How do you find a rectangle diagonal?
√(length² + width²).
Diagonal Calculator
Find plane diagonals or a cube’s face and space diagonals.
Select a figure
Choose rectangle, square, or cube and enter side lengths.
About this calculator
How to use Diagonal Calculator, plus the formulas and logic behind the results.
Tool description
Diagonals connect opposite corners. In a cube, a face diagonal stays on one face; a space diagonal goes through the interior.
How to use
- Select rectangle, square, or cube.
- Enter the required positive side length(s).
- Select Calculate.
- For a cube, compare face vs space diagonal.
Formulas & reference
Rectangle: √(L² + W²)Square: s√2Cube face: a√2; space: a√3
Diagonal facts
- 3-4-5 rectangle diagonal = 5
- Square diagonal ≈ 1.414 s
- Cube space diagonal ≈ 1.732 a
- Face diagonal lies on a square face
Calculation logic
Applies Pythagorean relations in 2D or 3D for the selected figure.
All side lengths must be positive.
Diagonal Calculator FAQ
Common questions about this calculator and how to use the results.
What is a square diagonal?
s√2.
What is a cube face diagonal?
a√2 on any square face.
What is a cube space diagonal?
a√3 connecting opposite vertices through the cube.
Are face and space diagonals different?
Yes—space diagonal is longer.
Must sides be positive?
Yes.