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Rectangle Calculator

Calculate rectangle area, perimeter, and diagonal from length and width for math, construction, or design.

What this tool does

Calculate rectangle area, perimeter, and diagonal from length and width for math, construction, or design. You fill in these fields: Length (any unit) and Width (same unit as length). It shows area, perimeter, and diagonal.

Inputs

  • Length (any unit) (units): A number. Must be at least 0.
  • Width (same unit as length) (units): A number. Must be at least 0.

How to use

  1. Enter the length (any unit) in units.
  2. Enter the width (same unit as length) in units.
  3. Read the result: area, perimeter, and diagonal. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • Length (any unit): 5 units
  • Width (same unit as length): 3 units

Results:

  • Area: 15 square units
  • Perimeter: 16 units
  • Diagonal: 5.830952 units

How this is worked out: Area = 5 x 3 = 15. Perimeter = 2 x (5 + 3) = 16. Diagonal = sqrt(5^2 + 3^2) = sqrt(34) = 5.8309519, which rounds to 5.830952.

Example 2. Inputs:

  • Length (any unit): 4 units
  • Width (same unit as length): 3 units

Results:

  • Area: 12 square units
  • Perimeter: 14 units
  • Diagonal: 5 units

How this is worked out: Area = 4 x 3 = 12. Perimeter = 2 x (4 + 3) = 14. Diagonal = sqrt(16 + 9) = sqrt(25) = 5, the 3-4-5 right triangle.

Example 3. Inputs:

  • Length (any unit): 0.5 units
  • Width (same unit as length): 2 units

Results:

  • Area: 1 square units
  • Perimeter: 5 units
  • Diagonal: 2.061553 units

How this is worked out: Area = 0.5 x 2 = 1. Perimeter = 2 x (0.5 + 2) = 5. Diagonal = sqrt(0.25 + 4) = sqrt(4.25) = 2.0615528, which rounds to 2.061553.

Example 4. Inputs:

  • Length (any unit): 7 units
  • Width (same unit as length): 7 units

Results:

  • Area: 49 square units
  • Perimeter: 28 units
  • Diagonal: 9.899495 units

How this is worked out: A square of side 7: area = 49, perimeter = 4 x 7 = 28, diagonal = 7 x sqrt(2) = 9.8994949, which rounds to 9.899495.

Limitations

  • Assumes a true rectangle with four right angles. Any other shape gives wrong results.
  • Both sides must use the same unit.
  • Does not handle other shapes or find a side from the area or perimeter.

FAQs

Q: What if one side is zero?
A: The calculator needs both sides greater than 0, because a rectangle with a zero side has no area.
Q: Can it find the sides from the area or the perimeter?
A: No. Enter both sides. The calculator finds the area, perimeter and diagonal from them.
Q: Do I need to choose a unit?
A: No. Use the same unit for both sides. The results are in that unit, and the area is in its square.
Q: Is the diagonal exact?
A: The diagonal is a square root, so long decimals are shown rounded.

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