Published 2026-09-29
Notations like "4:5" or "16:9" show up constantly, but it's easy to lose track of what they actually mean. A ratio describes the relationship between width and height — it's separate from the actual pixel dimensions.
Aspect ratio expresses width to height as the simplest possible whole numbers. "4:5" means for every 4 units of width there are 5 units of height — whether the actual size is 1080 × 1350px or 400 × 500px, the ratio is the same 4:5.
Divide both the width and the height by their greatest common divisor to get the simplest ratio.
| Pixel size | Calculation | Ratio |
|---|---|---|
| 1080 × 1350px | GCD 270 → 1080÷270 : 1350÷270 | 4 : 5 |
| 1920 × 1080px | GCD 120 → 1920÷120 : 1080÷120 | 16 : 9 |
| 1080 × 1920px | GCD 120 → 1080÷120 : 1920÷120 | 9 : 16 |
If working out the GCD feels like a hassle, it's just as useful to compute width ÷ height as a decimal. 4:5 is 0.8, 16:9 is about 1.78, and 9:16 is about 0.5625. If two photos share that number, they share the ratio.
| Ratio | Decimal (width ÷ height) | Typically used for |
|---|---|---|
| 1:1 | 1.0 | Square posts, profile pictures |
| 4:5 | 0.8 | Instagram feed portraits |
| 3:4 | 0.75 | Instagram grid-matched, Threads portrait |
| 9:16 | 0.5625 | Stories, Reels, short-form video |
| 16:9 | 1.78 | Landscape video, YouTube thumbnails, X images |
You don't need to memorize the numbers or do the math by hand. prezll's Crop tool ships with presets for the ratios used most often — pick the platform's name and it's matched exactly, no calculation needed. Free-form cropping is there for the rare case when you need a ratio outside the presets.
Crop to a ratio now