Vidlora guide
How to compress a video online
Compressing a video re-encodes it with a lower data budget, so the file shrinks while the picture stays recognisably the same. Done well, the saving is large and the loss barely visible; done badly, you get blocky, smeared footage. This guide explains what a compressor is changing, how CRF and presets behave with real numbers, how to choose settings for email, messaging and upload limits, and the situations where compression gives disappointing results.
7 minute read
What a video compressor changes
A compressor decodes your video and encodes it again with a different quality target. The resolution, duration and frame rate stay the same unless you change them separately; what changes is how many bits are spent describing each frame. Modern encoders such as H.264 and H.265 store full frames occasionally and describe the frames between them as small differences, discarding detail the eye is unlikely to miss. Audio is often re-encoded too, usually at a modest bitrate such as 128 kbps AAC, which is still clear for speech and most music. The output plays like the original, just with less data per second. Because the process is lossy, the new file cannot be turned back into the original, so keep the source until you are happy.
CRF explained
CRF stands for constant rate factor, a quality-based setting used by encoders such as x264 and x265. Instead of asking for a fixed bitrate, you choose a quality level and the encoder uses as many bits as each scene needs. Lower numbers mean higher quality and bigger files. For x264, CRF 18 is visually near-lossless, 23 is the default and a good balance, 28 is noticeably smaller with visible softening, and values above 30 look rough. A rule of thumb: raising CRF by about 6 roughly halves the bitrate. So a clip that is 100 MB at CRF 23 may land near 50 MB at CRF 29 and near 160 MB at CRF 18. Because CRF adapts to content, busy footage grows more than static footage at the same setting.
Presets: speed versus size
An encode preset controls how hard the encoder searches for efficient ways to represent the video. Presets run from ultrafast to veryslow. Faster presets finish quickly but need more bits for the same visual quality, while slower presets spend time to produce smaller files. The difference between medium and slow is typically 5 to 10 percent in file size at the same CRF; between ultrafast and medium it can be 30 percent or more. Vidlora's compressor exposes a quality target and an encode preset so you can choose. For a quick share, a faster preset is fine. For a large file you intend to keep or upload, a slower preset is worth the wait. Presets do not change the quality at a given CRF much; they change the size and time.
Settings for common goals
For email or a chat app with a 25 MB limit, work backwards from duration: 25 MB is 200 megabits, so a 2-minute clip can use about 1.6 Mbps in total, while a 10-minute clip must fit in about 330 kbps, which is poor quality at any resolution. If the target is too small, reduce the resolution to 720p or 480p as well as raising CRF. For social media uploads, a moderate setting around CRF 23 to 26 keeps quality high enough that the platform's own re-encode does not make it look worse. For archiving, keep CRF at 18 to 20. For screen recordings of text, avoid aggressive settings because compression artifacts around letters are easy to see. Try one setting, check a busy scene, and adjust.
Step-by-step
Open the video compressor and add your file. Choose a quality level, starting with the moderate default, and pick a preset that matches how long you are willing to wait. Start the compression and watch the progress. When it finishes, compare the original and new file sizes, then play the new file, paying attention to fast motion, dark scenes and any on-screen text, because those are where artifacts show first. If the result looks too soft, repeat from the original with a lower CRF. If it is still too large, lower the resolution first, then raise CRF. Never compress the already compressed result repeatedly, because each pass loses more detail. Keep the original until you have confirmed that the final file plays correctly on the device it is meant for.
When compression gives little benefit
Not every file shrinks dramatically. Footage already encoded efficiently, for example a video downloaded from a streaming platform at 2 Mbps, has little left to remove, and re-encoding may even produce a larger file. Screen recordings with flat colours compress very well, but those with scrolling text or noisy gradients can show banding. Grainy low-light footage and fast action such as sport resist compression because every frame is different. Very short clips are already small. If the file is only slightly smaller, consider trimming unneeded parts, lowering resolution or changing frame rate instead. Compare the bitrate in a metadata viewer first: if it is already below about 2 Mbps for 1080p, there is limited room. Setting expectations early saves you from repeated attempts that give the same result.
Quality checks and mistakes to avoid
The most common mistake is choosing a target file size without considering duration, which forces a bitrate too low for the content. Another is upscaling: compressing does not add detail, and raising resolution only enlarges the file. Watch for blocky areas in dark scenes, smeared motion, banding in skies, and audio that sounds hollow, which signal the setting was too aggressive. Do not compress a file for archive and delete the original. Check that audio stays synchronised and that the aspect ratio did not change. Compare on the same device where it will be viewed, since a phone hides problems that a monitor shows. When in doubt, pick the more conservative setting; a slightly larger file that looks right is better than a tiny one that looks poor.
Privacy and practical limits
When you use an online compressor, your file is uploaded, processed on a server and returned, so consider what the video contains. Vidlora deletes uploads after processing, but you should still avoid uploading sensitive footage such as identity documents or confidential meetings to any online service if an offline alternative is available. Check your connection too: a 1 GB file needs time to upload on a slow line, and mobile data may be costly. Large files can hit upload size limits; in that case, trim or lower the resolution locally first. Finally, remember that some formats and codecs cannot be handled by every tool. If a file will not upload, check its format in a metadata viewer and convert it to a standard MP4 first.
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