Geometry & Shapes
Rectangle Calculator
Rectangle: it uses area = length × width and perimeter = 2(length + width). Worked through below with real figures, and the calculator repeats the working on your own inputs — free, on-device, instant.
Rectangle Calculator takes your length and width and gives back the area and perimeter, applying standard area and perimeter formulas. It's a quick, browser-based way to find the area and perimeter of the shape.
About this calculator
Rectangle is easiest to get wrong in the setup, not the arithmetic. It uses area = length × width and perimeter = 2(length + width). It matters whenever a shape has to be measured, cut, covered or compared and the geometry has to be right first. The worked example on this page (A 8 × 5 rectangle: area = 40, perimeter = 26, diagonal = √(8²+5²) = √89 = 9.43 .) shows the full substitution; the calculator above runs the same working on your own numbers, in your browser, with nothing uploaded.
How to use
- Enter the length of the rectangle.
- Enter the width of the rectangle.
- Press Calculate to find the rectangle's measurements.
- Read the area, perimeter, and diagonal.
Why use Rectangle Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Accurate
Uses standard formulas where applicable; check critical results before relying on them.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Honest disclaimers wherever browser limits or regional rules apply.
Common uses
Use it for fencing and edging lengths
Use it when comparing two shapes
Use it when checking a homework answer
Use it for flooring and tiling estimates
Rectangle in design and layout work
Use it for sign and panel sizing
Technical notes
Rectangle Calculator uses area = length × width and perimeter = 2(length + width).
Results assume exact, idealised shapes; real-world measurement error is not included.
Nothing you type is uploaded — the calculation happens entirely on your device.
FAQ
How is the diagonal of a rectangle found?
The diagonal is the square root of the length squared plus the width squared, from the Pythagorean theorem.
What is the difference between area and perimeter?
Area is the space inside (length times width); perimeter is the distance around (twice the length plus twice the width).
Does it matter which side I call length?
No. Swapping length and width gives the same area, perimeter, and diagonal.
Does the Rectangle Calculator send my data to a server?
No. The Rectangle Calculator runs entirely in your browser, so nothing you enter is uploaded, stored, or shared.
Worked example
A 8 × 5 rectangle: area = 40, perimeter = 26, diagonal = √(8²+5²) = √89 = 9.43.