How big will the video be?
Estimating file size from bitrate and duration

From bitrate and running time to megabytes

Video size is decided by how many bits the encoder spends on each second of footage and how many seconds there are. Resolution and frame rate matter only because they push the bitrate you need up or down. If you know the bitrate and the length, the size follows with one multiplication and one division.

Bitrates are quoted in bits per second, while file sizes are quoted in bytes. That factor of eight is the part people forget, and it is why a 10 Mbps stream does not produce 10 MB every second but 1.25 MB.

A calculator does the same sum, and it is handy for checking an export dialog that shows an estimate before you commit to a long render.

What bitrate to expect for each resolution

Bitrate is a choice you make in the encoder, so there is no single correct figure. The ranges below are commonly cited starting points for H.264 at 30 frames per second; newer codecs such as HEVC and AV1 reach similar quality with noticeably less, and fast action needs more than a talking head.

Notice that 4K is not four times the size of 1080p in this table but about 4.3 times, because platforms recommend a bitrate that grows a little faster than pixel count. In practice the encoder decides: constant-quality modes such as CRF spend more bits on busy scenes and fewer on still ones, so the real size can land well off an estimate.

Spending a size budget on frame rate and resolution

Every extra pixel and every extra frame has to be paid for in bits. Sixty frames per second is not double the cost of thirty, because neighbouring frames are very similar and compress well, but platforms commonly recommend roughly one and a half times the bitrate. Higher resolution raises the need further.

A talking-head lecture or screen recording has little motion, so 720p at 30 fps is usually plenty. Sport or gameplay is the opposite: motion is where low bitrates fall apart, and the extra frame rate is visible. Choose the setting that matches the content, then use the formula to see what it costs.

Constant and variable bitrate change the estimate

With constant bitrate (CBR) every second gets roughly the same budget, so the formula is almost exact. Variable bitrate (VBR) and quality-based modes average out to a target but fluctuate, and an estimate is only as good as that average.

• CBR: size is predictable, which suits live streaming and broadcast delivery.
• Two-pass VBR: the encoder analyses the clip first and hits your average bitrate closely, at the cost of more encode time.
• CRF or quality-based: you set a quality level, not a size, so the file size is an outcome rather than an input.

When a size limit is the real constraint, such as an email or upload cap, work backwards and pick a bitrate that fits. That is the use of the next formula.

Working backwards: the bitrate that fits a size limit

That is thin for 1080p, so the answer in this case is a lower resolution, not a clever setting. Shrinking the picture often looks better than starving a large one of bits.

Why the downloaded file is not the size you computed

• Container overhead: MP4 and MKV headers and index data usually add a percent or two.
• Decimal versus binary units: 609.6 MB is also about 581 MiB, and a drive or app may report either.
• Multiple audio tracks, subtitles and chapters add their own bits.
• Hardware encoders in phones and cameras often run at a fixed high bitrate regardless of scene content.

Streaming, data packs and hours per gigabyte

Turn the same relationship around to see what a data pack buys. One hour at 5 Mbps moves 2.25 GB, at 8 Mbps it moves 3.6 GB, and at 2.5 Mbps it moves 1.125 GB. A 1.5 GB daily pack therefore lasts about 25 minutes of 8 Mbps video, or about 80 minutes at 2.5 Mbps.

Streaming services adapt the bitrate to your connection and screen, so the real figure sits somewhere between the tiers they offer. Many apps include a data-saver option that simply picks a lower tier.

• Lower the quality setting on mobile data, since small screens hide most of the loss.
• Download over Wi-Fi in advance where the service allows it.
• Check whether the figure on a plan is decimal GB, since 1 GiB is about 7 percent larger.

Using the number for uploads and storage

The same figure drives two practical questions. Upload time is size in bits divided by your upload speed, so a 610 MB file on a 20 Mbps uplink needs about four minutes at best, since 609.6 MB is 4,877 megabits and 4,877 ÷ 20 is roughly 244 seconds. Real-world throughput is lower than the line rate, so allow extra.

For archives, multiply size per hour by hours recorded. A camera writing 50 Mbps fills about 22.5 GB an hour, so a day of ten-hour shoots produces over 200 GB before backups. Run these numbers before buying cards and drives, not after.