Area & Perimeter Calculator

What this calculator does

Give it the measurements of a shape and it returns both the area (the space inside) and the perimeter (the distance around the edge). It handles four everyday shapes: rectangles, squares, triangles, and circles. For a circle, "perimeter" is called the circumference, and the calculator labels it that way so you don't have to remember the difference.

When you'd reach for it

Area tells you how much material fills a surface; perimeter tells you how much edging goes around it. That split matches real projects. You need area to buy paint, flooring, turf, mulch, or fabric, and to price a job by the square foot. You need perimeter to buy fence panels, baseboard trim, picture-frame moulding, or a garden border. A common gotcha is buying by the wrong one, ordering enough sod to cover a lawn when you actually needed the length of fence around it.

A worked example

Say a room is 12 ft by 10 ft. Area is 12 × 10 = 120 ft² of flooring, and perimeter is 2 × (12 + 10) = 44 ft of baseboard. A circle of radius 5 ft gives area πr² ≈ 78.5 ft² and circumference 2πr ≈ 31.4 ft. The tool does this arithmetic instantly so you can try a few sizes without redoing the math each time.

How to use it

1. Pick the shape you're measuring: rectangle, square, triangle, or circle.
2. Type in the dimensions the form asks for, such as length and width, base and height, or radius.
3. Keep every measurement in the same unit (all feet, or all centimeters) so the result is consistent.
4. Read off the area and perimeter, then adjust a number and recalculate to compare options.

FAQ

What units does it use?

Whatever you enter. The calculator is unit-agnostic, so feed it inches, meters, or yards. Area comes back in those units squared and perimeter in the plain unit.

How do I find a triangle's area if I only know the sides?

This tool uses base × height ÷ 2, so measure the perpendicular height from the base to the opposite corner rather than a slanted side.

Is my data private?

Yes. Every calculation runs inside your browser and nothing you type is uploaded or stored.

Is it free?

Completely free and supported by the ads on the page. There's no signup and no limit on how many shapes you calculate.