Aspect Ratio Calculator
Find the aspect ratio of any resolution, or compute the missing dimension when resizing — 16:9, 4:5, 3:2 and more.
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How Aspect Ratio Is Actually Calculated
An aspect ratio is just width and height expressed as the simplest possible whole-number pair, found by dividing both by their greatest common divisor (GCD). Take 1920×1080: the GCD of 1920 and 1080 is 120, so dividing both by 120 gives 16:9 — the ratio itself, independent of the actual pixel count. That's why 1920×1080 (Full HD) and 3840×2160 (4K) are both exactly 16:9: 3840 and 2160 share a GCD of 240, and 3840÷240 = 16, 2160÷240 = 9. Same shape, four times the pixels. This is the entire reason resolutions with a matching ratio can be scaled up or down without cropping or adding blank space — the geometry lines up perfectly at any size.
The common named ratios each map to a real-world use: 16:9 is the standard for modern displays, TV and online video (YouTube, most streaming); 4:3 is the older TV and monitor standard, boxier and taller relative to its width; 1:1 is a perfect square, long associated with Instagram's original grid format; 21:9 is an ultra-wide, cinematic ratio used in some monitors and anamorphic film formats; and 9:16 is 16:9 rotated, the vertical format for Stories, Reels and TikTok.
What to Watch Out For
- Mismatched ratios always cost you something. Displaying 4:3 footage on a 16:9 screen without cropping leaves black bars on the sides (pillarboxing); stretching it to fill the screen distorts every shape in the image instead.
- Converting between ratios means cropping, not stretching. Turning 16:9 footage into 9:16 for a vertical platform means cutting away the sides of the frame (a 1920×1080 frame yields a 608×1080 vertical slice) — anything important near the left or right edge gets lost, which is why vertical versions are best framed at the time of shooting, not fixed in post.
- Resizing while keeping proportions needs one formula. To scale a width to a new value without distortion: new height = new width × (original height ÷ original width) — this is exactly what the GCD-based ratio guarantees will look right at any size.
- The reduced ratio hides the resolution. Two files can both say "16:9" while one is 640×360 and the other is 7680×4320 — the ratio tells you the shape, not the detail or file size.
Frequently Asked Questions
How is an aspect ratio calculated from a resolution?
Divide both width and height by their greatest common divisor (GCD). For 1920×1080, the GCD is 120, so 1920÷120 : 1080÷120 = 16:9.
Why do 1920×1080 and 3840×2160 both count as 16:9?
Because their width-to-height relationship reduces to the same simplest ratio — 3840×2160 has a GCD of 240, giving 16:9 again. Same shape, four times the total pixels, which is exactly why 4K scales to Full HD (or vice versa) without cropping or letterboxing.
What resolution is 4:5 for Instagram?
The recommended portrait size is 1080×1350. From any width, height = width × 1.25, which is the ratio's height-to-width relationship (5 ÷ 4).
How do I convert 16:9 footage to 9:16 without distorting it?
You crop rather than stretch — a 1920×1080 frame yields a 608×1080 vertical slice from its center. Anything positioned near the left or right edge of the original frame will be cut off, so vertical crops work best when planned during filming.
What happens if I display content in the wrong aspect ratio for a screen?
Either the image is stretched, distorting every circle into an oval and every face into an odd shape, or it's shown at its correct ratio with black bars filling the gap — vertical bars (pillarboxing) for narrower content on a wider screen, horizontal bars (letterboxing) for wider content on a narrower screen.
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