Area of a Rectangle Calculator

Area of a rectangle formula

A rectangle has four right angles and two pairs of equal sides. Its area is the number of unit squares that fit inside, which is found by multiplying the two side lengths that meet at a corner.

A = width × lengthBoth sides must use the same unit; the answer is in that unit squared.

A square is just a rectangle whose sides are equal, so the same rule gives A = s × s. The square area calculator is a shortcut for that case.

How to calculate it

1. Measure the two sides that meet at a corner. Which one you call the width and which the length does not matter.
2. Type them into the Width and Length boxes. The calculator starts with 3 and 4 so you can see the layout.
3. Choose the unit under Metric: mm, cm, dm, meter, km, in, ft, yd or mile.
4. Press Calculate area. The result shows in the same unit squared, such as 12 cm².

Both sides use the one unit you pick under Metric. If one side was measured in meters and the other in centimeters, convert first, or use the general area calculator, which has a unit box for every side.

Worked examples

Small sheet. A card measuring 3 cm by 4 cm has an area of 3 × 4 = 12 cm².

Room to tile. A living room is 4.2 m by 3.6 m. Its area is 4.2 × 3.6 = 15.12 m². Square tiles that are 30 cm on a side cover 0.3 × 0.3 = 0.09 m² each, so you need 15.12 ÷ 0.09 = 168 tiles. Adding 10% for cuts and breakage gives 168 × 1.10 = 184.8, which rounds up to 185 tiles.

Imperial units. A deck measures 12 ft by 9 ft, giving 12 × 9 = 108 ft². That equals 108 ÷ 9 = 12 square yards, or about 10.03 m², because one square foot is 0.09290304 m².

Same perimeter, different areas. Four rectangles with a perimeter of 20 units have sides 5×5, 6×4, 8×2 and 9×1. Their areas are 25, 24, 16 and 9. For a fixed perimeter the square encloses the most space, which is why a long, thin plot holds less than a compact one. The rectangle perimeter calculator covers the other half of the question.

Units and scaling

Area scales with the square of the side. Doubling both the width and the length makes the area four times larger; tripling them makes it nine times larger. That is also why converting square units needs the factor squared.

For anything not listed, see the area unit converter.

Common mistakes

• Adding instead of multiplying. Width plus length is half the perimeter, not the area.
• Mixing units. Multiplying 2.5 m by 40 cm gives nonsense unless one is converted first. The correct answer is 2.5 × 0.4 = 1 m².
• Reading square units as plain units. An answer of 12 cm² is twelve small squares, not a length of twelve.
• Measuring a slanted shape. If the corners are not right angles, the figure is a parallelogram and needs its perpendicular height; use the parallelogram area calculator.

Practical applications

• Flooring, carpets, paint coverage and wallpaper, where materials are priced per square meter or square foot (see also the square footage calculator).
• Land and garden plots: how much seed, fertilizer or fencing area a rectangular bed needs.
• Printing and screens: the area of a poster, sheet of paper or display panel.
• Construction: floor slabs, roof panels and the size of a building footprint.