Every stretched logo and squashed face is an aspect-ratio crime. The fix is one proportion: scale one dimension, and the other must follow by the same factor. This calculator locks that relationship and solves whichever side you leave alone.
The everyday example
1920×1080 needs to fit a 1280-wide slot: 1280 × (1080/1920) = 720. Edit either new dimension and the other recomputes — the live preview box reshapes so you can see the ratio, not just read it. The ratio itself comes from dividing both sides by their GCD: 1920:1080 ÷ 120 = 16:9.
The ratios that run your screens
16:9 owns video (720p through 8K). 4:3 is the vintage TV and iPad shape. 21:9 stretches across ultrawide monitors and cinema. 9:16 is 16:9 turned upright — Stories, Reels, TikTok. 1:1 squares the Instagram feed. And A4 paper uses 1:√2, the unique ratio that survives folding in half — each preset chip loads real pixel dimensions.
Fit, fill, or distort
When content and container disagree, you choose a casualty: letterbox (fit entirely, bars appear), crop (fill entirely, edges lost), or stretch (everything survives, everything lies). Professional layouts pick from the first two; the third is how ovals happen to circles.
Beyond pixels
The same proportion math scales recipes and maps in the ratio calculator, and print sizing crosses into physical units via the length converter. For screen-area comparisons (is a 27″ 16:9 bigger than a 25″ 21:9?), diagonal and ratio together decide — width = diagonal × cos(atan(h/w)).
Solved dimensions round to whole pixels.