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Rectangle Calculator

Calculate rectangle area, perimeter, and diagonal

Area

80.00 sq ft

Perimeter

36.00 ft

Diagonal

12.81 ft

Units

Area

80.00

sq ft

Perimeter

36.00

ft

Diagonal

12.81

ft

Shape Preview

10 ft
8 ft
d = 12.8

Formulas Used

Rectangle Area

Area = length × width

The area of a rectangle is the product of its length and width.

Where:

length= The longer side of the rectangle
width= The shorter side of the rectangle

Rectangle Perimeter

Perimeter = 2 × (length + width)

The total distance around the rectangle.

Where:

length= The longer side of the rectangle
width= The shorter side of the rectangle

Rectangle Diagonal

Diagonal = √(length² + width²)

The diagonal length using the Pythagorean theorem.

Where:

length= The longer side of the rectangle
width= The shorter side of the rectangle

Example Calculations

1Rectangle: 10 × 8

Inputs

Length10
Width8

Result

Area80.00
Perimeter36.00
Diagonal12.81

Area = 10 × 8 = 80.00. Perimeter = 2 × (10 + 8) = 36.00. Diagonal = √(10² + 8²) = √(100 + 64) = √164 = 12.81.

2Rectangle: 15 × 6

Inputs

Length15
Width6

Result

Area90.00
Perimeter42.00
Diagonal16.16

Area = 15 × 6 = 90.00. Perimeter = 2 × (15 + 6) = 42.00. Diagonal = √(15² + 6²) = √(225 + 36) = √261 = 16.16.

Frequently Asked Questions

Q

How do you calculate the area of a rectangle?

Area of a rectangle = length × width. For example, a rectangle with length 10 and width 8 has area = 10 × 8 = 80 square units.

  • Formula: A = l × w — multiply the two side lengths together
  • A 12 × 15 ft room has an area of 180 sq ft (about 20 sq yd of carpet)
  • Convert sq ft to sq yd: divide by 9 (180 sq ft ÷ 9 = 20 sq yd)
  • Convert sq ft to sq m: multiply by 0.0929 (180 sq ft = 16.7 sq m)
  • For irregular shapes, break them into multiple rectangles and add the areas together
Q

How do you calculate the perimeter of a rectangle?

Perimeter = 2 × (length + width). For a rectangle with length 10 and width 8, perimeter = 2 × (10 + 8) = 36 units. Alternatively, add all four sides: length + width + length + width.

  • Formula: P = 2 × (l + w) or equivalently P = 2l + 2w
  • A 12 × 15 ft room has a perimeter of 54 ft — useful for baseboard and trim estimates
  • Fencing a 50 × 100 ft yard requires 300 ft of fencing (perimeter)
  • Subtract door/window openings from perimeter for trim and molding calculations
  • For a square (l = w), perimeter simplifies to P = 4 × side
Q

How do you calculate the diagonal of a rectangle?

Diagonal = √(length² + width²). Using the Pythagorean theorem, for a rectangle with length 10 and width 8, diagonal = √(10² + 8²) = √164 = 12.81 units.

  • Formula: d = √(l² + w²) — derived from the Pythagorean theorem (a² + b² = c²)
  • A 12 × 16 ft room has a diagonal of 20.0 ft (a 3-4-5 right triangle scaled by 4)
  • TV screen sizes are measured diagonally: a 55" TV is about 48×27" (16:9 ratio)
  • Use the diagonal to verify square corners: if d = √(l² + w²), corners are 90°
  • The 3-4-5 rule is a quick check: if sides are 3 and 4, diagonal must be exactly 5
Q

What is the difference between a rectangle and a square?

A square is a special type of rectangle where all four sides are equal. In a square, length = width, so area = side² and perimeter = 4 × side. A rectangle has two pairs of equal sides but they don't have to be the same length.

  • A square has all sides equal (l = w); a rectangle only requires opposite sides equal
  • Square area = s²; rectangle area = l × w — square is simpler to calculate
  • Square diagonal = s × √2 (about 1.414 × side); rectangle uses √(l² + w²)
  • Both have four 90° angles and two equal diagonals that bisect each other
  • A square maximizes area for a given perimeter — 100 ft of fencing encloses 625 sq ft as a square but only 600 sq ft as a 20×30 rectangle

Understanding Rectangle Calculations

A rectangle has four right angles and opposite sides that are equal. The area is length times width.

The perimeter is twice the sum of length and width. The diagonal can be found using the Pythagorean theorem.

A square is a special rectangle where length equals width, simplifying all calculations.

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Last Updated: Mar 26, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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