Aspect ratios explained: what 16:9, 4:3, 3:2 and 9:16 mean, and how to crop between them

An aspect ratio is a shape, written as width to height: 16:9 is a widescreen TV, 1:1 a square, 9:16 a phone held upright. Getting it wrong is how photos arrive squashed and videos get black bars, and getting it right is a piece of arithmetic the aspect ratio calculator does instantly. This guide covers where the ratios come from and how to move between them.

What an aspect ratio is

The ratio is width divided by height, reduced to small whole numbers. A 1920×1080 image is 16:9 (both divide by 120); 4032×3024 is 4:3; 3000×2000 is 3:2. It says nothing about size — a 16:9 thumbnail and a 16:9 cinema screen are the same shape — which is exactly the point: a ratio is what you must preserve when resizing, because changing width and height by different factors distorts the picture.

Where the common ratios come from

• 4:3 — early film and television, the first computer monitors, and still most phone camera sensors and iPads.
• 3:2 — 35 mm film (36×24 mm), inherited by DSLR and mirrorless sensors and by 6×4-inch prints.
• 16:9 — HDTV's compromise between 4:3 television and wider cinema; now laptops, monitors, YouTube and most video.
• 1:1 — Instagram's original format, still the safe default for feeds and profile images.
• 9:16 — 16:9 turned upright: Stories, Reels, Shorts, TikTok.
• 4:5 and 5:4 — Instagram's tallest feed ratio and large-format film respectively.
• 21:9 and 2.39:1 — ultrawide monitors and cinematic widescreen.
• 1:1.414 (√2) — A-series paper, whose special property is that halving a sheet keeps the ratio.

The arithmetic

Given one dimension and a ratio, the other is a proportion: for 16:9 at 1280 wide, height = 1280 × 9 ÷ 16 = 720. To find an image's ratio, divide both dimensions by their greatest common divisor (the GCD calculator): 1366 and 768 share 2, giving 683:384 — which is why many "16:9" laptop screens are not quite 16:9. To check whether two sizes share a ratio, compare width ÷ height as a decimal: 1.777 for 16:9, 1.333 for 4:3, 1.5 for 3:2.

Crop, letterbox or stretch

Moving a picture from one ratio to another means one of three things. Cropping removes edges — a 3:2 photo becomes 16:9 by trimming top and bottom, or 1:1 by trimming the sides; composition decides where. Letterboxing (or pillarboxing) keeps everything and pads with bars — right for video where cropping loses action. Stretching keeps everything, loses nothing, and makes people look wide; it is never right, and it is what happens when width and height are typed independently. The image cropper offers ratio presets that lock the shape while you choose the region; the video cropper does the same for footage, and the resizer keeps proportions by default.

Ratios platforms expect

Landscape video: 16:9. Vertical short-form: 9:16. Feed posts: 1:1 or 4:5 (Instagram), 1.91:1 for link previews (Social media image sizes and aspect ratios lists the pixel sizes). Profile photos: square, shown as a circle — keep the subject centred. Prints: 3:2 for 6×4, 5:4 for 8×10, 1:√2 for A4; a 4:3 phone photo printed at 6×4 loses a strip on two sides, which is the moment most people first meet aspect ratios in the wild.