Area Calculator

About this tool

The Area Calculator finds the area of a rectangle, square, triangle, circle, ellipse, trapezoid, parallelogram or circular sector and shows the formula worked out with your numbers. Pick a shape, choose the unit you measured in (millimeters, centimeters, meters, inches, feet or yards) and type the dimensions: the area appears in that unit squared and is converted to ft², m², in², yd², cm², mm², acres and hectares. Triangles can be solved from a base and height, or from three sides with Heron's formula, which first finds the semi-perimeter s = (a + b + c) ÷ 2. A sector takes a radius and the central angle in degrees. A labeled diagram shows which measurement goes where, and impossible inputs, such as three sides that cannot form a triangle, get a plain explanation instead of a number. All dimensions use one unit and the tool handles one shape at a time; for a floor made of several rooms, use the Square Footage Calculator. Calculations run in your browser and nothing is uploaded.

Why use it

• Eight shapes in one place, including ellipses, trapezoids and circular sectors.

• Shows the formula with your own numbers substituted, so you can check the math step by step.

• Converts the result to eight area units at once, from square millimeters to acres and hectares.

• Solves a triangle from its three sides with Heron's formula when the height is hard to measure.

• Explains impossible inputs, such as sides that cannot form a triangle or an angle over 360°, instead of returning a wrong number.

• Runs entirely in your browser with no signup.

How to use

1. Choose the shape from the Shape list. The diagram changes to show which measurement is which.
2. Pick the unit you measured in: feet, inches, yards, meters, centimeters or millimeters.
3. Enter each dimension. For triangles, parallelograms and trapezoids, use the perpendicular height, not a sloping side.
4. Read the area in your unit squared and check the worked formula under Formula with your numbers.
5. Use the grid below it for the same area in square feet, square meters, acres, hectares and four other units, or click Copy working.

When it helps

• Working out the area of one rectangular room, patio or deck before buying materials.
• Checking geometry homework for circles, trapezoids, parallelograms and triangles.
• Finding the area of a triangular garden bed or plot when you only know the three side lengths.
• Converting a lot or field measured in feet or meters into acres or hectares.
• Sizing a round or oval tabletop, rug or pond liner.

Examples

Tips

• Measure the height of a triangle, parallelogram or trapezoid at a right angle to the base. Using the sloping side overstates the area.
• Split an L-shaped or irregular area into rectangles and triangles, find each area and add them up.
• Know the diameter of a circle? Halve it for the radius: a circle 10 ft across has a 5 ft radius and an area of 78.5398 ft².
• An ellipse needs its semi-axes, half the full length and half the full width. A 12 × 8 ft oval uses a = 6 and b = 4 and covers 75.3982 ft².
• One acre is 43,560 ft², about 4,046.86 m², and one hectare is 10,000 m².

Frequently Asked Questions

Glossary

Area

The amount of flat surface inside a shape, measured in square units such as ft² or m².

Heron's formula

A way to find a triangle's area from its three sides: √(s(s − a)(s − b)(s − c)), where s is half the perimeter.

Semi-perimeter

Half the distance around a triangle, s = (a + b + c) ÷ 2.

Perpendicular height

The distance from a base to the opposite side or corner, measured at a right angle to the base.

Semi-axis

Half of the longest or shortest width of an ellipse. The area is π times the two semi-axes.

Circular sector

A slice of a circle between two radii and the arc that joins them, like a slice of pie.