Aspect Ratio Calculator: Resolution, PPI and Viewing Distance

How to use it

1. Find a ratio. Type a width and a height in pixels, or in any unit. The calculator reduces them to the smallest ratio, such as 1920 × 1080 to 16:9.
2. Resize at a ratio. Choose a ratio, or type one such as 16:9 or 2.39:1. Then type the new width or the new height. The calculator fills in the other side, in whole pixels.
3. Check a screen. Type the diagonal in inches and the resolution, or choose "This screen" or a common resolution. You get the pixel density (PPI), the pixel pitch and the size of the picture.
4. Check the distance. The last part uses the screen above. It gives the distances for two common guidelines, and the distance where single pixels stop being visible. Type your own distance to see how much of your view the screen fills.

Each result has a Copy button. The units switch changes lengths and distances between inches and centimetres. Screen diagonals stay in inches, because screens are sold in inches everywhere.

Common aspect ratios

"21:9" is a trade name. Philips first used it in 2009. The real ratios are 64:27 (2560 × 1080) and 43:18 (3440 × 1440) [1]. The calculator names both as 21:9. When a resolution is close to a common ratio but not exactly on it, the calculator says so. For example, 1366 × 768 reduces to 683:384, which is close to 16:9.

How the numbers are worked out

Aspect ratio. The width and the height are divided by their greatest common divisor. 1920 and 1080 share 120, so the ratio is 16:9. The decimal form is the width divided by the height: 1.78:1.

The other side. New height = new width × ratio height ÷ ratio width. A 16:9 picture 1280 pixels wide is 1280 × 9 ÷ 16 = 720 pixels high.

PPI. The diagonal in pixels is the square root of width² + height². PPI is that number divided by the diagonal in inches [2]. A 27-inch 2560 × 1440 monitor has 2937 pixels on the diagonal, so it has about 109 PPI.

Viewing distance. The calculator uses two guidelines. THX advises a seat where the screen fills about 40° of your view. A 30° view is often quoted as the SMPTE guideline [3]. Closer feels more like a cinema. Further suits a room where people also talk and read. The third distance is where one pixel is one arc minute, the detail limit of 20/20 sight [3]. Closer than that, a reader with good sight can see single pixels.

Which TV or monitor size?

Type your viewing distance in the last part. The calculator gives a range of screen sizes: the low end fills 30° of your view and the high end fills 40°. At 2.5 m from a 16:9 TV, that is about 60 to 82 inches.

For a monitor on a desk, about 60 to 80 cm is common. The pixel limit matters more there. A 27-inch 1440p monitor shows single pixels closer than about 80 cm. A 27-inch 4K monitor shows them only closer than about 54 cm.

3D screens and viewing distance

A 3D screen has its own best distance, set by the screen and not only by its size. The Owl3D Shift is a 14-inch 4K glasses-free 3D monitor with 315 PPI. You watch it from about 45 to 100 cm away, best at 50 to 70 cm [4]. Choose "Owl3D Shift" in the screen part to see its numbers.

Frequently asked questions

How do I calculate an aspect ratio?

Divide the width and the height by their greatest common divisor. 1920 × 1080 becomes 16:9, because both numbers divide by 120. Type the two numbers above and the calculator does it for you.

What is the height of a 16:9 picture?

Multiply the width by 9 and divide by 16. A 1280-pixel width gives 720, and a 1920-pixel width gives 1080.

Is 1920 × 1080 the same as 16:9?

Yes. 1920 × 1080 is exactly 16:9, and so are 1280 × 720, 2560 × 1440 and 3840 × 2160.

Is 2560 × 1080 really 21:9?

Not exactly. It is 64:27, about 2.37:1. 21:9 is the name the industry sells it under [1]. 3440 × 1440 is 43:18, about 2.39:1.

What is the difference between PPI and DPI?

PPI counts the pixels in one inch of a screen or a digital picture. DPI, for a printer, counts the dots of ink it puts in one inch, and a printer needs several dots to make one pixel's colour [2]. A mouse's DPI is something else: how far the pointer moves for one inch of hand movement. This calculator gives PPI.

What is a good PPI for a monitor?

It depends on the distance. At a normal desk distance, about 100 to 110 PPI shows single pixels to a reader with good sight. About 140 PPI and more looks sharp. Phones are held closer, so they use 400 PPI or more. The pixel-limit distance in the calculator gives the answer for your screen.

Is anything I type saved or sent?

No. Every sum runs in your browser. We count that a part of the calculator was used, never the numbers you type.