Understanding Video Aspect Ratios

Aspect ratio is the relationship between width and height, written as two numbers. It is a shape rather than a size: 16:9 describes a 1280 x 720 frame and a 3840 x 2160 one equally well.

How we got here

Early film and television were close to 4:3, a shape inherited from photographic plates and reproduced in television because that is what film looked like. When television started taking cinema's audience in the 1950s, the industry responded by getting wider, producing the cinema ratios still in use: 1.85:1 and the very wide 2.39:1.

Digital television settled on 16:9, which is 1.78:1, as a compromise between the old 4:3 and cinema's wider shapes. It became the default for essentially all video for two decades.

Then phones happened

Phones are held upright, and video shot on them is 9:16. For a while this was treated as a mistake to be corrected. It is now simply a format, and one that fills a phone screen properly rather than occupying a strip in the middle of it.

Square at 1:1 and portrait at 4:5 sit between the two, reading acceptably in a scrolling feed without committing entirely to either orientation.

The three ways to fit a different shape

1. Crop: fill the frame and lose what falls outside. Fine when the subject is central, ruinous when it is not.
2. Letterbox or pillarbox: add bars and keep everything. Nothing is lost except screen area.
3. Stretch: distort the picture to fit. Keeps every pixel and ruins all of them. Almost never the right answer.

The first two are legitimate decisions about what matters in a shot. The third is what happens when nobody made a decision.

Compose for the shape you will deliver

The most common failure is shooting wide and cropping to vertical afterwards. A landscape composition places the subject relative to space on either side; remove that space and the composition is gone, often along with the subject.

If a project needs both, either shoot with the crop in mind, keeping the subject centred and the important action within the narrower area, or shoot the two separately. Deciding afterwards is how you end up with footage that works in neither shape.

Pixel shape, briefly

Modern video uses square pixels, so the pixel dimensions and the shape agree. Some older and broadcast formats used non-square pixels, where a frame with a given pixel count displays wider or narrower than the numbers suggest. It is largely historical, and it explains occasional old footage that appears subtly stretched.

The aspect ratio calculator on this site scales one dimension while keeping the shape, reports the exact ratio, and names the nearest common one along with how far off it is.