Use 1080 x 1920 and stop thinking about it
9:16 is the aspect ratio: nine units wide for every sixteen tall. 1080 x 1920 is the resolution every platform recommends and the one they all accept without re-encoding penalties. Going higher — 1440 x 2560, say — gains you nothing, because the platforms downscale to 1080 wide anyway and you have paid the upload and the encode for pixels nobody sees. Going lower is visibly soft on a modern phone screen.
A 16:9 source loses most of its width, not its height
This is the part that surprises people. A 1920 x 1080 recording taken to 1080 x 1920 keeps the full height and throws away about 72% of the width — you end up with a 608-pixel-wide column of the original frame, upscaled. That is why a straight centre crop so often lands on an empty podium: the useful part of the frame is only about a quarter of it, and the speaker is rarely dead centre. Choosing WHERE that column sits is the single biggest quality decision in the conversion.
The alternatives to cropping, and when they are right
You can letterbox the 16:9 frame into the middle of a 9:16 canvas and fill the space above and below — usually with a blurred copy of the video, sometimes with a title. Nothing is cropped away, which suits a screen recording, a slide, or anything where the edges of the frame carry information. The cost is that the subject is now small on a phone. Crop when a person is speaking; letterbox when the frame itself is the content.
Frame rate, bitrate and the things that do not matter
Keep the source frame rate rather than converting — 30 to 24 or back introduces judder for no benefit. For bitrate, anything above roughly 8 Mbps at 1080 x 1920 is invisible after the platform re-encodes it, and every platform re-encodes. H.264 in an MP4 is accepted everywhere; more modern codecs gain you nothing here because the upload is transcoded regardless.
Questions
What size is a 9:16 video?
1080 x 1920 pixels. That is the resolution all three major short-form platforms recommend and the one that avoids both visible softness and wasted upload.
How much of a 16:9 video is lost when cropping to 9:16?
About 72% of the width. A 1920-pixel-wide frame becomes roughly a 608-pixel-wide column, which is then upscaled to 1080. The full height is kept. This is why where you place that column matters more than any other setting.
Should I crop or add blurred bars?
Crop when a person is speaking and the background does not matter — it puts the subject at full size on the screen. Use blurred bars when the whole frame carries information, such as a screen recording or slides, because cropping would cut the content itself.
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