Two ways to turn a landscape video vertical, and the one number that keeps the preview honest
A landscape video cannot become a vertical one without giving something up, and there are only two things to give up. Either the sides go, and the picture fills the new frame, or the edges of the frame go empty, and the whole picture stays. This tool offers both and names them by what they cost. Fill the frame crops to the largest window of the new shape that fits inside the original, so a 1920 by 1080 clip reframed to 9:16 becomes a 606 by 1080 slice, and you choose which slice by dragging. Show everything keeps the full picture, scaled down to fit the new shape, and fills the empty bands above and below with a heavily blurred copy of the same video, which is how most landscape clips end up on Reels and Shorts without black bars.
Every size is worked out in pixels, because a fraction of a frame is not a shape. A window half as wide and half as tall as a 1920 by 1080 frame is 960 by 540, which is 16:9, not the square it might sound like. So the crop starts from the side that has to be used in full, derives the other side from the target ratio, and rounds it down to an even number. Even, because the output is H.264 in the common yuv420p format, which stores colour at half resolution and refuses odd dimensions outright; down, because rounding up could push the window a pixel past the edge of the picture. The result is never more than two pixels away from the exact shape, and the tests state the guarantee in those terms rather than as a tolerance on the ratio, since the same two pixels shift a wide ratio more than a tall one.
The preview shows the crop without drawing a rectangle over the video. The stage on the page is the shape of the output, and the video inside it uses the browser’s object-fit cover with an object-position percentage. That percentage has a precise definition: the point that far across the picture is lined up with the point that far across the box. Worked through, it says the visible window starts at that fraction of the distance the picture can travel, which is exactly how the crop window is positioned for the export. So one number drives both. Dragging the picture changes it, the position slider changes it, and whatever the stage shows when you press export is the part of the frame that ends up in the file.
Show everything takes a small filter graph rather than a single crop. The video is split in two. One copy is scaled up until it covers the whole output, cropped to it and blurred with a box blur repeated twice, which turns it into colour and movement with no detail left. The other copy is scaled down until it fits inside the output, then rounded to even dimensions, and laid over the blurred copy in the centre. The output keeps the source’s short side as its own short side, so a 1080p clip becomes 1080 by 1920 vertical: the picture is only ever scaled down, and nothing is invented by stretching a small frame up. Square pixels are set explicitly on both paths, because a source recorded with non-square pixels would otherwise be stretched by players back into its old proportions.
Audio is handled the same way as in the other video tools. The file is asked whether it has a soundtrack before the command is built, because mapping one that is not there fails the whole export, and any audio is encoded to AAC so a WebM recording with Opus audio can go into the MP4. Like the others, the export re-encodes every frame, so it takes at least as long as the clip plays, and the first run downloads the video engine, about 30 MB, which is then cached. Nothing about the video leaves the browser tab.