How aspect ratio works
Aspect ratio is the shape of a picture: its width compared to its height. Two images with the same ratio can be scaled up or down into each other without stretching. Resizing means keeping width ÷ height constant, so once you fix one side, the other follows.
The formula
new height = new width × original height ÷ original width
new width = new height × original width ÷ original height
Example: 1920 × 1080 resized to 1280 wide
| Step | Result |
|---|---|
| Greatest common divisor of 1920 and 1080 | 120 |
| Ratio: 1920 ÷ 120 : 1080 ÷ 120 | 16:9 |
| New height: 1280 × 1080 ÷ 1920 | 720 |
Common aspect ratios
| Ratio | Typical sizes | Used for |
|---|---|---|
| 16:9 | 1280 × 720, 1920 × 1080, 3840 × 2160 | TVs, monitors, YouTube |
| 9:16 | 1080 × 1920 | TikTok, Reels, Shorts, Stories |
| 4:5 | 1080 × 1350 | Instagram portrait posts |
| 1:1 | 1080 × 1080 | Square posts, album art |
| 4:3 | 1024 × 768, 1440 × 1080 | Older TVs, iPads, many phone photos |
| 3:2 | 6000 × 4000 | DSLR and mirrorless photos |
| 16:10 | 1920 × 1200, 2560 × 1600 | Laptops, productivity monitors |
| 21:9 (≈ 64:27 or 43:18) | 2560 × 1080, 3440 × 1440 | Ultrawide monitors |
Frequently asked questions
How do I calculate aspect ratio?
Divide the width and height by their greatest common divisor. For 1920 × 1080 that's 120, giving 16:9. If the numbers don't simplify neatly, divide width by height to get a decimal ratio such as 1.78:1.
How do I find the height for a new width?
Multiply the new width by the original height and divide by the original width. Resizing 1920 × 1080 to 1280 wide: 1280 × 1080 ÷ 1920 = 720 pixels tall.
What aspect ratio is 21:9?
Ultrawide monitors marketed as 21:9 are really 64:27 (2560 × 1080, 2.37:1) or 43:18 (3440 × 1440, 2.39:1). Movies shot in 'scope' are 2.39:1, which is why they fill an ultrawide screen.
What size should video be for TikTok, Reels and Shorts?
Vertical 9:16 at 1080 × 1920 pixels. Instagram feed posts look best at 4:5 (1080 × 1350), and YouTube's standard is 16:9 (1920 × 1080 or 3840 × 2160).
Why do my dimensions need to be even numbers?
Most video codecs, including H.264 and H.265, need even widths and heights because color data is stored at half resolution. This calculator flags odd results so you can round to the nearest even number.
Rates and figures last checked October 2026.
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