Area and Perimeter Calculator
Calculate the area and perimeter of a square, rectangle, triangle, circle, or trapezoid from its dimensions, with the formula used displayed for each calculation—ideal for checking a geometry exercise.
How it works
Select the geometric shape, enter its dimensions in the unit of your choice (results will be in the same unit, squared for area), then click "Calculate". The exact formula used for the area and perimeter is displayed next to the result to help you understand the calculation.
Why does the triangle require both a height and sides?
The area of a triangle is calculated from its base and height (the perpendicular distance between the base and the opposite vertex), while the perimeter requires the length of all three sides. These two pieces of information cannot be automatically derived from each other for an arbitrary triangle.
Does a circle have a "perimeter"?
For a circle, we usually say circumference, calculated as 2 × π × radius. The tool still displays it in the "Perimeter" field to stay consistent with the other shapes, since it is indeed the length of the figure's outline.
All calculations are performed locally in JavaScript in your browser, with no data transmitted to a server.