YouTube re-encodes every upload, so pixel-precise cropping before you send the file avoids wasted bitrate on edges you wanted removed and keeps the framing you chose in both the 16:9 player and the 9:16 Shorts feed. Video Cropper lets you set that rectangle with exact source-pixel coordinates, draw the region into a Canvas at 30 frames per second, record the stream as WebM in the current tab, and download the file without ever uploading the source. The tool is not a time trim, a resizer, a compress-only pass, or a subject tracker, and it does not preserve subtitles, chapters, rotation tags, HDR signaling, or every container feature, so the result still needs a quick review in the destination player before the original is deleted.
YouTube's Accepted Frame Sizes and Aspect Ratios
The dimensions below are widely used YouTube targets that match the platform's 16:9 player and 9:16 Shorts player. Pick the one that matches where the clip will live, then plan the crop rectangle to fit inside the source.
| Use case | Aspect ratio | Width × height (px) | Total pixels |
|---|---|---|---|
| Legacy SD | 16:9 | 426 × 240 | 102,240 |
| SD | 16:9 | 640 × 360 | 230,400 |
| SD | 16:9 | 854 × 480 | 409,920 |
| HD | 16:9 | 1280 × 720 | 921,600 |
| Full HD | 16:9 | 1920 × 1080 | 2,073,600 |
| QHD | 16:9 | 2560 × 1440 | 3,686,400 |
| 4K UHD | 16:9 | 3840 × 2160 | 8,294,400 |
| Shorts | 9:16 | 1080 × 1920 | 2,073,600 |
The total-pixel column matters because Video Cropper's bitrate request is scaled to the cropped area at 30 fps: the larger the rectangle, the higher the requested bitrate inside the 180 kbps to 8 Mbps band. A 3840 × 2160 crop sits at the upper bound; a 1280 × 720 crop stays well inside it.
How Video Cropper Handles Pixels, Audio, and Output
The processing pipeline has three observable parts, each with a clear contract for what is and is not preserved.
Pixels. The tool reads the decoded width and height of the source, then accepts one fixed rectangle as X, Y, width, and height in whole source pixels. X and Y are offsets from the top-left corner of the source frame: X moves the rectangle right, Y moves it down. Both offsets must be untrimmed nonnegative whole numbers, and both dimensions must be at least 2 pixels. The rectangle must stay entirely inside the decoded frame, and the cropped area must not exceed 3840 × 2160 pixels. Invalid rectangles fail without shifting, clipping, rounding, or producing a partial file, so the result is either a complete WebM that decodes at the requested dimensions or no file at all. One rectangle is applied to every frame; the tool does not track a moving subject or change the rectangle over time.
Audio. The Canvas stream that the recorder captures may include an audio track when the browser exposes a capturable source through media capture, as described in the W3C Media Capture from DOM Elements specification. Support varies by browser and by source codec, and the source may play audibly during real-time processing, so the tool cannot guarantee that any specific input will end up with sound. When audio is missing from the output, the cause is almost always a browser that did not surface a track rather than a setting inside the tool itself.
Output. The recorder writes a fresh WebM file using the first available VP9, VP8, or generic WebM MediaRecorder the browser exposes, per the W3C MediaStream Recording specification. The requested bitrate is derived from the cropped pixel area at 30 frames per second and is bounded between 180 kbps and 8 Mbps, but this is a single-pass browser setting rather than a guarantee of target size or visual quality. Fast motion, noise, grain, the source codec, and the browser's encoder all influence the final bytes, so the same rectangle can produce different file sizes on different machines.
Cropping a YouTube Clip With Video Cropper
The tool runs locally and walks you through four decisions: pick a file the browser can decode, read the source dimensions, enter one rectangle, then wait for the real-time recording.
- Open Video Cropper and choose an MP4, WebM, MOV, M4V, or Ogg file from your device. Wait for the decoded width and height of the source to appear before touching any coordinate field — the source dimensions set every later boundary.
- Enter X and Y as pixel offsets from the source frame's top-left corner. X moves the rectangle right, Y moves it down. Both must be untrimmed nonnegative whole numbers.
- Enter width and height as whole source pixels. Each must be at least 2 pixels; X + width and Y + height must remain inside the source frame; the cropped area may not exceed 3840 × 2160 pixels. The defaults cover the full frame.
- Pick the Crop video action, then keep the tab open while the source plays at real time and the recorder captures the Canvas stream as WebM. A one-minute source takes about one minute to record.
- Download the WebM, then verify the duration, the rectangle position, the audio track, and the file's behavior in the player you intend to upload with. Keep the original until the entire output has been reviewed.
Worked example for a centered 16:9 crop. If your source is 1920 × 1080 and you want a 1280 × 720 rectangle that stays centered horizontally and vertically: X = (1920 − 1280) ÷ 2 = 320, Y = (1080 − 720) ÷ 2 = 180. The crop is 320, 180, 1280, 720. Verify that X + width = 320 + 1280 = 1600 ≤ 1920 and Y + height = 180 + 720 = 900 ≤ 1080, and that the cropped area 1280 × 720 = 921,600 pixels is well below the 3840 × 2160 = 8,294,400 pixel ceiling.
Input Limits and Output Constraints
The contract below comes directly from the tool's implementation. Anything outside the limits is rejected without producing a partial file.
| Parameter | Accepted range |
|---|---|
| File extension | MP4, WebM, MOV, M4V, Ogg (codec depends on the browser) |
| File size | Up to 500 MiB |
| Decoded duration | Above zero, at most 5 minutes |
| Source side | No side above 4096 pixels |
| Source area | At most 3840 × 2160 pixels |
| Crop offsets X, Y | Untrimmed nonnegative whole pixels |
| Crop width, height | Whole pixels, each at least 2 |
| Crop rectangle | X + width and Y + height must stay inside the source |
| Output area | At most 3840 × 2160 pixels |
| Output format | WebM using the first available VP9, VP8, or generic WebM recorder |
| Output bitrate | 180 kbps to 8 Mbps, scaled to the cropped area at 30 fps |
| Audio | Included only when the browser exposes a capturable track |
| Recording speed | Real time; a one-minute source takes about one minute |
The Canvas that receives the crop is opaque, so any area outside the rectangle is simply not drawn into the output frame, and the drawImage call follows the rules laid out in the WHATWG HTML canvas specification. Object URLs, timers, recorder tracks, and playback are stopped or released on replacement, error, stale completion, or unmount, which keeps a long session from leaving background tasks behind.
Verifying the WebM Before You Upload
The recorder re-encodes the canvas rather than copying the original codec, so the output is a fresh WebM file. Streaming MediaRecorder output may omit a finite container duration, so the tool parses Segment Info and TimestampScale from the Matroska container and inserts a big-endian Duration value in timestamp-scale ticks, the same standards-tested writer that Video Compressor uses, as described in the Matroska Media Container Format specification. The patched file must be nonempty and must decode at the requested dimensions before download is offered.
Before you upload to YouTube, play the downloaded file from start to finish. Confirm the rectangle sits where you intended on every frame, that the audio plays if the source had sound, and that the file behaves the same way in your usual player as it does in a fresh one. YouTube re-encodes uploaded clips, so a single-pass WebM with bounded bitrate is a clean source for that re-encode rather than a final master. If you also need to remove a middle section by time instead of by rectangle, a local cut workflow for YouTube clips covers that task without uploading either.
Two details the tool does not change: subtitles, chapters, rotation tags, HDR signaling, and many container metadata fields are not preserved, so re-add those after cropping if the destination needs them. And because the source plays audibly during real-time processing on browsers that expose a track, mute the tab if you don't want the clip to play through your speakers while the recorder runs. The same goes for tab throttling — switching to another tab or letting the screen sleep can pause media decoding and stretch the recording past the source's real duration.