How to compress a video for email: the size limits and the bitrate math

At the 8 Mbps that YouTube recommends for 1080p uploads, one minute of video is 60 MB. That is more than twice what Gmail will attach. Getting a clip into an email is a small sum: pick a target below your provider’s limit, work out the bitrate that fits it, and re-encode at that bitrate. Below are the numbers for each step, then how MediaKit does it on iPhone and Mac without sending the clip anywhere first.

How big a video can you email?

Limits as each provider’s help page stated them in July 2026: Gmail Help, Microsoft Support, and Apple’s pages on iCloud Mail limits and Mail Drop. Work and school accounts can be set lower by their administrators, and the receiving server applies its own limit. Microsoft’s Exchange documentation puts it plainly: a message must fit the limits of both sender and recipient. The smallest limit on the route is the one that counts.

Why a 25 MB limit does not take a 25 MB file

Email is a text format. Before a video can travel as an attachment it is Base64-encoded: every three bytes of the file become four characters of text, broken into lines of at most 76 characters (RFC 2045, section 6.8). The message that crosses the network is about a third larger than the file on your disk.

Whether a limit is measured before or after that encoding varies, and consumer help pages rarely say. Microsoft’s Exchange documentation does: Base64 adds about 33%, so a 64 MB message limit means a realistic maximum of about 48 MB. Its Outlook page adds that the 20 MB limit includes the email, not just the attachment.

So the working rule is three-quarters of the stated limit:

Undershooting costs a little picture quality. Overshooting means a bounce, or a link where you wanted an attachment.

How to work out the bitrate that fits

File size is bitrate multiplied by duration. Run it backwards:

1. Convert the target to kilobits. Megabytes × 8,000, counting a megabyte as 1,000,000 bytes.
2. Divide by the clip’s length in seconds. That is the total bitrate in kbps.
3. Subtract the audio. 128 kbps is a common stereo setting; 64 kbps is plenty for speech.
4. The rest is the video bitrate. Leave a few percent for the container’s own overhead.

A 90-second clip for Gmail: 18 MB × 8,000 = 144,000 kilobits. Divided by 90 seconds, that is 1,600 kbps. Take away 128 kbps of audio and about 1,470 kbps is left for the picture.

The same sum for common lengths, with 128 kbps of audio taken out first:

Figures are rounded down.

Which resolution and codec to choose

A bitrate is only generous or thin relative to the frame it has to fill. For scale, YouTube’s recommended upload settings for standard frame rates ask for 8 Mbps at 1080p, 5 Mbps at 720p, 2.5 Mbps at 480p and 1 Mbps at 360p. Those figures are high, because YouTube re-encodes everything it receives, but they show how fast the need falls with frame size: 1080p has 2.25 times the pixels of 720p.

A rough rule: if your video budget is well under half of YouTube’s figure for the clip’s resolution, step down one resolution. By that rule, a one-minute clip at 18 MB (2,270 kbps) sits better at 720p than at 1080p, and at 7.5 MB (870 kbps) it belongs at 480p or below. Three other settings move the result:

• Audio bitrate. For a talking-head clip, 64 kbps instead of 128 kbps hands the difference to the picture. On a five-minute clip at 15 MB, that is nearly a quarter more video bitrate.
• Frame rate. A 60 fps clip spreads its bits across twice as many frames as a 30 fps one. YouTube’s figures for 48 to 60 fps are about half as high again as for 24 to 30.
• Codec. Apple says HEVC compresses better than H.264, and recent Apple devices play it. But you don’t control where the attachment is opened, and H.264 in an MP4 plays almost everywhere. Apple’s page also notes that its devices might convert HEVC to H.264 when sharing to a device that can’t play it; for a file you have already compressed, it is simpler to pick H.264 yourself.

When to send a link instead

Compression has a floor, and the five-minute row of the table finds it: 270 kbps is about a quarter of what YouTube suggests for 360p. When the budget falls that low, a link serves the recipient better than an attachment.

• Mail Drop in Apple Mail and iCloud Mail sends files up to 5 GB as a download link that expires after 30 days. Mail offers it when a message is larger than your provider allows.
• Gmail swaps an attachment over 25 MB for a Google Drive link on its own.
• Outlook points you to compressing the file or sharing it from cloud storage.

The trade is simple: the recipient gets the full-quality original, and a copy sits on a server for as long as the link lives.

Doing it in MediaKit

MediaKit runs the arithmetic above on the device, on iPhone and on Mac, and never uploads the clip. Video Compress is one of the five tools that stay free after the three-day trial.

1. Open the clip. On a Mac, drag it onto the window. On iPhone, add it from Photos or Files, or send it from another app’s share sheet.
2. Choose the video Compress tool and set Mode to Target size.
3. Type the target from the three-quarters table, for example 18, with the unit on MB. MediaKit counts a megabyte as 1,000,000 bytes.
4. Open Advanced. Set Max resolution using the bitrate table, leave Codec on H.264 for email, and choose 64 kbps (voice) under Audio bitrate if the clip is mostly speech.
5. Run it, then check the size of the result before you attach it.

In Target size mode MediaKit does steps one to four of the sum itself: it divides the target by the clip’s length, takes out the audio bitrate you chose and holds back 3%, encodes, then measures the file it wrote and encodes again with a corrected bitrate if it missed. A 4K or 1440p source starts with Max resolution set to 1080p; for a small target, set it lower.

If you already use HandBrake for this job, the HandBrake comparison covers where the two differ, and the Compress Videos & Resize Video comparison does the same on iPhone. Why the studio keeps this work off other people’s servers is the subject of an earlier post, and the wider toolkit is in this one.