Cropping a video keeps one fixed rectangle of source pixels and discards everything outside it, so before beginners use a video crop tool they need to know that the rectangle is measured in whole source pixels, stays identical across every frame, and cannot exceed 3840 × 2160 output pixels. The crop is not a moving selection, it is not a resize, and it is not a trim-by-time: it is one rectangular mask applied to every decoded frame from the first frame to the last. Processing happens locally inside the browser tab, so the source file and the resulting WebM are not uploaded to any server, and the tab must stay open while the source plays because the tool records the cropped canvas stream at normal speed. Beginners should also expect a WebM output regardless of whether the source is MP4, WebM, MOV, M4V, or Ogg, and they should plan for processing time roughly equal to the source duration rather than waiting for an instant result. An accepted extension is not a codec guarantee either, because decoding depends on the browser you are using at that moment.
What Cropping Actually Changes in a Video File
A crop is a per-frame mask, not a single edit. The browser draws only the requested rectangular region of each source frame into an opaque canvas at 30 frames per second, records that canvas stream as WebM, and patches a finite Matroska Duration value after recording so players report a real length instead of a streaming timestamp. Because the rectangle is fixed, every frame loses the same outer pixels; nothing inside the rectangle is resampled or scaled, so the cropped region keeps its original sharpness pixel for pixel. Because the canvas matches the rectangle in size, the WebM contains only the cropped region with no surrounding area to be black or transparent, so the output dimensions equal the width and height you entered. And because the tool plays and records in real time, a 90-second source takes about 90 seconds, a four-minute source takes about four minutes, and a five-minute source pushes the upper edge of the safety budget.
The crop also changes what counts as the visible frame for downstream tools. If you resize the WebM afterward, the resize acts on the cropped dimensions rather than the original source dimensions. If you extract a frame from the WebM, the PNG is sized to the cropped rectangle, not the source frame. Beginners often expect cropping to behave like a trim or a resize, so getting the rectangle right on the first pass saves a second run through the encoder.
The Input Limits Beginners Run Into First
The cropper shares an explicit safety budget with the rest of the local video tools, and almost every beginner trip traces back to one of these values being out of range. The tool will not start a crop that violates them, and it will not shift, clip, round, or produce a partial file when the rectangle is invalid; it simply fails before drawing a single canvas frame.
| Limit | Allowed value | Why it matters for beginners |
|---|---|---|
| File size | Up to 500 MiB | Larger files are rejected before decoding so the tab does not stall. |
| Decoded duration | Above zero, up to five minutes | Real-time recording caps how long a single pass can run. |
| Source width or height | No side above 4096 px | Decoded frames larger than this can break canvas allocation. |
| Source area | No larger than 3840 × 2160 | Combined width × height budget for the source frame. |
| Output area | No larger than 3840 × 2160 | The cropped rectangle cannot exceed this budget either. |
| Accepted extensions | MP4, WebM, MOV, M4V, Ogg | An accepted extension is not a codec guarantee; decoding depends on the browser. |
Two practical consequences follow. First, a four-minute 4K MP4 sits comfortably inside the budget, but a four-minute 8K clip is rejected because one side exceeds 4096 px even if the file size and duration are fine. Second, a five-second 4 GB MOV is rejected because of size even though the duration is short, so trimming the source first is sometimes the only way in.
How X, Y, Width, and Height Behave in Source Pixels
All four inputs are whole, nonnegative pixel counts read from the decoded source frame. X and Y are offsets measured from the top-left corner of the source frame: X moves the rectangle to the right and Y moves it down. Width sets how many pixels the rectangle spans horizontally and height sets how many pixels it spans vertically. The rectangle is the area from (X, Y) to (X + width, Y + height), and it must sit entirely inside every decoded frame for the crop to start.
Width and height must each be at least two pixels, and X + width and Y + height must each be less than or equal to the source width and source height respectively. Output width × output height must also stay within 3840 × 2160 pixels, so a wide strip such as 5000 × 50 is not rejected for its 250,000-pixel output area but because its required source width of at least 5000 pixels exceeds the 4096 px side cap. There is no rounding, trimming, or auto-shift: an invalid rectangle is rejected as written.
Worked example. Suppose the source is 1920 × 1080 and you request X = 100, Y = 50, width = 800, height = 600.
- Right edge: X + width = 100 + 800 = 900, which is less than or equal to 1920 ✓
- Bottom edge: Y + height = 50 + 600 = 650, which is less than or equal to 1080 ✓
- Output area: 800 × 600 = 480,000 pixels, which is well under 3840 × 2160 = 8,294,400 pixels ✓
If the source were 800 × 600 instead, the same rectangle would fail at the right edge (900 > 800), and you would need to lower width or move X left before the crop would run. If you are unsure of the decoded dimensions, open the source in any player that reports frame size or check the file properties; the cropper reads the same numbers once the file is loaded.
How to Crop a Video Locally With the Video Cropper
Open the Video Cropper in a modern browser and follow these steps. Everything below happens in the current tab, so the source file and the WebM output both stay on your device.
- Choose a browser-decodable video (MP4, WebM, MOV, M4V, or Ogg) and wait for its decoded width and height to load; the tool needs those numbers before it will accept any rectangle.
- Enter whole-pixel X, Y, width, and height values that stay inside every source frame, keeping width and height at two or more pixels and the output area at or below 3840 × 2160.
- Select Crop video. The source begins to play locally while the tool draws only the requested region into a canvas and records that canvas stream as WebM. Keep the tab open and do not navigate away during this pass.
- Wait for the recording to finish; processing time is roughly equal to the source duration, so a one-minute source takes about one minute.
- Download the WebM, then open it in your destination player and confirm duration, crop position, audio, and compatibility before deleting the original.
If the crop button refuses to start, re-check the four coordinates first: an off-by-one pixel, a width below two, or a rectangle that crosses the frame edge is the most common cause for a beginner pass.
What the Cropper Preserves and What It Drops
The cropper is honest about what it carries over. Pixels inside the rectangle survive untouched, and audio survives when the browser exposes a capturable track through media capture. Almost everything else does not, because the tool does not copy the original container; it records a new WebM from the canvas stream and patches the duration after the recording ends.
| Property | Carried into the WebM? | What to do about it |
|---|---|---|
| Pixels inside the rectangle | Yes, unchanged | Re-encode or resize after cropping if you need a different frame. |
| Audio track (when capturable) | Yes, appended to the canvas stream | Verify both sound and picture in the output. |
| Subtitles, chapters, attachments | No | Re-add them in a downstream tool if the destination needs them. |
| Rotation tags, HDR signaling, color metadata | No | Re-tag the WebM if your player depends on those signals. |
| Original container, codec, bitrate | No | Treat the WebM as a fresh encode at a bounded bitrate. |
The output bitrate is requested from the cropped pixel area at 30 frames per second and is bounded between 180 kbps and 8 Mbps, but that is a single-pass browser setting rather than a target size or a quality guarantee; fast motion, noise, grain, the source codec, and the browser encoder all change the result. Keep the original file until you have reviewed the entire WebM, including audio, in the destination player.
When a Single Static Rectangle Is the Right Choice
A static rectangle is the right tool when the unwanted area is also static across the clip. Black borders from a mis-captured screen recording, watermarks anchored to a corner, a fixed logo bar, or platform framing that no longer matches the destination aspect ratio all live in the same pixels frame after frame, so one rectangle handles the entire video. It is also the right tool when the destination requires a known frame, because the pixel-exact inputs are predictable and repeatable: enter the same X, Y, width, and height on the same source and the result is the same WebM.
It is the wrong tool when the unwanted element moves with the subject, when you only want to shorten the clip, when you need to scale every pixel by a percentage, when you need to obscure a face or logo dynamically, or when you need to remove an object from a moving background. For those cases, use a trimmer to cut time, a resizer to change dimensions, a frame extractor to grab a still, or a dedicated masking tool to track moving subjects; the cropper does not replace any of them, and pairing it with one of those tools is often the cleanest workflow. For a wider pre-flight routine before you start, the pre-crop checklist walks through the same set of inputs and limits in checklist form, which is a useful complement to the steps above.
For a deeper look, see What to Compare Before You Use Video Crop in a Browser.