Area and perimeter calculator

Pick a shape, enter its measurements and get the area and perimeter.

How it works

Area measures the surface a shape covers and perimeter the length of its edge. Use the same unit for every measurement: the area comes out in that unit squared.

For triangles we use the three sides and Heron’s formula, so you don’t need the height.

Circle: A = π·r², P = 2·π·r
Triangle (Heron): A = √(s(s−a)(s−b)(s−c)), s = (a+b+c)/2

Frequently asked questions

How do I work out the square metres of a room?

Multiply length by width. If it’s L-shaped, split it into two rectangles and add their areas.

Why does my triangle give an error?

Those sides can’t close into a triangle: each side must be shorter than the other two added together.

More maths calculators

Scientific calculator — Sines, logarithms, powers, roots and factorials, with history.
Percentages — X% of an amount, what percentage, increases, decreases and change.
Rule of three — Direct and inverse proportion, with the working.
Fractions — Add, subtract, multiply, divide and simplify fractions.
Powers, roots and logarithms — Any exponent, nth root and logarithm in any base.
Equations — Linear, quadratic and systems of two equations.
Mean, median and mode — Mean, median, mode, range, variance and standard deviation of a list.
GCD and LCM — Greatest common divisor, least common multiple and prime factors.
Roman numerals — Convert numbers to Roman numerals and back.
Random numbers — Random numbers between two values, with or without repeats.