Area Calculator — Triangle, Circle, Rectangle & More

About the Area Calculator

This works out the area — and the perimeter or circumference — of the common geometric shapes, including triangles solved from either three sides or a base and height. It's a handy helper for geometry homework, DIY and home projects (flooring, paint, turf, fabric), gardening beds, and any job where you need to measure a shape. Pick a shape, enter its dimensions, and get the results instantly. Need more than area — logs, powers, trig? Try the Scientific Calculator.

How it works — the formulas

Triangle (3 sides) — Heron's formula: with s = (a+b+c)⁄2, area = √(s(s−a)(s−b)(s−c)); perimeter = a+b+c.
Triangle (base & height) — area = ½ × base × height.
Rectangle — area = length × width; perimeter = 2(length + width).
Square — area = side²; perimeter = 4 × side.
Circle — area = πr²; circumference = 2πr.
Parallelogram — area = base × height; perimeter = 2(base + side).
Trapezoid — area = ½ × (a + b) × height.

Assumptions and behaviour

Dimensions are unitless — enter everything in the same unit (all cm, all inches, etc.) and the area comes out in that unit squared. Mixing metric and imperial measurements first? Convert them with the Length Converter.
For a triangle from three sides, the sides must satisfy the triangle inequality (any two sides must sum to more than the third), or the shape is impossible and it's flagged.
Base-and-height triangle and trapezoid modes give area only, since the perimeter needs the slanted side lengths.
Circle uses the full-precision value of π; results are rounded for display.

Limitations

It covers common flat (2D) shapes; it doesn't do 3D solids (volume/surface area), regular polygons beyond these, or irregular shapes.
Area only is available where the perimeter can't be found from the inputs (base-and-height triangle, trapezoid) — supply all sides for a full perimeter.
It assumes your measurements are accurate and in consistent units; mixing units gives a meaningless result.
For a triangle, three valid sides must actually form a triangle, or no area exists.

Privacy

The calculation runs entirely in your browser. Nothing is uploaded or stored.

Frequently asked questions

Is this area calculator free?

Yes — free, no sign-up, no limits, runs in your browser.

Which shapes can it handle?

Triangles (from three sides or base and height), rectangles, squares, circles, parallelograms and trapezoids — with area and, where possible, perimeter.

How do I find a triangle's area from its three sides?

It uses Heron's formula: half the perimeter s = (a+b+c)/2, then area = √(s(s−a)(s−b)(s−c)). Just enter the three side lengths.

What units does it use?

Whatever you enter — it's unitless. Keep every dimension in the same unit and the area is in that unit squared (e.g. cm gives cm²).

Why does it say my triangle is impossible?

Three lengths only form a triangle if each pair of sides sums to more than the third (the triangle inequality). If not, no triangle — and no area — exists.

Can it calculate volume of 3D shapes?

No — it handles flat 2D shapes only, giving area and perimeter, not the volume or surface area of solids.

Is my data uploaded?

No. Everything runs locally in your browser; nothing is sent or stored.

Triangle / Area Calculator

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