Calculate area and perimeter for a circle, rectangle or triangle. Choose a shape and enter your measurements.
For the circle, r is the radius, i.e. the distance from the center to the edge. The diameter is twice the radius.
For the rectangle, you find the diagonal by applying the Pythagorean theorem to the length and width.
For the triangle, you only need the base and height (the perpendicular distance from the base to the opposite corner) to find the area. The perimeter of a triangle, on the other hand, requires all three side lengths, not just base and height — this calculator therefore doesn't ask for perimeter in triangle mode.
If you don't know the height but know all three side lengths, you can use Heron's formula instead. This calculator uses the simpler formula base × height ÷ 2, which requires knowing the height.
The perimeter is the sum of all the sides of the shape. For a triangle, the base and height aren't the same as the three side lengths, so you need to measure or calculate all three sides separately to find the perimeter.
No, the formulas above only apply to circles, rectangles and triangles with a known base and height. Irregular shapes require other methods, such as splitting the shape into several known sub-shapes.