Simplifying a ratio
An aspect ratio is width divided by height, written as two whole numbers with a colon. To get it, divide both dimensions by their greatest common divisor. For 1920 x 1080 the greatest common divisor is 120, giving 16:9. For 1024 x 768 it is 256, giving 4:3. The decimal form is often more useful for comparison: 16:9 is 1.7778, 4:3 is 1.3333, and 3:2 is 1.5. Two images with the same decimal ratio can be scaled to each other with no distortion, whatever their pixel counts.
Not every resolution reduces to a friendly pair. The common laptop panel 1366 x 768 has a greatest common divisor of only 2, so it simplifies to 683:384. Its decimal ratio is 1.7786, which is 0.05 percent away from true 16:9, close enough that the panel is sold and used as widescreen. That is why this calculator reports both the exact reduced ratio and the nearest standard one.
Resizing without distortion
To scale proportionally, multiply by the ratio of the new dimension to the old. New height equals new width times original height divided by original width. Worked example: a 1920 x 1080 image resized to 1280 wide needs a height of 1280 x 1080 / 1920 = 720 pixels, which is the familiar 720p frame. Going the other way, a target height of 1440 needs a width of 1440 x 1920 / 1080 = 2560.
When the result is not a whole number, round rather than forcing a value. A 4000 x 3000 photo scaled to 1000 wide gives exactly 750, but scaled to 1080 wide gives 810.0 exactly; a 1.91:1 target from a 3:2 source will not land on integers, and rounding by half a pixel is invisible. Forcing both dimensions to values that do not match the source ratio is what produces stretched faces and squashed logos. If the shape must change, crop instead of stretching, or add letterbox bars.
Which ratio to use where
Video and displays standardized on 16:9 with HDTV, and 720p, 1080p, 1440p and 4K UHD are all that ratio at 1280, 1920, 2560 and 3840 pixels wide. Still photography kept 3:2 from 35mm film, which is why a DSLR photo does not quite fill a 16:9 screen and why 4x6 prints match your camera but 8x10 prints crop it. Tablets and older cameras use 4:3. Ultrawide monitors sold as 21:9 are really 64:27 at 2560 x 1080 or 43:18 at 3440 x 1440.
Social platforms pushed the frame vertical. TikTok, Instagram Reels and YouTube Shorts use 9:16, normally 1080 x 1920. Instagram feed posts are 1:1 at 1080 x 1080 or 4:5 portrait at 1080 x 1350, the tallest the feed allows. Link previews using Open Graph want 1200 x 630, roughly 1.91:1. Cinema uses 1.85:1 for standard widescreen and 2.39:1 for anamorphic scope, which is why films on a 16:9 television show black bars top and bottom.