Rectangle Area Calculator
Calculate area, perimeter, and diagonal of a rectangle
How to Use This Rectangle Area Calculator
Enter the length
Input the longer side of the rectangle. Use any unit — the calculator works with all measurements.
Enter the width
Input the shorter side. Both values must be positive numbers.
Get instant results
The calculator shows area, perimeter, and diagonal length automatically.
Rectangle Formulas
The area formula multiplies length by width. Perimeter adds all four sides. The diagonal uses the Pythagorean theorem since it forms a right triangle.
Rectangle Examples
Room: 12 ft × 10 ft
Area: 120 sq ft | Perimeter: 44 ft | Diagonal: 15.6 ft
TV Screen: 32" × 18"
Area: 576 sq in | Perimeter: 100" | Diagonal: 36.7" (diagonal size)
Garden Plot: 5 m × 3 m
Area: 15 sq m | Perimeter: 16 m | Diagonal: 5.8 m
Square: 8 cm × 8 cm
Area: 64 sq cm | Perimeter: 32 cm | Diagonal: 11.3 cm
Practical Applications
Flooring and Carpet
Calculate how much material you need for a rectangular room. Add 10% for waste.
Paint Coverage
Find wall area to estimate paint needed. One gallon typically covers 350-400 sq ft.
Fencing
Use perimeter to determine how much fencing material you need for a rectangular yard.
Frequently Asked Questions
Is a square a rectangle?
Yes. A square is a special rectangle where all four sides are equal. The same formulas work — just use the same value for length and width.
How do I find the diagonal of a rectangle?
Use the Pythagorean theorem: diagonal = √(length² + width²). The diagonal splits the rectangle into two right triangles.
What units should I use?
Any consistent units work. If length and width are in feet, area is in square feet and perimeter is in feet. The calculator does not convert between units.
Can I use decimals?
Yes. Enter decimals like 10.5 or 3.14. The calculator handles fractional measurements and gives precise results.
What if I only know area and one side?
Divide area by the known side to find the other. For example, if area is 120 sq ft and length is 12 ft, width = 120 ÷ 12 = 10 ft.
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