Aspect Ratio Calculator
Keep images and videos in proportion. Enter a ratio and one dimension to compute the other instantly.
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An aspect ratio is the proportional relationship between the width and the height of a rectangle, written as two numbers separated by a colon, such as 16:9 or 4:3. It describes shape independently of size: a 16:9 image is the same shape whether it is 1920 by 1080 pixels on a monitor or 4 metres by 2.25 metres on a billboard. Because the ratio fixes the relationship between the two dimensions, knowing the ratio and just one dimension is enough to work out the other. This calculator does exactly that: you choose a ratio, type in either the width or the height, and it computes the missing value while keeping everything in proportion.
Frequently asked questions
Is my data sent anywhere?
How does the calculator work out the missing dimension?
Can I enter a custom ratio?
Why is my computed dimension not a whole number?
What is the difference between 16:9 and 4:3?
What ratio should I use for social media?
Does aspect ratio change the file size of an image?
How do I keep an image in proportion when resizing?
About Aspect Ratio Calculator
Aspect ratios matter to almost anyone who works with images, video, or layout. Photographers crop to 3:2 or 4:3 to match print and sensor formats. Videographers and YouTubers deliver in 16:9 for standard widescreen, 9:16 for vertical mobile video, and 21:9 for cinematic ultrawide. Web designers size hero images and embeds so they scale cleanly across devices without distortion. Anyone resizing an image for a social media post, a presentation slide, or an email header needs to keep the ratio intact, because stretching one dimension without adjusting the other squashes or elongates the picture. Doing the arithmetic by hand is easy to get wrong, especially with awkward custom ratios, which is where a dedicated calculator saves time and avoids distorted results.
This tool runs entirely in your browser. Pick one of the common presets (16:9, 4:3, 21:9, 1:1, 3:2) or type a custom ratio of your own, choose whether you know the width or the height, enter that value, and the matching dimension appears immediately. You can copy either result to the clipboard with a single click. No measurements are sent to any server, nothing is stored, and the page keeps working offline after it has loaded.
A couple of points worth knowing. Aspect ratio is about shape, not absolute size, so a 1:1 ratio is always a square regardless of how large it is, and a 21:9 frame is always far wider than it is tall. The computed dimension may not be a whole number; for example, a 1000-pixel-wide image at 16:9 works out to 562.5 pixels tall, and you will usually round to the nearest whole pixel. Rounding by a pixel here and there is harmless in practice, but if you are tiling or stitching images precisely, prefer ratios and base sizes that divide cleanly to avoid accumulating small errors.
Why Screens Settled on 16:9
The aspect ratios we use today are the product of more than a century of competing technical and commercial decisions. Early silent films were shot in a roughly 4:3 frame, formalised as the Academy ratio of 1.375:1 in 1932 once a soundtrack strip had to be accommodated alongside the image. This near-square shape carried over directly into television, which is why 4:3 dominated home screens for decades. For most of the 20th century, if you owned a TV, you owned a 4:3 box.
Widescreen cinema emerged in the 1950s as a deliberate weapon against the threat of television. To lure audiences back into theatres, studios introduced dramatically wider formats such as CinemaScope (around 2.35:1) and the slightly less extreme 1.85:1, giving the big screen a sweeping, immersive look that a boxy living-room set could not match. This created a long-running mismatch: films shot wide had to be cropped, letterboxed, or pan-and-scanned to fit 4:3 televisions, a compromise that frustrated directors and viewers alike.
The 16:9 ratio that now dominates was effectively a negotiated compromise. When engineers were designing standards for high-definition and widescreen television in the 1980s, they sought a single shape that could reasonably accommodate the wide range of existing cinema formats with the least objectionable cropping. The mathematician Kerns Powers is credited with demonstrating that 16:9 (1.78:1) sat neatly between the older 4:3 and the wide cinematic ratios, making it the best all-round fit. It was standardised for HDTV and, with the arrival of flat-panel displays, online video, and smartphones held sideways, 16:9 became the default shape of the modern screen. The recent rise of vertical 9:16 video for mobile feeds is, in a sense, the same ratio turned on its side for a new way of holding the device.