Rectangle diagonal calculator
Leave one of length, width or diagonal empty and the tool fills it in with the Pythagorean theorem.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter length, width, diagonal and length unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
diagonal² = length² + width², so d = √(l² + w²) and a missing side = √(d² − known side²). Diagonal angle = atan(width ÷ length).
Example
Inputs: Length = 8, Width = 6, Length unit = Centimeters (cm). Result: Diagonal: 10 cm; Length: 8 cm; Width: 6 cm.
Limits and caveats
- An advertised screen size refers to the visible area and may differ slightly from the outer frame.
- The unit choice only labels the results.
Questions about this tool
How do I find the diagonal of a rectangle?
Add the squares of length and width and take the square root. An 8 × 6 rectangle has a diagonal of 10.
How can I check that a room is square?
Measure the length and width, then compare the measured diagonal with the one calculated here. If they match, the corners are right angles.
Can I get a screen width from its diagonal?
Yes, if you know the height. Enter the diagonal and the height and leave the width empty.