Area and perimeter calculator

How it works

Formula: Circle: area = π × r², perimeter = 2 × π × r

Each shape has its formula: length × width for a rectangle, π × r² for a circle, base × height for a parallelogram, (base 1 + base 2) × height ÷ 2 for a trapezoid. For a triangle with three known sides, Heron's formula gives the area without the height. The perimeter is the length around the shape; for a circle, it is the circumference 2 × π × r.

Worked examples

• A 12 × 11 ft bedroom

  Area: 132 ft², perimeter: 46 ft.

• A round table, 24 in radius

  Area: 1,810 in², circumference: 150.8 in.

• A triangle of 5, 6 and 7 in

  Heron's formula: 14.7 in².

Common mistakes

• Using the diameter as the radius.

  With the diameter, the area is 4 times too big: 7,238 in² instead of 1,810 in².

• Mixing units.

  Put everything in the same unit before multiplying: 12 ft × 6 in is 6 ft², not 72.

• Multiplying a parallelogram's base by its slanted side.

  The area is base × height, the height being perpendicular to the base.

Area and perimeter formulas

Frequently asked questions