half of the rectangle (or parallelogram) that shares its base and height. That is why you multiply base by height and then halve the result — the triangle fills half of that box.
area = (base × height) ÷ 2The base can be any of the three sides; the height is always the straight-line distance from that base to the opposite corner, measured at a right angle. All three base-and-height pairs give the same area, so you simply pick the pair that is easiest to measure.
Suppose a triangle has a base of 10 and a perpendicular height of 6.
Multiply base by height
Multiply the two measurements: 10 × 6 = 60.Halve the result
Divide by two, because a triangle is half its bounding box: 60 ÷ 2 = 30.Read the area
The triangle covers 30 square units — square metres if you measured in metres, square feet if in feet.
The number you get is an area, so it is always expressed in square units of whatever you entered: a base and height in metres produce square metres, while feet produce square feet. The single most common mistake is using a slanted side instead of the perpendicular height — that always overstates the area, because the slanted side is longer than the true right-angle distance to the base. If your figure looks too big, check that the height really meets the base at a right angle. Remember too that any of the three sides can be the base as long as you pair it with its own matching height; all three pairings give the same answer, which is a handy way to double-check a measurement. When you know all three side lengths but no height, Heron's formula is the alternative route to the same area. Real-world uses are everywhere: land plots and triangular gardens, roof gables, boat sails, gable-end walls, and of course school geometry homework.
The formula is exact; the care is all in the measurement.
Measure the perpendicular height
This calculator assumes the height you enter is the true perpendicular distance to the base, measured at a right angle. Using the length of a sloping side instead will overstate the area. The tool is also unit-agnostic — keep the base and height in the same unit, and read the result as that unit squared. If you only have the three side lengths and no height, use Heron's formula instead of this base × height method.