Rectangle Size Calculator
Choose which two values you know and enter them. The calculator solves for the length and width, then gives the area, perimeter, diagonal and aspect ratio.
- Rectangle sides
- Area
- Diagonal
Use the same unit for both values.
Area = L × W, perimeter = 2(L + W), diagonal = √(L² + W²). A 4 × 3 rectangle has an area of 12, a perimeter of 14 and a diagonal of 5. Knowing only the area 12 and the length 4 gives the same width 3.
Rectangle formulas
Two values solve it. Search for a formula.
| Find | Formula |
|---|---|
| Area | L × W |
| Perimeter | 2 × (L + W) |
| Diagonal | √(L² + W²) |
| Width from area | A ÷ L |
| Width from perimeter | P ÷ 2 − L |
| Width from diagonal | √(d² − L²) |
Any unit.
Length for an area of 100
Fixed area. Slide the length.
The rectangle is drawn to scale.
How to solve a rectangle
Two values to all. Step through it.
Choose
Which two you know.
Check units match before you start.
Rectangle, square or golden
Shapes. Compare them.
Any ratio.
Equal sides.
Classic.
A square has the smallest perimeter for an area.
Before you use it
Checks. Tick each item.
Tick each item as you go.
Round only at the end.
What you get
Rectangle sizing. Each tile explains one result.
Length and width.
Square units.
Edge.
Corner to corner.
Aspect.
Diagonals help you check a frame is square.
Frequently Asked Questions
No question matches that search.
How do you find the area of a rectangle?
Multiply the length by the width.
How do you find the diagonal of a rectangle?
Take the square root of length squared plus width squared.
How do you find the width from the area?
Divide the area by the length.
Can I find the sides from area and perimeter?
Yes. They are the two roots of a quadratic equation.
What is the perimeter of a rectangle?
Twice the length plus twice the width.
How do I check a rectangle is square?
Measure both diagonals, they should be equal.
Any rectangle solved
Enter any two values and get every size of the rectangle at once.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.