Aspect ratio explained, and why platforms crop your images

Shape, not size

Aspect ratio is width divided by height, expressed as two whole numbers. 1920 × 1080 is 16:9. So is 1280 × 720, and 3840 × 2160. Those are three different resolutions — three different amounts of detail — and one shape.

That separation is the useful part. A platform asking for 16:9 is describing the shape of the hole it will put your image into. It will usually specify a minimum resolution separately, because the two requirements are independent: an image can be the right shape and too small, or big enough and the wrong shape.

What happens when the shape is wrong

Almost never an error message. Almost always a crop.

Upload a 4:3 photograph where a 16:9 banner is expected and the platform takes a 16:9 region out of it — usually the centre, because that is the only choice an algorithm can make without understanding the picture. That is how a group photograph loses the people at the ends, and how a portrait ends up as a shot of somebody’s chin.

Occasionally the opposite happens: the image is stretched to fill the space. This is rarer and much worse, because everything in the picture is subtly the wrong proportion and viewers can tell without being able to say why.

Either way, the decision is taken by the destination rather than by you, which is the entire argument for setting the ratio yourself before uploading.

The ratios you will actually meet

Ratio	Where it turns up
1:1	Profile pictures, avatars, Instagram squares, app icons
4:3	Older cameras, tablet screens, presentation slides
3:2	35 mm film, most DSLRs, 6×4 inch prints
16:9	YouTube video and thumbnails, HD and 4K screens
21:9	Ultrawide monitors, cinematic video
9:16	Stories, Reels, TikTok, phone wallpapers
4:5	Instagram portrait posts — the tallest feed post allowed
2:3	Portrait prints, book covers, Pinterest pins
3:4	Portrait photos from older cameras
1.91:1	Link previews on Facebook and LinkedIn
5:4	8×10 inch prints
7:5	7×5 inch prints

Four of those have a page of their own, with the ratio already applied and the crop frame ready: cropping to a square for avatars and profile pictures, cropping to 4:5 for the tallest post a feed shows in full, cropping to 16:9 for thumbnails and slides, and cropping to 9:16 for stories and vertical video.

Why your camera does not produce any of them

Phones mostly shoot 4:3, because that is the shape of the sensor. Cameras mostly shoot 3:2, inherited from 35 mm film. Neither is 16:9, 1:1, 4:5 or 9:16 — the shapes screens and social platforms use.

So a crop is not an unusual event. It is what happens to essentially every photograph that goes anywhere. The question is never whether to crop but where.

Ratios do not always simplify usefully

1000 × 667 simplifies to 1000:667, which is arithmetically correct and tells you nothing. It is 3:2 to within a rounding error, and 3:2 is the answer you wanted.

This happens because pixels are whole numbers and ratios are not. 1000 pixels wide at 16:9 wants a height of 562.5, and neither 562 nor 563 is exactly 16:9. The difference is far too small to see and it means “exactly 16:9” is often a slight overstatement. The calculator reports the exact simplification and the nearest named ratio separately, and says when a result has been rounded.

Two ways to change the shape

Cropping removes pixels until the frame is the right shape. You keep full resolution in the part that survives, and you lose the rest permanently.

Padding adds space until the frame is the right shape. Nothing is lost, and the picture occupies less of the final image — which on a feed of thumbnails can mean it reads as small.

Neither is correct in general. Crop, or add borders? works through the cases where each one wins, and both tools are a click away from the calculator.

A practical habit

If you know where an image is going, frame the photograph with room around the subject. Every crop then becomes a choice rather than a rescue, and one original can serve a square post, a 4:5 feed post and a 16:9 banner without any of them cutting into anything.

Where a platform has published exact requirements — dimensions, ratio and file size — they are collected on sizes by platform, each with a link to the documentation it came from and the date it was read.