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When you set a target width and height, the video is scaled down (or up) to fit entirely inside that box while keeping its original aspect ratio, then any leftover space is filled with black bars (letterboxing on the top/bottom, or pillarboxing on the sides) so the output exactly matches the dimensions you requested. This means the original footage is never stretched or squashed to fill the frame — if your source and target aspect ratios don't match, you'll see black bars rather than distorted video.

This matters most when your target size has a very different shape than your source. A horizontal 16:9 video resized into a 9:16 vertical target won't magically become full-frame vertical footage — the scaled video fills the target's full width and shrinks to fit vertically, leaving it centered with black bars on the top and bottom (since width, not height, ends up being the limiting dimension), rather than cropping into the frame. If you specifically want a full-bleed vertical crop instead of letterboxing, the site's separate Crop Video tool lets you select exactly which portion of the frame to keep.

For a straightforward resolution change where the aspect ratio already matches — say, downscaling a 4K (3840x2160) recording to 1080p (1920x1080), both 16:9 — there's no letterboxing at all, since the shapes already match and the video simply scales cleanly to the new size.

Width and height can each be set from 64px up to 3840px (width) or 2160px (height), covering everything from a tiny preview thumbnail size up to 4K. As with other tools on the site, there's also a CRF quality slider and preset alongside the resize settings, since resizing re-encodes the video.