Rectangle calculator
Find a rectangle’s area, perimeter and diagonal from its length and width. Keep both dimensions in the same unit.
Result for the values shown.
Calculations run in your browser.
What to enter
| Input | Meaning and units |
|---|---|
| Length | Enter length using the definition in the method and example. |
| Width | Enter width using the definition in the method and example. |
Understanding your result
Read Area (square units), Perimeter (units), Diagonal (units) in the units and model stated on this page. The formula A = lw · P = 2(l + w) · d = √(l² + w²) defines the relationship; measurement accuracy and model applicability are separate from numerical precision.
Common mistakes
- Using a different mathematical definition or operation than the one stated by this tool.
Check your calculation
- Use the worked example as a known reference case, then change one input at a time and check the direction and units of the response.
Calculation checks, sources and review limits
How much area does a rectangle cover, and how far is it around the edge?
Enter length and width in the same unit to calculate area, perimeter and diagonal. These answer different measurement questions about the same rectangle.
Common uses
- Measure a rectangular surface.
- Calculate a rectangular boundary or diagonal.
How it works
Area multiplies length by width. Perimeter is twice their sum. The diagonal follows the Pythagorean theorem. Metre inputs give square metres for area and metres for perimeter and diagonal. Zero dimensions give a degenerate rectangle.
Worked example
A 3 by 4 rectangle has area 12, perimeter 14 and diagonal 5.