Rectangle Calculator

Two measurements give you everything about a rectangle.

Rectangle

Area60
Perimeter
34
Diagonal
13

Working

  1. Area = 12 × 5 = 60
  2. Perimeter = 2 × (12 + 5) = 34
  3. Diagonal = √(12² + 5²) = 13

Formulas

area = l × w

perimeter = 2(l + w)

diagonal = √(l² + w²)

Worked example

12 × 5

  1. Area: 12 × 5 = 60
  2. Perimeter: 2 × 17 = 34
  3. Diagonal: √(144 + 25) = √169 = 13

Area 60, perimeter 34, diagonal 13.

Checking a right angle

Builders use the diagonal to check that a frame is square: if both diagonals of a rectangle are equal, its corners are right angles. The 3-4-5 rule is the same idea — a triangle with sides 3, 4 and 5 always contains a right angle.

Screen sizes

TV and monitor sizes are the diagonal. A 16 : 9 screen with a 55-inch diagonal is about 47.9 × 27 inches.