How to use
- Choose two sides if you measured a room or a picture and want the corner-to-corner length.
- Choose a diagonal if you know a screen size in inches and an aspect pair such as 16 and 9.
- Fill only the boxes that match the mode. The other boxes are ignored.
- Press Calculate. Keep the same unit on every length you care about.
How it's calculated
From two sides, the diagonal is the square root of side A squared plus side B squared. That is the Pythagorean theorem for a right angle.
From a diagonal and an aspect pair W:H, the aspect hypotenuse is the square root of W squared plus H squared. Width = diagonal × W ÷ that hypotenuse. Height = diagonal × H ÷ that hypotenuse.
Worked example
A rectangle 30 by 40 has a diagonal of 50, the 3-4-5 triangle scaled by 10. A 55 inch screen at 16:9 is about 47.94 wide and 26.96 high, in the same inches as the diagonal. The aspect page reduces a width and height you already measured. This page builds the width and height from the diagonal.
Assumptions
A flat rectangle and a right angle. Bezel and stand are not subtracted from a screen size. Units are not converted.
FAQ
Is a 55 inch TV 55 inches wide?
No. 55 inches is the diagonal of the viewable area in the way sets are marketed. The width is shorter, about 47.9 inches at 16:9.
Can I use feet for a room?
Yes. A 30 foot by 40 foot room has a 50 foot diagonal. Do not mix feet and inches in the two side boxes.
What aspect pair is 4:3?
Enter 4 and 3 as the aspect width and height, and the diagonal you measured.
Does this measure area?
No. Area is width times height. Square footage for a floor is the square footage calculator, and shapes are on SqftKit.