How this calculator works
A screen ratio is the width in pixels divided by the height in pixels, often written as two whole numbers. Reducing a resolution by its greatest common divisor shows the shape in its simplest form: 1920 by 1080 reduces to 16:9, while 1920 by 1200 reduces to 8:5, which is the same shape as 16:10. Many popular resolutions are rounded by manufacturers and do not reduce to a standard ratio at all. In the matching mode the page compares your decimal ratio with the standard ratios and reports the closest one plus the percentage difference, so you can see how far a panel such as 1366 by 768 departs from a true 16:9 frame. In the sizing mode a chosen ratio and one known side give the other side.
Formula
ratio = width / height, reduced by the greatest common divisor. nearest standard ratio = the standard with the smallest absolute difference from that decimal. difference = (your ratio / standard ratio - 1) x 100 percent. For a chosen ratio w:h and a known width W, height = W x h / w; for a known height H, width = H x w / h.
Worked example
A 1366 by 768 laptop screen divides by 2 to give 683:384, a decimal ratio of 1.778646. The nearest standard is 16:9 at 1.777778, so the screen is only 0.0488 percent wider than a true 16:9 frame. A 1920 by 1200 monitor divides by 240 to give 8:5, which is exactly 16:10 and has zero difference. A 1280 by 1024 monitor reduces to 5:4, another exact standard. For a true 16:9 screen 1920 pixels wide, the height is 1920 x 9 / 16 = 1080 pixels.
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Frequently asked questions
Why is 1366 by 768 not exactly 16:9?
Panel makers chose 1366 by 768 because it is close to 16:9 and fits economically on a production line. The exact 16:9 width for 768 pixels of height is 1365.33, so the panel is rounded up to the next whole pixel.
Is 21:9 a real ratio?
The label is a marketing convention for ultrawide screens. Most 21:9 monitors are closer to 64:27, which is 2.370, or 2.35:1, the shape of many widescreen films. The calculator uses 21:9 as the standard label because that is how the screens are sold.
Does the nearest standard ratio affect how video looks?
Yes. A screen that is slightly wider than 16:9 shows tiny bars or a slight overscan when playing 16:9 video, depending on the player settings. The difference for 1366 by 768 is under one tenth of a percent, so it is usually invisible.
Can I use this for a projector or a phone?
Yes. Any rectangular display can be described the same way. Phone screens often use 19.5:9 or 20:9, which are taller than 16:9; the matching mode will report the closest standard and the difference.