Compress a video without uploading it
Make MP4, MOV, WebM and MKV files smaller in your browser. Pick a size, see the result, download. Nothing leaves this device.
Processed on your device. Your video is never uploaded. How we know
Drop a video and I’ll take a look
Drop a video here, paste it with Ctrl+V, or choose one from your device.
ClipHeron runs in your browser. We don’t receive your videos, their names or anything in them. Ads on this page load separately and are not given your files. What happens to your video
How to compress a video to a target size like 25 MB
ClipHeron re-encodes your video inside this browser tab. Choose a target size, such as 16 MB for WhatsApp or 25 MB for an email attachment, and the tool works out the video bitrate that fits, drops the resolution only when it has to, and tells you the predicted size before you commit. Prefer a quality preset instead? Pick Smaller, Balanced or Best and read the prediction line.
The guide to platform size limits explains where the presets come from, and the bitrate and file size guide has a calculator for working out any size by hand.
- 1
Add your video
Drop an MP4, MOV, WebM or MKV onto the page, paste it, or choose it from your device. The type is read from the file’s bytes, so a wrong extension doesn’t matter. The file card shows size, resolution, frame rate, codecs and duration.
- 2
Pick a goal
Target size: type a number in MB or tap a platform preset. Quality preset: choose Smaller, Balanced or Best. Adjust resolution, frame rate and audio if you want to; the prediction line updates as you go.
- 3
Compress, compare, download
Press Compress and watch the progress bar. When it finishes, drag the divider between before and after at any point in the video, then download the MP4. The button states the exact size.
What this tool does
- Target size in MB with WhatsApp, Discord and email presets, or Smaller, Balanced and Best quality presets
- Predicted size, output resolution and time estimate shown before anything is encoded
- Resolution steps down automatically when a target can’t be met at the source size, and says so
- Output is always a standard MP4 (H.264 and AAC) with fast start, playable everywhere
- Hardware encoding through the browser where available, software encoding where not
- Before and after comparison at any point in the video, with a shared scrubber
- HDR footage converted to standard color; portrait phone video stays upright
- No watermark, no resolution ceiling, no file cap beyond what your device can hold
What a 25 MB target does to a 3-minute 4K phone video
Take a typical iPhone clip: 3 minutes 12 seconds, 3840×2160, 29.97 fps, HEVC video with AAC audio, 1.42 GB. Ask for 25 MB and the prediction line works like this (1 MB = 1,000,000 bytes):
| Step | Figure |
|---|---|
| Bit budget with a 4% safety margin | 25 MB × 8 × 0.96 = 192 Mb |
| Audio kept at 128 kbps for 192 s | 24.6 Mb |
| Container overhead (about 4 kbps) | 0.8 Mb |
| Left for video | 166.6 Mb ÷ 192 s = 0.87 Mbps |
| Lowest sensible bitrate at 2160p / 1440p / 1080p | 2.5 / 1.2 / 0.6 Mbps |
| Output resolution | 1080p (“Will output 1080p to fit 25 MB”) |
| Predicted size | (0.87 Mbps + 132 kbps) × 192 s ÷ 8 ≈ 24.0 MB |
The same clip asked for 16 MB (the WhatsApp preset) lands at 720p, because 0.5 Mbps is below what 1080p needs to look acceptable. A 60-second 1080p30 clip asked for 10 MB keeps its 1080p resolution at 1.15 Mbps and comes out at about 9.6 MB. These are the planning figures the prediction line uses; the encoder’s rate control lands within a few percent, and a second pass runs automatically if the first overshoots the target by more than 3%.
Bitrate × time = size
A video file is mostly a stream of bits per second. Multiply the bitrate by the duration and you have the file size, give or take the audio track and a little container overhead. That is the whole trick behind a target size: ClipHeron solves that equation backwards, subtracts the audio, and asks the encoder for exactly the video bitrate that fits.
Bitrate on its own doesn’t decide quality. A 4K frame has four times the pixels of a 1080p frame, so the same bitrate spread over 4K looks much worse than it does at 1080p. Below a floor for each resolution (roughly 2.5 Mbps for 2160p, 1.2 Mbps for 1440p, 0.6 Mbps for 1080p, 0.3 Mbps for 720p) the picture turns to blocks, so the compressor steps down to the next resolution instead and tells you in the prediction line. It never upscales.
The quality presets use the same arithmetic the other way round: a bitrate ladder by resolution and frame rate. Balanced is about 4 Mbps for 1080p30 and 2 Mbps for 720p30, with 60 fps footage given 1.5 times as much. Smaller halves those figures roughly; Best nearly doubles them.
Hardware, browser software and ClipHeron software encoders
The file card names the encoder in use because speed differs by an order of magnitude. Hardware means the browser’s WebCodecs API is handing frames to your device’s own video chip, the same one the camera app uses; a 1080p minute can take a few seconds. Browser software means WebCodecs is available but running on the CPU. ClipHeron software is the engine’s own H.264 encoder, compiled from Rust to WebAssembly, used when the browser offers nothing: slower, but it works everywhere and produces the same output on every device.
Whichever path is used, the output is the same kind of file: MP4 with H.264 video (High profile by default, Baseline under More options for very old devices) and AAC-LC audio, with the index at the front of the file so it starts playing before it has fully downloaded. The H.264 level is chosen from the output size and frame rate: 4.0 for 1080p30, 4.2 for 1080p60, 5.1 for 2160p30.
Platform limits behind the presets
The chips on the compressor come from a dated table of each platform’s published limit: WhatsApp 16 MB for media sent inline, Discord 10 MB without Nitro, Gmail 25 MB per message, Outlook.com 20 MB, Slack 1 GB, Telegram 2 GB. Limits change, so each row records the page it came from and the date it was read. The sharing guide has the full table with notes on what each limit actually applies to.
Aim a little under the limit. Platforms count bytes their own way, and a file that is 15.9 MB on your computer can be 16.1 MB by the time a messaging app has looked at it. The 4% safety margin built into the budget covers most of that.
HDR to standard color, and the “never bigger” rule
Recent phones record HDR video (HEVC Main 10 with HLG or PQ transfer, including Dolby Vision profile 8.4). H.264 output is 8-bit standard dynamic range, so ClipHeron tone-maps those sources to BT.709 and says so with a one-line notice. The result looks right on ordinary screens; on an HDR display it is a touch less vivid than the original.
With a quality preset, a file that is already an efficient H.264 MP4 can come out the same size or larger, because the encoder spends bits copying old artefacts. If the output is at least 98% of the input size, ClipHeron keeps the original bytes, tells you it was already small for its quality, and suggests the Target size goal instead. Target size always produces the encoded output, because you asked for a number.
Good to know
- Keep your original. Lossy compression can’t be undone, and compressing a compressed file loses more each time.
- Phones recommend inputs under about 500 MB and process 4K sources at 1080p output to stay within memory; computers handle 2 GB comfortably and up to 4 GB with a warning.
- Variable frame rate screen recordings keep their timestamps; frames are never duplicated or interpolated.
- Audio-only inputs, and files whose video codec browsers can’t decode (Xvid, DivX, WMV), are refused with a specific message rather than a blank tab.
- Metadata is not carried over except duration-critical boxes: no location, device model or software tag. “Keep creation date” under More options re-adds only the creation time.
Related tools and guides
Questions people ask
Is there a file size limit?
Not a fixed one, because nothing is uploaded. The limit is your device’s memory: about 2 GB is comfortable on a computer (up to 4 GB with a warning), about 500 MB on a phone. The drop zone shows the guidance for the device you are on, and anything larger is refused before it starts rather than crashing mid-way.
Is my video uploaded?
No. The compressor runs in your browser. We don’t receive your videos, their names or anything in them. The how it works page shows two tests you can run yourself, including switching off Wi-Fi.
Will the quality drop?
Compression always discards some detail; the question is whether you can see it. Use the before/after divider at a few points in the video before you download. If it looks too soft, raise the target or pick the Best preset. Cutting the parts you don’t need with the trimmer is the best compression of all.
Why is it slow on my phone?
Phones have less memory and often no hardware H.264 encoder exposed to the browser, so the software path does the work. A minute of 1080p can take a couple of minutes. Keep the tab open; the screen is kept awake while a job runs.
Why did the resolution change?
Because the target you asked for leaves too few bits per second for the original resolution to look acceptable. The prediction line says so before you start (“Will output 720p to fit 10 MB”). Raise the target, remove the audio, or trim the clip to keep more resolution.
Can I compress for WhatsApp exactly?
Tap the WhatsApp 16 MB preset. The budget keeps a 4% margin under the figure and a second pass runs if the first overshoots, so the file lands under 16 MB. WhatsApp may still re-compress it on its side; that is out of anyone’s hands.
Does it work offline?
After the first visit, yes: the page and the engine are cached by your browser, so you can switch off Wi-Fi and keep compressing. The ads and analytics simply don’t load.
Why MP4 only?
Because an MP4 with H.264 video and AAC audio plays on every phone, computer, TV and chat app made in the last fifteen years. Smaller formats exist (HEVC, AV1, VP9) but each fails somewhere. If your source is WebM or MKV and you only want MP4 without shrinking it, use Convert to MP4.
What happens to your video
Processed on your device. Your video is never uploaded. ClipHeron runs in your browser. We don’t receive your videos, their names or anything in them. Ads on this page load separately and are not given your files.
Decoding, encoding and the before/after comparison all run in a background worker on your device, with hardware codecs where the browser offers them. The output is written to the browser’s private temporary storage while the job runs, then handed to your download and deleted. Your goal, target and resolution choices are saved in local storage for next time; files, names and thumbnails never are.
The page does load advertising and analytics scripts, separate from the tool and described in our privacy policy. They are never given your files.
Sources and review
Eight video tools, one rule: the file never leaves your device
ClipHeron is a small set of video tools that run entirely in the browser. Drop in a video, make it smaller, cut a piece out, resize or crop it for a feed, turn it the right way up, mute it, turn it into a GIF, convert it to MP4 or pull the audio out. There is no upload, no queue, no account, no watermark and no “free minutes” meter, because there is no server doing the work. Your browser downloads the engine once, and from then on the processing happens on your own computer or phone.
The heron is the idea behind the name: tall, precise and patient. It stands still, watches, and strikes once, cleanly. ClipHeron does one precise thing to a video at a time and does it without fuss, and the heron on the page tells you how far along it is.
It is for people sending a video that is “too big for WhatsApp”, students and teachers cutting the bit they need out of a recording, anyone with iPhone footage that won’t open on a Windows PC, streamers whose OBS recording came out as MKV, and podcasters who only need the sound. The site is run by Infinity Square Labs Ltd, a small UK company that builds free, privacy-first web tools; read more about us and how we know nothing is uploaded.
Runs in your browser. An open-source video engine compiled to WebAssembly, plus your device’s own codecs through WebCodecs.
Nothing to upload. No server ever receives your files or file names. Nothing to retain, so no retention policy to trust.
Check it yourself. Load a tool, switch off Wi-Fi, and it keeps working. How it works
Convert shortcuts: MOV to MP4 · WebM to MP4
Guides
- How to make a video smaller for WhatsApp, email or Discord · Every platform’s size limit in one table, and the three ways to get under it.
- MP4 vs WebM vs MOV: which video format to use · Containers and codecs explained, with a table of what plays where.
- How to make a GIF from a video under 8 MB · The size maths behind GIFs, and the order to turn the knobs in.
- iPhone HEVC video won’t play on Windows? Here is why, and the fixes · Why iPhone .mov files fail on a PC, the camera setting to change, and how to convert the ones you already have.
- Keyframes explained: why a video trim snaps · I-frames, P-frames and GOPs, with a diagram of where a cut can land.
- Video bitrate and file size: calculator and guide · Bitrate × time = size. A two-way calculator and typical bitrates by resolution.