How to use the aspect ratio calculator
- Open the Ratio menu and pick a preset, 16:9, 9:16, 4:3, 3:2, 1:1, 21:9 or 4:5, or choose Custom… to type your own, like 239:100 for cinematic 2.39:1.
- Enter the dimension you already know in the Width or Height box. The other field is filled in instantly to keep the ratio exact.
- Change either number whenever you like, the ratio stays locked, so the paired value updates as you go. The result is always whole pixels.
Why aspect ratio matters
Aspect ratio is the proportion between an image's width and its height, written as two numbers like 16:9. It's independent of size: a 1280×720 thumbnail and a 3840×2160 4K frame are both 16:9, because scaling one gives you the other. That's exactly why the ratio matters more than the raw pixel count. Get it right and your image fills its frame cleanly at any size; get it wrong and the platform steps in, cropping the sides off a landscape photo you posted as a vertical story, or slapping black bars around a square video on a widescreen player.
The practical payoff is avoiding rework. Say you're exporting a banner at 1920 wide and need it to stay 16:9: type 1920 and the calculator returns 1080, so you set the canvas once and never discover a squashed result later. Designers use it to size responsive containers, video editors to match export settings to a delivery spec, photographers to plan a crop before printing, and anyone building a thumbnail to hit a platform's expected shape. Because the math is proportional, it also tells you the largest image of a given ratio that fits inside a fixed space, handy when you're fitting artwork into a layout slot.
Common aspect ratios and their resolutions
- 16:9, 1280×720 (HD), 1920×1080 (Full HD), 2560×1440 (QHD), 3840×2160 (4K UHD). The default for YouTube, TVs, monitors and most video.
- 9:16, 1080×1920. Full-screen vertical for TikTok, Instagram Reels, YouTube Shorts and Stories.
- 1:1, 1080×1080. The square that suits profile pictures and Instagram feed posts.
- 4:5, 1080×1350. The tallest portrait Instagram allows in the feed, so it claims more screen than a square.
- 4:3, 1024×768, 1600×1200. The classic ratio of older monitors, many slide decks and a lot of tablet screens.
- 3:2, 3000×2000, 6000×4000. The native shape of most DSLR and mirrorless cameras, and standard 4×6 photo prints.
- 21:9, 2560×1080, 3440×1440. Ultrawide monitors and cinematic letterboxed video.
Frequently asked questions
How do I calculate height from width for 16:9?
Multiply the width by 9 and divide by 16. For 1920 wide: 1920 × 9 ÷ 16 = 1080. This calculator does that instantly for any ratio and works both directions, so entering a height gives you the matching width too.
What does an aspect ratio like 16:9 actually mean?
It's the proportion of width to height, not a size. 16:9 means for every 16 units across there are 9 down, so 1280×720, 1920×1080 and 3840×2160 are all 16:9, each is just a scaled version of the same shape.
How do I enter a decimal ratio like 2.39:1?
Choose Custom… and use whole numbers that keep the proportion, 239:100 for 2.39:1, or 47:20 for a cinema 2.35:1. The calculator only needs the ratio between the two values, not tidy integers.
Which aspect ratio should I use for social media?
9:16 for TikTok, Reels and Shorts; 1:1 or 4:5 for Instagram feed posts; 16:9 for YouTube; and roughly 1.91:1 (1200×630) for the link-preview image that appears when a page is shared.
Why does my photo get cropped or letterboxed?
Because its ratio doesn't match the frame it's shown in. A platform either crops the overflow (cutting the edges to fill the space) or letterboxes it (adding bars to fit the whole image). Exporting at the target ratio in the first place avoids both.
Does resizing to a new ratio distort the image?
Changing the pixel dimensions to a different ratio and forcing the image to fill them stretches it. To change ratio without distortion you crop instead. Use this tool to find the exact target dimensions, then apply them in the image resizer.