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Rectangle Calculator: Area, Perimeter and Diagonal

This free rectangle calculator gives the area, perimeter, diagonal and aspect ratio from the length and width. You can also find a missing side from the area.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

A rectangle is the shape of rooms, screens, doors and paper. Its area, perimeter and diagonal are the usual questions.

Enter the length and width, or the area and one side.

How to Use This Tool

  1. Enter the value or values asked for in the boxes.
  2. Choose the unit label if there is a choice.
  3. Press Calculate.
  4. Read every result in the table and copy what you need.

How the Calculation Works

Area = l × w, perimeter = 2(l + w), diagonal = √(l² + w²). From an area A and a side a, the other side is A ÷ a. The ratio divides both sides by their greatest common divisor.

Worked Example

A rectangle 12 by 5 has an area of 60, a perimeter of 34, a diagonal of 13 and a ratio of 12:5.

Common Uses

  • Rooms, plots and screens.
  • Frames and doors.
  • Material estimates.
  • Homework.

Tips and Common Mistakes

  • A square corner has equal diagonals.
  • Use the same unit for both sides.
  • Area is in square units.
  • Measure carefully: a small error in a length is squared in an area and cubed in a volume, so a 5 percent error in a side can become a 10 percent error in the area or a 15 percent error in the volume.
  • Round only at the end. If you copy a result into another calculation, copy the full number from the tool rather than a rounded one, so small errors do not build up.

Next, you may need the trapezoid calculator, rhombus calculator, cube and box calculator, circle calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

Area = length × width, perimeter = 2(length + width) and diagonal = √(length² + width²).
Divide the area by the known side.
The length and width reduced to simple numbers, such as 16:9 for a screen.
Checking that a corner is square, and sizing screens, frames and doors.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.