How screen size is calculated
Diagonal² = Width² + Height² A screen's diagonal, width, and height form a right triangle, so the same theorem used for any right triangle applies here. Combined with the aspect ratio (which fixes the proportion between width and height), that's enough to solve for the exact width and height from the diagonal alone — the number printed on the box.
Looking for the aspect ratio of your own screen instead?
If you have a width and height (in pixels, inches, or any unit) and want to know what ratio that simplifies to, use the Aspect Ratio Calculator instead.
Frequently asked questions
How big is a 55 inch TV?
A standard 55-inch 16:9 TV is about 47.9 inches wide and 27.0 inches tall, with a screen area of roughly 9 square feet.
Why is the diagonal size printed on the box, not the width and height?
It's an industry convention that goes back to the era of curved CRT tube screens, where width and height weren't consistent or easy to measure the same way diagonal was. It's stuck ever since, even though flat screens no longer have that constraint.
What aspect ratio is my phone?
Modern smartphones vary quite a bit by model, commonly somewhere between 19:9 and 21:9. Use the "19.5:9 (Common Smartphone)" preset as a reasonable starting point, or select Custom Ratio and enter your phone's exact specification if you know it.