How this instrument works
A portrait aspect ratio describes the shape of a vertical image or video frame — one that's taller than it is wide — as a simplified ratio of width to height, such as 9:16. It's the mirror image of the familiar landscape ratios like 16:9, just rotated: the same two numbers, with width and height swapped.
Reducing a resolution like 1080x1920 to 9:16 uses the greatest common divisor (gcd) of the width and height. Dividing both numbers by their gcd finds the smallest whole-number pair that describes exactly the same proportions — 1080 and 1920 both divide evenly by 120, leaving 9 and 16, and no smaller whole-number pair describes that same shape.
Portrait ratios matter for mobile-first video because platforms enforce them at upload: Instagram Stories, TikTok and YouTube Shorts are all built around 9:16 vertical frames sized to fill a phone screen held upright, and content shot or exported in the wrong ratio typically gets cropped or letterboxed to fit.
- Enter your vertical frame's pixel dimensions into Width (px) and Height (px) — width should be the smaller, portrait-orientation number.
- Read the simplified ratio from Simplified ratio — width part and Simplified ratio — height part, shown together as width:height.
- Compare the result against a target platform ratio, such as 9:16 for Stories, Reels and Shorts, to check whether your footage needs cropping before upload.
- If your footage was shot landscape and needs converting to portrait, note that this instrument only reduces a ratio — cropping or padding the actual frame is a separate step in your video editor.
Worked example — a 1080x1920 vertical video export
Enter 1080 into Width (px) and 1920 into Height (px) — the standard Full HD portrait export used for Instagram Stories, TikTok and YouTube Shorts. The greatest common divisor of 1080 and 1920 is 120. Dividing both numbers by 120 gives Simplified ratio — width part = 9 and Simplified ratio — height part = 16, so the frame reduces to a 9:16 ratio.
That 9:16 result is exactly the ratio every major short-form video platform expects for full-screen vertical playback, which is why 1080x1920 has become the default export preset in most phone camera apps and video editors for social content.
Questions
Why is 9:16 the standard portrait video ratio?
Because it's the vertical rotation of 16:9, the dominant landscape video and TV standard, so footage shot on a 16:9-sensor phone held upright naturally fills a 9:16 frame. Instagram Stories, TikTok and YouTube Shorts all standardised on 9:16 for full-screen vertical playback, which made 1080x1920 (or its higher-resolution multiples like 1440x2560) the default portrait export size across nearly every phone camera app and mobile editor.
How is a portrait ratio different from a landscape ratio like 16:9?
It's the same two numbers with width and height swapped — 16:9 landscape becomes 9:16 portrait. The underlying gcd-reduction math is identical either way; only the orientation, and which dimension is larger, changes. A frame that's 1920 wide and 1080 tall is landscape 16:9; rotate the device and the same sensor produces a 1080-wide, 1920-tall frame, which is portrait 9:16.
My phone screen resolution doesn't reduce to a clean ratio like 9:16 — why?
Many phones use resolutions that are close to, but not exactly, a simple ratio, because the panel size is chosen for other engineering reasons and then cropped or letterboxed by software to standard content ratios. An iPhone 13's 1170x2532 screen, for example, reduces to 195:422 rather than a tidy ratio — the display simply isn't manufactured to hit round numbers, even though the apps running on it typically render standard 9:16 vertical content within that physical panel.
Do I need to physically crop my footage to change its aspect ratio?
Yes — this instrument only tells you what ratio a given pixel width and height already describes; it doesn't alter the footage itself. Converting landscape footage to a portrait ratio for Stories or Shorts means cropping, padding with blurred or solid-colour bars, or re-filming vertically in your camera or video-editing software, which is a separate step from the arithmetic this page performs.
What's the difference between aspect ratio and resolution?
Resolution is the exact pixel count, like 1080x1920; aspect ratio is the simplified shape that resolution describes, like 9:16. Many different resolutions share the same aspect ratio — 1080x1920, 1440x2560 and 2160x3840 are all 9:16 — because what matters for framing and cropping is the proportion between width and height, not the absolute pixel count.