mimicThe math
Aspect ratio is width ÷ height, reduced by their greatest common divisor: 1920×1080 ÷ 120 = 16:9. To resize, multiply the new width by height ÷ width.
Crop keeps the largest box of the new shape inside your frame. Pad keeps the whole frame and adds bars to reach the new shape. Both are rounded to even pixels, which 4:2:0 video encoding needs.
| From 1920×1080 to | Crop to | Picture kept | Pad to |
|---|---|---|---|
| 9:16 | 608×1080 | 31.7% | 1920×3414 |
| 1:1 | 1080×1080 | 56.3% | 1920×1920 |
| 4:5 | 864×1080 | 45.0% | 1920×2400 |
| 16:9 | 1920×1080 | 100% | 1920×1080 |
Which shape for which platform
| Platform | Shape |
|---|---|
| TikTok, Reels, Shorts | 9:16 vertical |
| Instagram Reels (accepted range) | 9:16 to 1.91:1 |
| YouTube Shorts | Square or vertical |
| YouTube video | 16:9 standard |
| 1:2.4 to 2.4:1 |
Worked example
A 3840×2160 (4K) clip for TikTok.
9:16 crop: 2160 × 9 ÷ 16 = 1215 → 1216×2160, cutting 1312 px from each side. 31.7% of the picture is kept.
Then scale to 1080×1920 for upload: 1216 → 1080 is a 0.89× downscale, so it stays sharp.
Sources
Questions
What aspect ratio is 1080×1920?
9:16, vertical. It's the full-screen size for TikTok, Reels and Shorts.
How do I turn a 16:9 video into 9:16?
Crop a 608×1080 slice from a 1920×1080 frame (or 1216×2160 from 4K) and keep the speaker inside it, or pad it to 1920×3414 with bars. Cropping fills the screen; padding shows everything but small.
Is 4:5 or 9:16 better for Instagram?
Reels play full screen at 9:16. 4:5 is the tallest shape for feed posts. Reels accept anything from 9:16 to 1.91:1.
Why even numbers?
Most video uses 4:2:0 chroma subsampling, which stores colour at half the width and height, so encoders need even dimensions.