How this calculator works
Every image and screen is a rectangle, and its aspect ratio is the width divided by the height, expressed as two whole numbers separated by a colon. A 1920 by 1080 frame reduces by the greatest common divisor of 120 to 16:9. Reducing keeps the shape but drops the pixel count, which is why 1280 by 720, 1920 by 1080 and 3840 by 2160 all report 16:9. When you start from a ratio instead, one known side fixes the other: the unknown side equals the known side multiplied by the other ratio term. Pixels should be whole numbers, so a rounded result may be a pixel off from a true ratio before rounding.
Formula
aspect ratio = width : height, reduced by the greatest common divisor. For a known ratio w : h and a known width W, the height is H = W x h / w. For a known height H, the width is W = H x w / h. total pixels = W x H. megapixels = total pixels / 1,000,000.
Worked example
A 1920 by 1080 video has a greatest common divisor of 120, so 1920 / 120 = 16 and 1080 / 120 = 9, giving 16:9 and a decimal ratio of 1.777778. It holds 2,073,600 pixels, or 2.0736 megapixels. If you instead need a 16:9 frame with a width of 3840 pixels, the height is 3840 x 9 / 16 = 2160 pixels, which is the familiar 4K frame. A 1280 by 1024 monitor reduces to 5:4 because 1280 / 256 = 5 and 1024 / 256 = 4.
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Frequently asked questions
Is 1920 by 1080 the same ratio as 2560 by 1440?
Yes. Both reduce to 16:9, so they are the same shape at different pixel counts. The larger frame simply has more detail; it does not change the proportions.
Why did my exact ratio come out with large numbers?
The greatest common divisor of an odd pixel size is often 1, so a frame such as 1366 by 768 reduces only to 683:384 even though it is close to 16:9. Manufacturers round panel sizes, so near-16:9 resolutions are not always an exact 16:9.
Does cropping change the aspect ratio?
Yes. Cropping removes pixels from at least one side, so the remaining rectangle almost always has a different ratio. If you crop width and height by the same proportion, the ratio stays the same and only the resolution falls.
Can I use this for print sizes?
Yes, because print sizes are rectangles too. A 4 by 6 inch print is 2:3, and an 8 by 10 inch print is 4:5, which is why the same photograph often needs cropping to fit a standard frame.