Why 16:9 became universal
Widescreen television settled on 16:9 as a compromise between the 4:3 of old broadcast and the wider 2.35:1 of cinema. It is now the default for monitors, phones in landscape, and nearly all streaming video.
Ratios are conventionally written in lowest terms. 1920×1080 reduces to 16:9 by dividing both by 120 — the greatest common divisor.
Resizing without distortion
To scale an image without stretching, both dimensions must change by the same factor. Fix one and the other follows from the ratio, which is what the new width and height fields do.
Fractional results should be rounded to whole pixels. Rounding both independently can shift the ratio by a hair; for video encoding, round to even numbers, since most codecs require dimensions divisible by two.
Letterboxing and cropping
Displaying content at a different ratio than the screen forces a choice. Letterboxing adds black bars and preserves the whole frame; cropping fills the screen and discards the edges. Cinema at 2.39:1 shown on a 16:9 television is the everyday example of the first.
Cropping is destructive, so it matters what was framed near the edges. Broadcasters have historically shot with "safe areas" in mind precisely so that a frame survives being cropped to a different ratio.
Pixels, resolution and ratio
Resolution and aspect ratio are independent. 1920x1080 and 1280x720 are both 16:9 despite differing sharply in pixel count. Ratio describes shape; resolution describes detail.
Most codecs require even pixel dimensions, and some require multiples of four or sixteen. When resizing, round to an even number rather than accepting a fractional result from the ratio arithmetic.
Frequently asked questions
What ratio should I use for social media? +
1:1 or 4:5 for feed posts, 9:16 for stories and short-form video, 16:9 for landscape video. Vertical formats dominate mobile-first platforms.
What aspect ratio is a phone screen? +
Modern phones are typically 19.5:9 or 20:9 — considerably taller than the 16:9 that dominated a decade ago, which is why older video letterboxes on them.
Why is cinema wider than television? +
Widescreen cinema formats were introduced in the 1950s specifically to differentiate the theatrical experience from television, and the convention stuck.