Utilnaro

Geometry formulas

Rectangle area and perimeter

Calculate rectangle area, perimeter, or a missing side from the available dimensions.

Formula

A=ab;P=2(a+b) A = a × b; P = 2(a + b)

Area is the product of adjacent side lengths, while perimeter is twice their sum.

Rectangle area and perimeter calculator

What do you want to calculate?

Known values

The result updates automatically.

Rectangle diagram
abA ?
  • a · Side a: known value
  • b · Side b: known value
  • A · Area: value being calculated (?)
  • P · Perimeter: related value

Result

15 m²

Substitution and calculation

A = a × b = 15 m²

Symbols and units

SymbolQuantitySISupported units
AareaSurface enclosed by the rectangle.m²m², cm², mm², km²
PperimeterTotal length of all four sides.mm, cm, mm, km
afirst sideLength of one pair of opposite sides.mm, cm, mm, km
bsecond sideLength of the adjacent pair of opposite sides.mm, cm, mm, km

Formula rearrangements

Calculate area

A = a × b

Calculate perimeter

P = 2(a + b)

Calculate side a from area

a = A / b

Calculate side a from perimeter

a = P / 2 - b

Worked examples

Area of an 8 by 5 rectangle

A rectangle has sides of 8 cm and 5 cm. Calculate its area.

  1. 1Step 1: Use A = a × b.
  2. 2Step 2: Substitute a = 8 cm and b = 5 cm.
  3. 3Step 3: Multiply to obtain 40 cm².

Result: 40 cm²

Missing side from perimeter

A rectangle has a perimeter of 30 m and one side is 6 m. Find the other side.

  1. 1Step 1: Use a = P / 2 - b.
  2. 2Step 2: Divide 30 m by 2 to obtain 15 m.
  3. 3Step 3: Subtract 6 m to obtain 9 m.

Result: 9 m

Sources and review