The math behind the auto-scaling
If you know one dimension, the other follows from the ratio. For a 16:9 video with a width of 1080 pixels: height = (1080 × 9) ÷ 16 = 607.5, rounded to 608 pixels. Type either number into the calculator and the other appears automatically.
Two ways to write the same ratio
Ratios show up in two common forms: whole numbers like 16:9, or a decimal against 1, like 1.78:1 — both describe the exact same shape. Whole-number form comes from dividing both sides by their greatest common factor (1920 and 1080 both divide evenly by 120, leaving 16:9); decimal form just divides width by height directly. Real screen resolutions rarely reduce to a clean whole-number ratio on their own — a 2560×1080 ultrawide monitor technically simplifies to 64:27, which is why the "Find My Ratio" tool above also matches your dimensions against the nearest common named ratio, not just the exact mathematical reduction.
Why 9:16 dominates mobile
Vertical video fills the entire phone screen without rotation, which is why every short-form platform standardized on 9:16 at 1080×1920 pixels. Shooting at this size natively — rather than cropping a landscape clip — keeps your subject centered and your resolution sharp. Modern phone screens themselves lean even taller than that: most current iPhones sit around 19.5:9, taller and narrower than the classic 16:9 of older displays, which is one reason full-screen video looks slightly cropped on newer phones unless it's exported at the phone's actual ratio.
Even movies don't always agree on one ratio
Christopher Nolan's Oppenheimer was shot on two different film formats in the same movie — large-format IMAX scenes at a tall 1.43:1, and anamorphic 65mm scenes at a wider 2.20:1 — switching ratio mid-film depending on the scene. Standard theaters without true IMAX projectors show the whole film cropped to one uniform widescreen ratio, which is why some viewers seek out specific IMAX screenings to see the frame the way it was actually shot.
Safe zones matter too
Remember that platforms overlay UI elements (captions, buttons, usernames) on your video. Keep critical content — faces, text, product shots — inside the middle ~80% of the frame so nothing important is hidden behind interface elements.