What Is Bitrate? The One Number That Decides Your Video File Size

Bitrate explained, the arithmetic that tells you exactly how large a video will be, what a codec actually is, and why H.265 is smaller but you probably still want H.264.

Bitrate is how much data is spent on each second of video or audio, measured in kilobits or megabits per second. It is the single number that decides how large a file is and how good it looks.

Almost every question about video file size turns out to be a question about bitrate.

What is bitrate?

Bitrate is the amount of data used per second of playback. A video at 5 megabits per second spends five million bits — about 625 kilobytes — on every second it runs.

Everything else you can adjust matters only because it changes how far that budget has to stretch. Resolution, frame rate and codec are all ways of deciding how much work the bitrate has to do.

For a sense of scale: a video call runs at roughly 1 to 2 Mbps, streaming a film in 1080p is around 5 Mbps, a phone recording in 4K is often 45 Mbps or more, and a Blu-ray disc goes up to about 40 Mbps.

How do I calculate video file size?

Multiply the bitrate by the length, then divide by eight to get bytes. That is the entire calculation, and it is exact.

Written out: file size in megabytes = (total bitrate in megabits per second x length in seconds) / 8.

So a ten-minute video at 5 Mbps:

Remember to use the total bitrate — video plus audio. Audio is usually 128 to 256 kbps, which is small enough to ignore for a rough estimate and large enough to matter when you are trying to hit an exact size.

This also works backwards, which is the genuinely useful direction. If you need a ten-minute video to fit in 25 MB:

That is a low number, and knowing it is low before you start is worth more than any quality slider. It tells you immediately that you should drop the resolution too.

What is a good bitrate for 1080p?

Between about 4 and 8 Mbps for ordinary footage, and more if there is a lot of motion. There is no single right answer because the right bitrate depends on what is in the picture.

Reasonable starting points at 30 frames per second:

Double these for 60 frames per second, because there are twice as many frames to pay for. And spend toward the top of each range for anything with fast motion, confetti, rain, fire or heavy camera movement — all of which are expensive because almost every pixel changes every frame.

A useful way to think about it: a still shot of someone talking against a plain wall needs very little, because most of the frame is identical to the last one. A drone shot over a forest needs a great deal.

What is a codec?

A codec is the method used to compress video or audio — the rules for turning pictures into a small stream of data and back again. The word is a contraction of coder-decoder.

The distinction that confuses everybody is between a codec and a container:

So an .mp4 file does not tell you how the video inside it was compressed, any more than a box tells you what is in it. Most MP4s contain H.264 video and AAC audio, but they do not have to.

This is why a file sometimes will not play even though the format looks familiar: the container is supported and the codec inside it is not.

H.264 vs H.265: which should I use?

H.265 produces files roughly 30 to 50% smaller at the same quality, but H.264 plays on everything. For anything you are sending to someone else, use H.264.

The practical rule: H.264 for anything leaving your hands, H.265 or AV1 for archiving your own material where you control what plays it.

MP4 vs MOV: what is the difference?

Almost nothing. Both are containers built on the same underlying standard, and both usually hold H.264 video and AAC audio.

MOV is Apple's version, created for QuickTime, and it is what an iPhone records by default. MP4 is the international standard that grew out of it. They are close enough that many tools treat them identically, and converting between them is often just a matter of relabelling the file rather than re-encoding anything.

Use MP4 when sending a file to someone, because a few Windows applications still look at a .mov extension and refuse. Otherwise the difference will not affect you.

Why is my video file so large?

Almost always because it was recorded at a high resolution and a high bitrate by a device that had no reason to be careful.

Phones record generously. A modern iPhone in 4K at 60 frames per second produces roughly 400 MB per minute. That is deliberate — it is the master copy, meant to be edited from, not sent. The same footage at 1080p, which is all anyone will see on a phone screen anyway, is a small fraction of the size.

The other common cause is a screen recording. Those are often enormous relative to how little happens in them, because recorders tend to use a fixed high bitrate regardless of content.

How do I make a video smaller?

Reduce the resolution first, then the bitrate. Resolution is the more powerful lever and the one people reach for last.

Halving the height of a video quarters the number of pixels in every frame. That is a far bigger saving than turning the quality down at full size, and it generally looks better too, because the encoder has fewer pixels to spend its budget on and can afford more for each.

In practice:

Lonecto's video compressor does this in your browser, and it works the other way round from most: you give it the size you need — under 25 MB for an email, under 10 MB for a form — and it calculates the bitrate from the length of your video. Tested across seventy-two combinations of target size, length and audio, the result always came in under the number.

How do I send a large video?

Compress it to the limit you are up against, rather than trying to work around the limit.

The common limits worth knowing: most email attachments cap at 25 MB, and messaging apps re-compress video you send them anyway — often badly, and with no say from you. Sending something already small and already the right resolution gives a better result than sending a large file and letting the app decide.

For genuinely large files that cannot be compressed enough — an hour of 4K, a wedding, raw footage for an editor — a link from cloud storage is the right answer rather than any amount of compression.

Does compressing a video reduce the quality?

Yes, always, by definition — compression works by discarding information. How much you notice depends entirely on how far you push it.

Halving the size of a phone video is usually invisible, because the phone was generous to begin with. Squeezing a ten-minute 4K clip into 10 MB is not, and no tool can make it so — that is 0.13 Mbps, which is less than a video call.

The useful habit is to check the output size before you spend the time. If the arithmetic says the bitrate will be under about 0.5 Mbps at 1080p, drop the resolution instead — you will get a better-looking result at the same size.

Questions

What does kbps mean? Kilobits per second — one thousand bits of data for each second of playback. Mbps is megabits, a thousand times more. Note bits, not bytes: divide by eight to get file size.

Is a higher bitrate always better? No. Above the point where the source material has detail left to describe, extra bitrate only makes the file larger. Re-encoding a 128 kbps audio track at 320 kbps is the clearest example — 2.5 times the size, and it cannot sound better.

What is CRF? A setting in many encoders that targets constant quality rather than constant bitrate, letting the bitrate rise and fall with what is on screen. It usually produces a better file, at the cost of not knowing the size in advance — which is exactly the wrong trade when you have a hard upload limit.

Why does my compressed file look blocky? The bitrate was too low for the resolution, so the encoder ran out of budget and started grouping pixels together. Lower the resolution and the same bitrate will look considerably better.

Does frame rate affect file size? Yes, roughly proportionally — 60 fps needs about twice the bitrate of 30 fps for the same quality. Halving the frame rate is a real saving, though it changes how the video feels more than lowering the resolution does.

What bitrate does YouTube use? YouTube re-encodes everything you upload to its own settings, so your upload bitrate mostly determines how much quality survives that second pass. Uploading at a generous bitrate is worthwhile; the file YouTube serves will be its own regardless.

Can I compress a video without losing any quality? Only by changing the container rather than the video — for instance rewrapping a MOV as an MP4, which copies the streams untouched. That saves very little. Any real size reduction means re-encoding, and re-encoding always costs something.