The ratios in common use and where they came from
16:9 became the television and web video standard as a compromise between the 4:3 of early television and the wider cinema formats, chosen so that the common ratios could be accommodated with the least total waste. It remains the default for video, presentations and most monitors.
Cinema uses wider formats: 1.85:1 for standard widescreen and 2.39:1 for anamorphic scope, which is why films shown on a 16:9 screen carry black bars top and bottom. 4:3 survives in some photography and in older archive material. 1:1 came from Instagram's early constraint and persists as a social format, while 9:16 vertical is now dominant for phone-native video.
Photography works to its own set: 3:2 from 35 mm film, which most interchangeable-lens cameras retain, and 4:3 from medium format, which most phone sensors use. This is why a phone photograph does not fit a standard 6 × 4 print without cropping — the sensor ratio and the print ratio genuinely differ.
Scaling, letterboxing and pixel counts
Scaling within a ratio is a single multiplication, and the useful check is that both dimensions scale by the same factor. 1920 × 1080 halves to 960 × 540, which is still 16:9. If one dimension is rounded and the other is not, the ratio drifts and the image distorts — subtly at first, and visibly after a few steps.
Fitting content into a differently shaped frame forces a choice. Letterboxing preserves the whole image and adds bars; cropping fills the frame and loses edges; stretching fills the frame and distorts, which is almost never acceptable and is immediately noticeable on faces.
Remember that pixel count scales with the square of the linear dimension. Doubling both width and height quadruples the pixels, and therefore roughly quadruples memory use and encoding time. Moving from 1080p to 4K is four times the pixels, not twice, which is why the file sizes and processing costs jump so sharply.
Where ratios are enforced
Platforms crop to their own ratios, and doing the crop yourself is nearly always better than accepting an automatic centre crop that may cut the subject. Common targets: 16:9 for video thumbnails and presentations, 1.91:1 for link preview cards, 1:1 and 4:5 for social feeds, 9:16 for stories and short-form video.
Print has its own fixed sizes, and they do not form a single family. The ISO A series is uniquely consistent at 1:√2 throughout, which is why A4 halves cleanly into A5. US paper sizes and standard photographic print sizes do not share a ratio, so a set of prints at 6 × 4, 7 × 5 and 10 × 8 requires three different crops of the same photograph.
In CSS, the aspect-ratio property reserves space before an image loads, which prevents the layout shifting as content arrives — a direct improvement to Cumulative Layout Shift and one of the cheapest fixes available for it.