Free online tool
Aspect Ratio Calculator
Enter the target ratio and one dimension — get the matching dimension instantly.
Where aspect ratios matter
Aspect ratio is the relationship between a rectangle's width and height. Get it right and your image, video, or container fits the medium perfectly; get it wrong and you get black bars, awkward crops, or a layout that jumps as content loads. Common ratios include 16:9 for YouTube and modern displays, 1:1 for square social posts, 9:16 for TikTok and Reels, 4:5 for Instagram feed, and 21:9 for ultrawide cinema footage.
How this calculator works
Enter the ratio width and ratio height (for example 16 and 9), then type the pixel width you have. The tool computes the matching pixel height using simple cross-multiplication: height = width × ratioH / ratioW. It's rounded to the nearest whole pixel because subpixel dimensions cause blurry rendering in most browsers. Try any of the preset ratios if you're not sure what to use for a given platform.
SEO and Core Web Vitals tie-in
Declaring correct width and height attributes on <img> and <video> tags lets the browser reserve the right amount of space before the file loads. That eliminates layout shift, which is one of the three Core Web Vitals Google measures. So this calculator isn't just a design convenience — it's a small ranking factor. Whenever you insert media, use the ratio to figure the width/height pair, then paste those into your HTML.
+What is the aspect ratio of a 4K TV?
3840×2160 pixels — that's 16:9, the same ratio as 1080p HD.
+Should Instagram Reels be 9:16?
Yes. Full-screen mobile video is 9:16. Feed square posts stay at 1:1 or 4:5 vertical.
+Why round to a whole pixel?
Sub-pixel dimensions get antialiased by the browser, causing blurriness. Whole pixels stay crisp.
Keeping proportions intact across screens, prints and social feeds
An aspect ratio is just a width-to-height relationship reduced to its simplest form, and getting it wrong means either awkward cropping or letterboxing — this tool solves for the missing dimension so the ratio stays exact.
Different platforms and mediums favor different ratios for very concrete reasons, and knowing those reasons makes it much easier to pick dimensions that won't need re-cropping later.
The core math
If you know the ratio W:H and one dimension, the other follows directly: for 16:9 at a width of 1920, height = 1920 x 9/16 = 1080. For a 4:5 portrait crop (common on Instagram) starting from a 1600px-wide image, height = 1600 x 5/4 = 2000. Cross-multiplication is all that's happening under the hood, but doing it correctly by hand for odd starting numbers is where mistakes creep in.
16:9 versus 4:5 versus 1:1 in practice
16:9 (roughly 1.78:1) suits horizontal video and desktop displays; 4:5 (0.8:1) is taller than it is wide and is optimized for mobile feed scrolling, showing about 25% more vertical content than a square 1:1 crop in the same feed width. Converting a 16:9 landscape photo directly to 4:5 without recomposing will typically cut off the left and right edges significantly, since you're going from a wide rectangle to a tall one.
A caution on rounding
Pixel dimensions must be whole numbers, so a ratio like 21:9 applied to a width of 1000 gives a height of 428.57, which has to round to 428 or 429 — a tiny discrepancy that's invisible on screen but can cause a one-pixel border or letterbox line to appear when the file is placed into a strict container that expects the ratio exactly.
Where these specific ratios come from
16:9 became the television and monitor standard in the early 2000s as a compromise between the boxier 4:3 of old CRT TVs and the ultra-wide theatrical ratios like 2.39:1 used in cinema. 1:1 square format traces back to Instagram's original photo grid before the platform later added support for taller crops, and 9:16 (16:9 rotated) now dominates vertical video for Stories and Reels because it fills a phone screen held upright without any letterboxing.
Simplifying a ratio to its lowest terms
Given raw pixel dimensions like 3840x2160, the aspect ratio simplifies by dividing both numbers by their greatest common divisor (240 in this case), giving 16:9 — the same math taught in grade-school fraction reduction, just applied to screen dimensions instead of pie slices. This is useful for identifying, from dimensions alone, whether an unfamiliar video file is standard widescreen, ultrawide, or something nonstandard that a device might not render correctly.
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