An aspect ratio looks like a simple pair of numbers, but the way it's calculated — and the rounding that happens when you scale by it — explains a lot of confusing image and video sizing problems.
What an Aspect Ratio Actually Means
An aspect ratio describes the proportional relationship between a width and a height, not their actual size. A 16:9 ratio just means the width is always 16 units for every 9 units of height — it says nothing about whether that's 1920×1080 pixels or 160×90 pixels. Two images can look completely different in physical size while sharing the exact same ratio and, therefore, the exact same shape.
How a Ratio Gets Simplified
To go from raw dimensions like 1920×1080 to a clean ratio like 16:9, both numbers are divided by their greatest common divisor (GCD) — the largest number that divides evenly into both. 1920 and 1080 share a GCD of 120, so dividing both by 120 gives 16 and 9. This is the same simplification process used for reducing any fraction, just applied to two dimensions instead of a numerator and denominator.
Common Ratios You'll Run Into
16:9 is the standard for widescreen video, most TVs, and YouTube uploads. 4:3 was the old standard-definition TV and early computer monitor shape, boxier than modern screens. 1:1 (square) is common for social media profile photos and some Instagram posts. 9:16 is vertical video — the shape of Reels, TikTok, and Stories. 21:9 is ultrawide, used for cinematic video and some gaming monitors. Picking the wrong one for a platform is a common reason uploaded video or images get cropped or letterboxed unexpectedly.
Why Rounding Can Distort an Image
When you know a ratio and one dimension, the matching dimension is found with simple cross-multiplication — but the result isn't always a whole number. Scaling a 16:9 video to a 1000px width, for example, gives a height of exactly 562.5 pixels, which doesn't exist as a physical pixel. Whatever software does the resizing has to round that to 562 or 563, which technically shifts the actual ratio by a tiny, usually invisible amount. This is normal and unavoidable at odd widths — it's only a real problem when a chain of multiple rounding steps compounds into a noticeably stretched result.
Find Your Ratio Instantly
Enter a width and height to get the simplified ratio, or enter a ratio plus one known dimension to calculate the matching missing dimension, with the free Aspect Ratio Calculator — no sign-up, and nothing you enter leaves your browser.
FAQ
Why do 1920×1080 and 1280×720 have the same aspect ratio? Because both reduce to 16:9 when divided by their greatest common divisor — 1920/120 = 16, 1080/120 = 9, and 1280/80 = 16, 720/80 = 9. Same proportion, different pixel counts.
What's the difference between 16:9 and 16:10? They look similar but aren't interchangeable — 16:9 is standard for widescreen video and most TVs, while 16:10 is slightly taller (common on some laptop and monitor displays), and stretching content built for one to fit the other distorts it.
Why does scaling by a ratio sometimes give a fractional pixel value? Because dimensions can scale to non-integer results — for example, a 1000px width at 16:9 gives a height of 562.5. Any real image or video needs whole pixels, so that value has to be rounded, which introduces a tiny proportion error.
Is the data I enter sent anywhere? No — all the math happens entirely in your browser using JavaScript, so nothing you enter is ever sent to a server.