Resize MP4
Resizing an MP4 to a different resolution is always a re-encode. Every frame gets scaled to the new dimensions and re-compressed. Here's the ffmpeg command for common target sizes, the quality trade-offs, and where Wave contributes.
What actually changes
Resizing (changing video resolution) requires the encoder to actually re-render every frame at the new dimensions. This differs from every other MP4 operation on this site because it can't be done via container edits - the sample data itself must be recomputed. The output file is fundamentally different from the source; only the timing structure survives unchanged.
The ffmpeg command with a scale filter: `ffmpeg -i input.mp4 -vf "scale=1280:720" -c:v libx264 -crf 22 -c:a copy output.mp4` re-encodes to 720p. Common targets: `scale=1920:1080` for 1080p, `scale=1280:720` for 720p, `scale=854:480` for 480p, `scale=640:360` for 360p. To preserve aspect ratio while capping one dimension: `scale=-2:720` scales to 720p tall with proportional width (rounded to even numbers, which most codecs require). For portrait: `scale=720:-2` for 720p wide with proportional height.
The size and quality trade-offs are proportional to pixel count. Downscaling from 1080p to 720p roughly halves the pixel count, so at equivalent quality (same CRF value) the output file is about half the size. Downscaling from 4K to 1080p is a quarter of the pixels; at same CRF, roughly a quarter of the size. Downscaling also makes the encoder's job easier (smaller frames encode more efficiently per pixel), so the actual size drop is often larger than the pixel-count ratio suggests. For 4K → 1080p at CRF 22, expect the file to drop to roughly 20-30% of the source size. Full bitrate + quality trade-off math in Video bitrate explained.
Wave's current build doesn't expose a resize UI. The conversion path preserves source dimensions (a 1080p input becomes a 1080p output). A dedicated resize mode with resolution presets or custom dimension input is on the roadmap - the WebCodecs H.264 encoder already supports it; what's missing is the UI wiring. Until then, HandBrake's "Dimensions" tab or the ffmpeg command above handles it locally.
Three steps.
-
01
Step 1
For 720p output: ffmpeg -i input.mp4 -vf "scale=1280:720" -c:v libx264 -crf 22 -c:a copy output.mp4.
-
02
Step 2
For 1080p output: ffmpeg -i input.mp4 -vf "scale=1920:1080" -c:v libx264 -crf 22 -c:a copy output.mp4.
-
03
Step 3
For aspect-preserving downscale to 720p max height: ffmpeg -i input.mp4 -vf "scale=-2:720" -c:v libx264 -crf 22 -c:a copy output.mp4.
Answers, specifically.
What CRF value should I use?
CRF 22 is a good default for downscaling - visually transparent on most content. CRF 20 for higher quality (larger file), CRF 24-26 for smaller files with acceptable quality loss on high-detail content. Range is 0 (lossless) to 51 (worst); 18-28 is the practical band.
Does Wave resize videos?
Not currently. The Wave converter preserves the source resolution. A dedicated resize mode with target resolution input is on the roadmap. For today: HandBrake or ffmpeg locally handle it.
HandBrake to resize?
Yes and straightforward. HandBrake → Load source → Dimensions tab → Width/Height (linked by default to maintain aspect ratio). Start Encode. HandBrake defaults to a quality-based encode; adjust the RF slider in the Video tab for the size/quality trade.
iOS Photos to resize?
iOS Photos doesn't offer explicit resize. The Share sheet lets you pick "Actual size", "Large", "Medium", or "Small" for sharing, which does resize the version being shared. To resize the file on disk, use a Shortcut ("Resize Video" community Shortcut) or an app like Compressor for iOS.
Will resize affect audio quality?
No, because -c:a copy tells ffmpeg not to touch the audio track. The audio bytes are copied through untouched.
Do I need even numbers for width and height?
Yes for H.264 and HEVC. Both codecs require even dimensions because they encode in 4:2:0 chroma subsampling with 2-pixel chroma blocks. The -2 shorthand in ffmpeg's scale filter (scale=-2:720) automatically rounds down to the nearest even number.
Downscale 4K to 1080p - how much smaller?
Typically 20-30% of the source size at same CRF. 4K at 40 Mbps → 1080p at CRF 22 lands around 8-12 Mbps for the video track. If the source was already efficiently compressed, the ratio is smaller; if the source was over-bitrated for its content, the ratio is larger.