How an aspect ratio is worked out
An aspect ratio is the width compared with the height. To get the tidy form, both numbers are divided by their greatest common divisor: 1920 and 1080 share a divisor of 120, which leaves 16:9. A 1080 × 1350 portrait post simplifies to 4:5, and a 4000 × 3000 photo to 4:3.
Some real sizes don't simplify neatly. The old 1366 × 768 laptop screen is exactly 683:384, which is within a tenth of a percent of 16:9, so the calculator tells you both. The decimal form, 1.78:1 for 16:9, 2.39:1 for widescreen cinema, is the easiest way to compare shapes that don't share round numbers.
Finding the missing width or height
For a known width, the height is width × ratio height ÷ ratio width. At 4:5, a 1080-pixel-wide post is 1350 tall; at 9:16 it's 1920. Type into either box and the other one follows, or pick a standard size from the table.
Leave Round to even numbers on for video. Most video is stored with 4:2:0 colour, which keeps colour at half resolution in both directions, so H.264 and similar encoders expect even widths and heights and may refuse or pad odd ones. Images don't care, so switch it off when you're sizing a picture.
Why some standard sizes aren't exact
A perfect 16:9 frame 480 pixels tall would be 853.33 pixels wide, which no screen can show, so 480p video is usually 854 × 480, the nearest even width. Small mismatches like that are normal, which is why the calculator calls a size a match when it's within a pixel or so of a named ratio, and tells you the exact simplified ratio as well.
Megapixels are simply width × height ÷ 1,000,000: 1920 × 1080 is about 2.1 MP and 3840 × 2160 is about 8.3 MP. Four times the pixels means encoders need much more data to keep 4K looking clean, the bitrate calculator shows what that does to file sizes.
The ratios creators use most
- 16:9: YouTube, TVs, and the video mode of nearly every camera. 1920 × 1080 and 3840 × 2160 are the everyday sizes.
- 9:16: Shorts, Reels, TikTok and Stories, usually 1080 × 1920. Check what the on-screen buttons cover with the safe zone checker.
- 4:5, portrait feed posts at 1080 × 1350, which take up more of a phone screen than squares.
- 1:1, square posts, profile pictures and podcast or album art.
- 4:3 and 3:2, photos. Phones usually shoot 4:3; most interchangeable-lens cameras shoot 3:2.
- 21:9, ultrawide monitors. It's a rounded name: 2560 × 1080 is really 64:27 (2.37:1) and 3440 × 1440 is 43:18 (2.39:1).
- 1.85:1 and 2.39:1, cinema's "flat" and "scope" shapes. Digital cinema masters them at 1998 × 1080 and 2048 × 858 in 2K.
Changing a video's aspect ratio
There are two ways to fit a video into a new shape, and the calculator helps with both. Crop to fill the frame and lose the edges, the video cropper lets you choose which part stays. Or fit the whole picture inside the new frame with bars or a blurred background, which is what the video resizer does. For photos and thumbnails, the image resizer offers both, with every platform's size built in.