How aspect ratio scaling works
An aspect ratio is simply the proportion between width and height. To resize without distortion you keep that proportion fixed. If you know the original size and one new dimension, the other follows directly:
new height = new width × ( original height ÷ original width )
new width = new height × ( original width ÷ original height )
The simplified ratio is found by dividing both numbers by their greatest common divisor (GCD). 1920 and 1080 share a GCD of 120, which reduces to 16:9.
Worked example
You have a 1920×1080 image and want it 1280 pixels wide. Keep the height blank and solve:
Ratio factor: 1080 ÷ 1920 = 0.5625.
New height: 1280 × 0.5625 = 720.
Result: 1280×720, still a clean 16:9.
Tip: if you enter both new width and height the calculator will warn you when they don't match the original ratio, so you can spot distortion before exporting.
Common video and screen ratios
16:9 is the standard for HD and 4K video and most monitors. 4:3 is the older television and slide format. 21:9 is ultrawide cinematic, and 1:1 is the square format popular on social media. Tap a preset above to load it into the original-size fields instantly.