Area Calculator
Calculate the area of common shapes: rectangle, triangle, circle, trapezoid, ellipse and more. Free and instant — the formula and working are shown, all in your browser.
Enter all lengths in the same unit — the area is in that unit squared (e.g. cm → cm²).
How to use the area calculator
- Pick a shape. Choose from rectangle, square, triangle (by base and height or by three sides), circle, ellipse, trapezoid, parallelogram, rhombus, circular sector or regular polygon.
- Enter the measurements. The input fields change to match the shape's formula. Use the same unit for every length — decimals are fine.
- Read the area. The result appears instantly, together with the formula and your numbers substituted in, ready to copy into homework or notes.
About this tool
This free area calculator covers the plane shapes you meet in school geometry and everyday measuring jobs. Each shape uses its standard formula: length × width for a rectangle, ½ × base × height for a triangle, πr² for a circle, ½(a + b)h for a trapezoid, π × a × b for an ellipse, base × height for a parallelogram, half the product of the diagonals for a rhombus, (θ/360) × πr² for a circular sector, and n·s² / (4·tan(π/n)) for a regular polygon with n sides. When you only know a triangle's three sides, it switches to Heron's formula, computing the semi-perimeter first and validating that the sides can actually form a triangle.
Use it to estimate flooring, paint, lawn seed or fabric, to check geometry homework with the working shown, or simply to avoid remembering eleven formulas. Everything is calculated locally in your browser at double precision — nothing you type is uploaded, stored or logged, and the page keeps working offline once loaded.
Frequently asked questions
What units does the area come out in?
Whatever unit you enter, squared. Measure in metres and the area is in square metres (m²); measure in inches and it is in square inches. Just make sure every length in one calculation uses the same unit — mixing cm and m will give a wrong result.
How do I find the area of a triangle from its three sides?
Select “Triangle (three sides)”. The tool applies Heron's formula: first the semi-perimeter s = (a + b + c) / 2, then A = √(s(s − a)(s − b)(s − c)). If the three lengths break the triangle inequality (one side as long as or longer than the other two combined), the tool tells you no such triangle exists.
What is the difference between the ellipse's a and b?
They are the semi-axes: half the widest diameter (a) and half the narrowest diameter (b), measured from the centre. The area is π × a × b. If a = b the ellipse is a circle and the formula reduces to πr².
Which measurements does a trapezoid need?
The lengths of the two parallel sides (a and b) and the perpendicular height h between them — not the slanted legs. The area is the average of the parallel sides times the height: ½ × (a + b) × h.