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rectangle · circle · triangle · trapezium

Area calculator

Pick a shape, enter its dimensions, and read the area, the perimeter where it is determined, and the formula with your numbers in it.

Short answer: a 4 × 5 rectangle is 20 square units with a perimeter of 18; a circle of radius 1 is π ≈ 3.14159265; a triangle with sides 3, 4 and 5 is exactly 6.

Reading the formula line

Every row prints the general formula and then the substituted version, so a homework answer can show the working rather than only the result. Where a quantity is not determined by the inputs — a triangle’s other two sides from base and height — the perimeter is left blank instead of guessed.

Worked example

A room 4.2 m by 3.6 m: area 15.12 m², perimeter 15.6 m. Paint covers about 10 m² per litre per coat, so one coat needs 1.5 litres — which is the calculation this page feeds into.

Area questions

Which shapes are covered?

Rectangle, square, circle, triangle by base and height, triangle by three sides using Heron’s formula, and trapezium. Each shows its formula and the numbers substituted, so the answer can be checked by hand.

How do I find a triangle’s area without the height?

Use the three-side option: Heron’s formula takes the semi-perimeter s = (a + b + c) ÷ 2 and computes √(s(s−a)(s−b)(s−c)). Three sides of 3, 4 and 5 give an area of exactly 6.

Why is no perimeter shown for some shapes?

Because it cannot be derived from the inputs: a triangle’s base and height do not determine its other two sides, and a trapezium’s parallel sides and height say nothing about its legs. The page leaves the field blank rather than inventing a number.

What units does the answer come in?

Whichever you entered, squared. Metres in give square metres, feet in give square feet — no conversion is applied, because mixing units silently is how area answers end up wrong by a factor of ten.

Related tools

Volume calculatorCubes, boxes, cylinders, spheres and cones.Unit converterSquare metres, feet, hectares and acres.Density calculatorTurn a volume into a mass.