Area Calculator (rectangle, circle, triangle…)

Area formulas

Area measures the surface a flat shape covers. Every formula below is a variation on one idea — multiply a width by a height — adjusted for the shape's geometry. Pick a shape and enter its dimensions; the answer is in whatever square units your inputs use (centimetres in, cm² out).

Rectangle: A = w × h  ·  Square: A = a²

Triangle: A = ½ × b × h  ·  Parallelogram: A = b × h

Circle: A = πr²  ·  Ellipse: A = πab

Trapezoid: A = ½ × (a + b) × h

How they relate

A triangle is half a parallelogram with the same base and height, which is where the ½ comes from — and a parallelogram is a rectangle with its top slid sideways, so its area is still base × height. A trapezoid averages its two parallel sides and multiplies by the height. An ellipse is a stretched circle, so πr² becomes πab using its two semi-axes.

Height means perpendicular height

The commonest mistake with triangles, parallelograms, and trapezoids is using a slanted side as the height. The height must be measured at right angles to the base, not along an edge — using the sloping side gives an answer that's always too large.

Worked example

A triangle with a 6 cm base and 4 cm perpendicular height has an area of ½ × 6 × 4 = 12 cm². A circle of radius 3 cm covers π × 3² ≈ 28.27 cm². To convert the result into other units, use thearea converter.

Real shapes are rarely the ideal ones

These formulas are exact for the shapes they name. The uncertainty is in deciding that your shape is one of them, and in the measurement — rooms are not square, walls are not straight, and a floor measuring 3.60 m on one side is often 3.65 m on the other. Measure both, use the larger, and for an awkward outline break it into rectangles and circles and total those rather than averaging it into a single figure. That approach also leaves you with numbers you can check afterwards.