Aspect ratios explained
An aspect ratio describes the shape of an image or screen — the relationship between its width and its height. Here is what the common ratios mean and when to reach for each.
What an aspect ratio actually is
An aspect ratio is written as width : height. A 16:9 image is 16 units wide for every 9 units tall. It is a proportion, not a size: a 1280 × 720 photo and a 1920 × 1080 photo are both 16:9, because both reduce to the same ratio when you divide width and height by their greatest common divisor.
That is the key idea — a ratio is the simplified form of a pixel size. 1920:1080 divides by 120 to give 16:9. If you only remember one thing, remember that the ratio fixes the shape while the resolution sets the detail. (More on that in aspect ratio vs. resolution.)
The common aspect ratios at a glance
| Ratio | Orientation | Typically used for |
|---|---|---|
| 16:9 | Landscape | YouTube, TVs, most laptop and monitor screens, HD/4K video |
| 9:16 | Portrait | Instagram Reels & Stories, TikTok, YouTube Shorts, phone-first video |
| 4:3 | Landscape | Classic TV and monitors, slide decks, many compact and tablet cameras |
| 1:1 | Square | Instagram feed posts, profile pictures, album art, avatars |
| 3:2 | Landscape | 35 mm and most DSLR/mirrorless photography, 4×6 prints |
| 4:5 | Portrait | Instagram portrait posts — the tallest feed post the app allows |
| 21:9 | Landscape | Ultrawide monitors and cinematic / letterboxed video |
| 2.39:1 | Landscape | Anamorphic widescreen cinema (the classic "movie" look) |
When to use each
16:9 — the default for video and screens
If you are not sure, 16:9 is the safe choice for anything that plays on a screen: YouTube uploads, presentations shown on a TV, webinars, and most embedded video. Thumbnails for 16:9 video are usually 1280 × 720.
9:16 — vertical, phone-first
Turn 16:9 on its side and you get 9:16, the full-screen portrait shape used by TikTok, Reels, Stories and Shorts. Design these for a phone held upright; 1080 × 1920 is the standard size.
1:1 and 4:5 — the social feed
Square (1:1) is the timeless feed format. On Instagram specifically, a 4:5 portrait post takes up more vertical space in the feed, so it often gets more attention. 1080 × 1080 and 1080 × 1350 are the go-to sizes.
3:2 — photography
Most interchangeable-lens cameras shoot 3:2 because it matches the classic 35 mm film frame. It also crops cleanly to common print sizes like 4×6.
4:3 — presentations and older formats
4:3 is the legacy TV and monitor shape. You will still meet it in slide decks, some documents, and older or industrial cameras.
21:9 and 2.39:1 — cinematic
Ultrawide ratios feel filmic and suit ultrawide monitors, but they letterbox (black bars) on standard 16:9 screens. Use them deliberately, for mood, not by default.
Rule of thumb: match the ratio to where the content will be seen. Landscape screens want 16:9, phones want 9:16, and the social feed wants 1:1 or 4:5. Pick the ratio first, then let the dimensions follow.
From ratio to actual dimensions
Once you know the ratio, getting real pixel dimensions is easy: fix one side and the other is determined. For 16:9, a 1920 px width forces a 1080 px height (1920 × 9 ÷ 16 = 1080). That is exactly what the RatioForge calculator does — type a width or a height, pick a ratio, and read the matching dimension. If you want to do it by hand, see how to calculate an aspect ratio.
Try it on your own numbers
Pick a ratio, enter a width or height, and get the matching dimension instantly.
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