Understanding aspect ratio
Aspect ratio describes shape, not size. Once you can read one, most dimension problems get simpler.
What an aspect ratio is
An aspect ratio is the relationship between the width and the height of a rectangle, written as two numbers separated by a colon. A 16:9 frame is 16 units wide for every 9 units tall. The units cancel out, so the same ratio describes a phone screen, a cinema projection and a postage stamp — anything with that shape.
Because a ratio carries no size information, two images with the same ratio can have wildly different pixel counts. 1920 × 1080 and 3840 × 2160 are both 16:9. One has four times the pixels of the other.
How a ratio is simplified
To simplify a pair of dimensions, divide both by their greatest common divisor. For 1920 × 1080 that divisor is 120, which gives 16:9. For 2560 × 1080 it is 40, which gives 64:27 — a ratio most manufacturers market as 21:9, even though 21:9 is not what the panel actually measures.
This distinction matters. The Aspect Ratio Calculator always shows the exact simplified ratio and only names a common ratio when the match is exact, rather than nudging your dimensions towards a familiar label.
Landscape, portrait and square
- Landscape — the width is greater than the height, so the ratio's first number is larger.
- Portrait — the height is greater than the width. 9:16 is 16:9 stood on end.
- Square — width and height are equal, giving 1:1.
Reversing the two dimensions inverts the ratio. That is why a video shot vertically on a phone is 9:16 and leaves bars on either side when shown on a 16:9 television.
Common aspect ratios
| Ratio | Decimal | Typically used for |
|---|---|---|
| 1:1 | 1 | Square. Common for social posts and album art. |
| 5:4 | 1.25 | Used by 8 × 10 in prints and some older displays. |
| 4:3 | 1.3333 | Classic television, many cameras and tablets. |
| 3:2 | 1.5 | Standard 35 mm photography and many DSLR sensors. |
| 16:10 | 1.6 | Common on laptops and productivity monitors. |
| 16:9 | 1.7778 | The standard widescreen video and display ratio. |
| 2:1 | 2 | Univisium. Used by some films and streaming productions. |
| 64:27 | 2.3704 | The exact ratio most 21:9 ultrawide displays use. |
| 32:9 | 3.5556 | Super ultrawide. Two 16:9 displays side by side. |
| 9:16 | 0.5625 | Vertical video for phones and short-form platforms. |
| 4:5 | 0.8 | Portrait social posts and 8 × 10 in prints. |
Decimal ratios in cinema
Film conventionally writes ratios as a decimal against 1: 1.85:1 for standard widescreen, 2.39:1 for modern anamorphic scope. These are used exactly as written rather than converted into whole-number fractions, because the decimal is the specification. TheResize Calculator accepts them directly.
Resizing without distortion
To keep proportions when resizing, both dimensions must change by the same factor. For a ratio A:B:
heightequalswidth × B ÷ A
widthequalsheight × A ÷ B
Ratios do not always divide into whole pixels. 16:9 at 1000 px wide gives 562.5 px, so a practical answer has to round to 562 or 563 — a difference small enough to be invisible, but worth knowing about rather than hiding.
When shapes do not match
Putting content of one ratio into a frame of another leaves you two choices: fill the frame and crop away what does not fit, or fit the whole thing in and accept unused bars. TheCompare & Crop tool shows exactly how much area each option costs.