Screen Size Calculator
Screens are sold by their diagonal measurement, which tells you surprisingly little about how wide or tall they actually are. This calculator turns a diagonal and an aspect ratio into the real width, height, and viewing area. Enter the diagonal in inches and the aspect ratio (such as 16 and 9), and it applies the Pythagorean theorem to return the dimensions of the rectangle. Use it to check whether a TV fits a wall niche, compare two screens by true area, or convert a phone's diagonal into width and height.
Screen size formula
r = sqrt(ratioW^2 + ratioH^2)
Width = diagonal * ratioW / r
Height = diagonal * ratioH / r
Area = width * height
Area (sq cm) = area (sq in) * 6.4516
This is the Pythagorean theorem: the diagonal is the hypotenuse of the right triangle formed by the screen's width and height. Scaling each side by the diagonal over the ratio's own hypotenuse recovers the true dimensions.
Common aspect ratios
- 16 to 9: standard HD and 4K televisions and most monitors.
- 21 to 9: ultrawide monitors for productivity and cinematic content.
- 4 to 3: legacy displays and some tablets.
- 19.5 to 9 and similar: tall ratios used by many smartphones.
- One square inch equals exactly 6.4516 square centimeters per the NIST definition.
Screen size: frequently asked questions
How do I find screen width and height from the diagonal?
A screen's size is quoted as the diagonal. Given the aspect ratio width to height, the width equals diagonal times the ratio width divided by the square root of (ratio width squared plus ratio height squared), and the height follows the same pattern. This is the Pythagorean theorem applied to the rectangle.
What is aspect ratio?
Aspect ratio is the proportion of width to height, such as 16 to 9 for modern TVs and monitors, 4 to 3 for older displays, or 21 to 9 for ultrawide. Enter the two numbers and the calculator handles the rest. The ratio itself has no units.
Why is my 16 by 9 TV not 16 inches wide?
Because the diagonal, not the width, is measured. For a 55 inch 16 by 9 screen the actual width is about 47.9 inches and the height about 27 inches. The diagonal stretches across the rectangle, so it is always longer than either side.
How is screen area computed?
Once width and height are known, area is simply width times height. A larger diagonal at the same aspect ratio gives a disproportionately larger area, because area grows with the square of the linear size.
Does this work for phones and ultrawide monitors?
Yes. The math is identical for any rectangular display. Just enter the correct aspect ratio: 19.5 to 9 for many phones, 21 to 9 for ultrawide, 16 to 10 for some monitors. The calculator accepts decimal ratio values.
Official sources
- National Institute of Standards and Technology: inch to centimeter conversion.
- Federal Communications Commission: digital television display formats.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.