How to Use This Tool
Enter the bitrate and how long you are recording. The answer includes the audio track and shows the size both as storage is sold and as your computer will display it.
The one calculation
File size is bitrate multiplied by duration, divided by eight. The divide-by-eight is there because bitrate is quoted in bits per second and file size is measured in bytes, and there are eight bits in a byte.
Miss it and your estimate is eight times too large, which is the most common mistake here by a wide
margin. The lower-case b in Mbps means bits and the upper-case B in MB means
bytes — a distinction that carries an enormous amount of weight for one character.
Audio is small but not nothing
An audio track at 192 kbps adds about 86 MB per hour. Against a 3.6 GB video that is under 3%, which is why it is routinely left out of estimates and why leaving it out is usually fine. It stops being fine for low-bitrate video: at 1 Mbps the same audio is nearly 20% of the file. The panel above shows the share so you can see when it matters.
Two GB, again
A 3.60 GB file shows as 3.35 GB in Windows, because Windows divides by 1024 and still writes GB. Both figures describe the same file. Storage is sold in decimal units, so if you are working out whether a recording fits on a card, the decimal figure is the one to compare against the number on the box — and the card's usable space will be very slightly less after formatting.
What this estimate cannot know
It assumes a constant bitrate. Most modern encoders use variable bitrate, spending more on complex scenes and less on simple ones, so the real file lands near the average rather than exactly on it. A locked-off shot of a static scene can come in well under; fast motion, confetti, rain, foliage in wind all push over.
Container overhead adds a fraction of a percent for MP4 or MKV. Multiple audio tracks, subtitles and chapter data add their own bitrates. For planning storage, add ten to twenty percent headroom and you will not be caught out.