Aspect Ratio Calculator: Screen Size and Area
Heading to the field? Install this site from your browser menu and this tool works with no signal. See how
Aspect Ratio Calculator
Simplifies any width and height into its ratio, fills in a missing dimension, and works out the real width, height and area of a screen from its diagonal. That last part is where the surprises are, because a diagonal on its own tells you almost nothing about size.
Jump to a common resolution
Same diagonal, different shape
| Ratio | Width (in) | Height (in) | Area (sq in) | Against 16:9 |
|---|
Every row and chip is a live link you can bookmark or share.
How to use this tool
Type a width and a height and the ratio appears reduced to its smallest whole numbers. The scale box tells you what height keeps the same shape at a different width, which is what you want when resizing an image without distorting it.
The diagonal box does something different and more useful than it looks. Give it a screen size in inches and it works out the actual width, height and area of that screen, then compares the area against another shape with the identical diagonal.
A diagonal tells you less than you think
Screens are sold by diagonal, and two screens with the same diagonal can differ enormously in how much picture you actually get.
A 27 inch 16:9 monitor is 23.53 by 13.24 inches, giving 311.5 square inches. A 27 inch 4:3 monitor is 21.60 by 16.20 inches, giving 349.9 square inches. That is 12.3 percent more screen for the same advertised number. Go the other way to a 27 inch ultrawide at 64:27 and you get 24.88 by 10.49 inches, which is 261.1 square inches, or 16.2 percent less than the 16:9.
The rule behind it is simple: for a fixed diagonal, the closer the shape is to square, the larger the area. A square is the maximum and long thin shapes are the minimum. This is why the switch from 4:3 to 16:9 televisions quietly shrank the picture unless you also bought a bigger number, and why ultrawide monitors feel smaller than the inches suggest.
Why 21:9 is not really 21:9
Ultrawide monitors are marketed as 21:9, but the common resolutions do not reduce to that. 2560 by 1080 simplifies to 64:27, and 3440 by 1440 simplifies to 43:18. Neither is 21:9.
64:27 is not arbitrary. It is 16:9 multiplied by 4:3, which makes it exactly three steps along the same progression that gave us the other broadcast ratios. 21:9 is just an easier thing to print on a box, and it is close enough that nobody complains.
A worked check you can repeat on paper
Take 1920 by 1080. The greatest common divisor is 120. Divide both and you get 16 by 9, which is why every full HD screen is described that way.
Now the physical size of a 27 inch one. The diagonal of a 16 by 9 rectangle is the square root of 16 squared plus 9 squared, which is the square root of 337, about 18.358. Scale that to 27 inches by dividing: 27 divided by 18.358 is 1.4708. Multiply by 16 for the width, 23.53 inches, and by 9 for the height, 13.24 inches. Multiply those together and the area is 311.5 square inches.
Repeat with 4 and 3 and you get 21.60 by 16.20, area 349.9. Same diagonal, more picture, and nothing on either box would tell you.
Mistakes that catch people out
Comparing screen sizes by diagonal alone. Two screens with the same number can differ by more than a quarter in actual area once the shapes differ. Compare area, not the marketing figure.
Assuming the pixels are square. Most modern formats use square pixels, but older standard definition video did not. A 720 by 480 frame is not 3:2 on screen, because the pixels themselves were stretched.
Confusing resolution with shape. 1280 by 720 and 3840 by 2160 are both 16:9. One has nine times the pixels of the other and they are the identical shape, so scaling between them never distorts anything.
Trusting the advertised ultrawide ratio. 21:9 is a label rather than a measurement. Work from the actual resolution if the exact shape matters.
Common resolutions and their ratios
The resolutions people look up most. Each opens the calculator on that size with its own address.
How this was checked
The ratio reduction was checked against known resolutions. 1920 by 1080 and 3840 by 2160 both reduce to 16:9, 1024 by 768 to 4:3, 1280 by 1024 to 5:4, 2560 by 1080 to 64:27 and 3440 by 1440 to 43:18. The last two are the ones commonly mislabelled as 21:9, and the calculator reports what the numbers actually say.
The screen dimension maths was verified by hand from Pythagoras: a 27 inch 16:9 comes out at 23.53 by 13.24 inches, and multiplying gives 311.5 square inches. The comparison figures follow from the same calculation repeated with different ratios, giving 12.3 percent more area for 4:3 and 16.2 percent less for 64:27 at the same 27 inch diagonal.
The missing dimension calculation was checked both directions: 16:9 at a width of 1920 gives a height of 1080, and at a height of 1440 gives a width of 2560.
Related Calculators and Reference
- Area of 2D Shapes Calculator for areas of shapes other than rectangles.
- Distance and Pythagoras Calculator for the diagonal calculation this rests on.
- Ratio and Proportion Calculator for ratios used as mixes and scales rather than shapes.
- Prime Factorisation Calculator for the greatest common divisor that does the reducing here.
- Length Converter for turning these inches into millimetres.
- Hex to RGB Converter: color codes in RGB, HSL and CMYK.