How bitrate decides your recording time
Why bitrate, not resolution, sets how much footage fits on a card, and how to pick a setting that balances quality against storage.
Bitrate is the number that fills your card
Storage is consumed by data per second, and that number is the bitrate, measured in megabits per second. Resolution and frame rate influence which bitrate a camera chooses, but two clips at the same bitrate take the same space even if one is 1080p and the other 4K. This is why the calculator asks only for storage and bitrate: those two figures are all that recording time depends on. Once you know the bitrate, the card size tells you the rest.
Bits and bytes, the eight that trips people up
Bitrate is quoted in megabits while storage is sold in megabytes, and there are 8 bits in a byte. The tool bridges the gap by multiplying gigabytes by 8 to convert capacity into bits before dividing by the bitrate. Forgetting the factor of 8 is the most common mistake in a hand estimate, and it makes footage look eight times larger or smaller than reality. Keeping bits and bytes straight is the whole trick to getting recording time right.
Typical bitrates by format
A 1080p mirrorless clip often sits around 50 Mbps, giving 375 MB per minute and roughly 2 hours 50 minutes on a 64 GB card. Consumer 4K commonly lands near 100 Mbps, which is 750 MB per minute, so the same card holds about 85 minutes. High end cinema modes and 10 bit codecs can reach 400 Mbps or more, dropping a 64 GB card to under half an hour. Plugging your own figure into the tool shows exactly where your camera falls on that scale.
Planning a shoot around card swaps
Once you know minutes per card, you can decide how many cards a day of filming needs and when to swap. Add a safety margin because variable bitrate footage runs long during action, and because you never want to fill a card to the last byte. A good rule is to plan for about 90 percent of the calculated time so you always have headroom. The MB per minute figure also tells you how long each offload to a laptop or SSD will take.