The Area Of A Rectangle Calculator
Rectangle area is the first formula most people learn and the one most often got wrong, because the units, the perimeter and the diagonal all get mixed into the same sentence.
Area
40
- Perimeter
- 26
- Diagonal
- 9.434
- Aspect ratio (long side / short side)
- 1.6
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
The formula this calculator uses
area = length x width
- A
- Area covered, in square units
- l
- Length of the longer side, in metres or feet
- w
- Width of the shorter side, in the same unit
How to check the result by hand
- 1
Measure the length and the width
Take the longer side as the length and the shorter as the width. It makes no difference to the area which one you call which, because multiplication is commutative, but it keeps the working readable.
- 2
Put both measurements in the same unit
Convert before multiplying. A length in metres and a width in centimetres cannot be multiplied directly, and doing so is the single most common source of answers that are out by a factor of a hundred.
- 3
Multiply length by width
Eight metres times five metres is forty. Nothing else is needed: no division, no addition, and no adjustment for the shape being long and thin.
- 4
Write the answer in square units
Forty square metres, not forty metres. The unit is the length unit multiplied by itself, so metres times metres gives square metres and feet times feet gives square feet.
- 5
Sanity check against the perimeter
The perimeter is two times the length plus the width, which for this rectangle is twenty six metres. If your perimeter is smaller than your area in the same unit, something has gone wrong with the units.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate The Area Of A Rectangle page.