How to Shrink a Video for Email and Messengers

Why videos are so large

A video is many pictures per second (usually 24, 30 or 60 frames) plus sound. Even compressed, it takes a lot of space, and file size is determined mainly by two things: how much data is stored for each second (the bitrate) and how long the video is.

A simple formula: size ≈ bitrate × duration ÷ 8. For example, 1 minute at 8 Mbps is about 480 megabits, or about 60 MB. One minute at 2 Mbps is about 15 MB. Phone bitrates vary by model and settings, so the exact figure is best read from the file's properties.

What sizes you are aiming for

Email services often allow roughly 20–25 MB of attachments. Gmail, for example, has a 25 MB limit, but attachments are encoded for sending and grow in the process, so it is safer to plan for about 18–20 MB. Messengers and social networks have their own limits, which change over time, so check the current rules before you send.

A practical goal: if you want to send by email, aim for a file no larger than roughly 15–20 MB. If that is not possible, consider a cloud link, because some videos are simply too long to fit in an email.

Resolution: 1080p, 720p, 480p

1080p is 1920 × 1080 pixels, 720p is 1280 × 720 and 480p is usually about 854 × 480. Going from 1080p to 720p leaves about 44 percent of the pixels, and going to 480p about 20 percent, so at the same quality level the file can be much smaller.

On a small phone screen or in a chat, the difference between 1080p and 720p is often barely noticeable, so 720p is a sensible compromise in many cases. 480p works when small size matters more than sharpness.

Bitrate and CRF in plain words

Bitrate is how much data is spent per second. A lower bitrate means a smaller file, but also more visible blockiness and blur in demanding scenes, for example when there is a lot of motion.

CRF (constant rate factor) is an alternative used in H.264 encoding: you choose a quality level and the encoder picks the bitrate itself. The higher the CRF number, the smaller the file and the lower the quality. Around 23 is often cited as the default and 18–28 as the usual range. If your converter does not offer such settings, it is enough to choose lower quality or a lower resolution.

Format: why MP4 with H.264

MP4 with the H.264 video codec and AAC audio is the most widely supported combination: nearly all phones, computers, browsers and messaging apps open it. If you have a .mov (iPhone), .mkv, .avi or .webm file, it is worth converting it to MP4 first.

Do not assume that changing the format alone will shrink the file a lot: that depends on the encoding settings. Also note that a GIF is usually larger than an MP4 of the same short clip, so GIF is fine as a simple animation format but not for reducing size.

Step by step

First look at the file size and duration. Then pick the converter that matches your file's format, for example MOV to MP4, and upload the file. If quality or resolution options are available, choose 720p or medium quality, and convert.

After converting, compare the new size, watch the clip and check the sound. If the file is still too large, try an even lower resolution or cut out the parts you do not need, because a shorter clip is always smaller. If you only need the audio, you can use the MP4 to MP3 converter.

• 1. Check the file size and duration.
• 2. Convert to MP4 if needed.
• 3. Choose 720p or lower quality.
• 4. Compare the size and watch the result.
• 5. If still too large, lower the resolution or trim the clip.

How long conversion takes in a browser

When you convert in a browser, your own device's processor does the work, so the time depends on how powerful the device is, how long the clip is and its resolution. A long 1080p clip can take noticeably longer than the clip itself, especially on an older computer or a phone.

It is best to start with a short test clip, keep the tab open and avoid running other heavy programs. For very large or long recordings, a dedicated desktop program may be a better fit.

A sample calculation

Suppose you want a 2-minute clip to fit into about 20 MB. 20 MB is 160 megabits (20 × 8), and dividing by 120 seconds gives about 1.33 Mbps for the whole file. Subtracting audio (about 0.13 Mbps if the AAC audio is 128 kbps) leaves about 1.2 Mbps for the picture. An original at 8 Mbps would take 2 × 60 MB = 120 MB, so you would need to shrink it roughly six times.

To make planning easier, here is a table of the maximum total bitrate that fits into 18 MB (144 megabits) by duration:

Common problems

If the file did not shrink after conversion, the usual cause is that a high quality was chosen or the original was already heavily compressed. Then try a lower resolution or a shorter clip. Some newer phones record in HEVC (H.265), which is smaller than H.264 at the same quality, so converting to H.264 can even make the file bigger. In that case lower quality or resolution matters more than changing the format.

Also check that a vertical video has not turned sideways after conversion and that the sound stays in sync with the picture. Whenever possible, open the result before sending and watch at least the start, middle and end.

Checklist before sending

• Is the file MP4 (H.264 and AAC)?
• Is the size comfortably within your email or app limit?
• Do picture and sound work from start to finish?
• Does the recipient really need top quality, or is 720p enough?