Area Calculator - Circle, Rectangle & Triangle Area

Calculate the Area of Any 2D Shape

An area calculator finds how much flat space a two-dimensional shape covers, measured in square units. Pick a shape, type in its dimensions, and the area appears the instant you finish — no formula to memorize and no arithmetic by hand. This tool covers nine common shapes, from a plain square to a regular polygon.

Area measures a two-dimensional surface, so the answer is always in square units — cm², m², in², and so on — matching the unit you choose for the inputs.

Shapes You Can Measure

Curved Shapes

Circle and ellipse — set by a radius, or by the semi-major and semi-minor axes.

Quadrilaterals

Square, rectangle, trapezoid, parallelogram, and rhombus.

Triangle & Polygon

Triangle, plus any regular polygon with n equal sides.

How to Calculate Area Step by Step

Select the Shape

Choose the shape you want to measure. The input fields update on their own to ask for exactly the dimensions that shape needs.

Enter the Dimensions

Type in the required values — a radius, sides, base and height, or diagonals, depending on the shape you picked.

Read the Result Instantly

The area is worked out in real time as you type, with the matching formula and a step-by-step substitution shown below it.

Switch Units If Needed

Use the dropdown to change between mm, cm, m, in, and ft. The result switches to the matching square unit.

Keep units consistent. Enter every dimension of a shape in the same unit — mixing cm and m, for example, gives a wrong area. The unit you pick is only a label; the numbers are not converted for you.

Area Formulas for Every Shape

Each shape uses a standard geometric formula. The calculator applies these automatically, but here is the full reference so you can check the math or work it out on paper.

Shape	Formula	Inputs Needed
Circle	`A = π × r²`	radius r
Square	`A = a²`	side a
Rectangle	`A = l × w`	length l, width w
Triangle	`A = ½ × b × h`	base b, height h
Trapezoid	`A = ½ × (a + b) × h`	bases a, b, height h
Parallelogram	`A = b × h`	base b, height h
Rhombus	`A = ½ × d₁ × d₂`	diagonals d₁, d₂
Ellipse	`A = π × a × b`	semi-axes a, b
Regular Polygon	`A = ¼ × n × a² × cot(π/n)`	sides n, side length a

Worked example: a circle with radius r = 5 cm gives A = π × 5² = π × 25 ≈ 78.539816 cm².

Features

Nine 2D Shapes

Circle, square, rectangle, triangle, trapezoid, parallelogram, rhombus, ellipse, and regular polygon — all in one tool.

Real-Time Results

The area recalculates the moment you change any value — no submit button, no waiting.

Visual Shape Preview

A diagram of the selected shape with its dimension labels helps you match each input to the right measurement.

Multiple Units

Work in mm, cm, m, in, or ft, with the result shown in the matching square unit.

Formula & Steps

Each shape shows the exact formula used, with a step-by-step breakdown so you can follow the calculation.

One-Click Clear

Reset every input and the result in a single tap to start a fresh calculation.

Results are shown with up to 6 decimal places, precise enough for schoolwork, DIY projects, and engineering estimates alike.

Private by design: every calculation runs locally in your browser — no dimensions are uploaded to a server.

Frequently Asked Questions

What is area and how is it measured?

Area is the amount of flat space a two-dimensional shape covers. Because it spans two dimensions, it is always measured in square units — square centimeters (cm²), square meters (m²), square inches (in²), and so on.

Which shapes can I calculate the area of?

Nine 2D shapes: circle, square, rectangle, triangle, trapezoid, parallelogram, rhombus, ellipse, and regular polygon. Each one shows its own input fields and formula.

Do I use the radius or the diameter for a circle?

Use the radius — the distance from the center to the edge, which is half the diameter. The circle formula is A = π × r², so entering the diameter by mistake makes the area four times too large. If you only know the diameter, divide it by 2 first.

What units does the area result use?

The area follows the unit you enter, squared. Dimensions in cm give an area in cm²; enter them in m and you get m², and likewise for mm², in², and ft². The tool does not convert between units, so use one unit for every dimension of a shape.

What is the difference between area and perimeter?

Area measures the flat space inside a shape (in square units), while perimeter measures the distance around its outline (in linear units). A rectangle 4 × 3 has an area of 12 square units but a perimeter of 14 units.

How do I find the area of an irregular shape?

Split the irregular shape into the simple shapes this calculator supports — usually rectangles and triangles — find each part's area, then add them together. Subtract any cut-out regions the same way.

Why does a triangle need the height?

The area of a triangle is ½ × base × height, where the height is the perpendicular distance from the base to the opposite vertex. The side lengths alone do not give the area unless that height is known.

How accurate are the calculations?

Results are shown with up to 6 decimal places, with trailing zeros trimmed. Calculations that use π rely on the full constant, so circle and ellipse areas are mathematically exact within that rounding.