Bitrate to bytes, and the two different gigabytes
Video storage is one conversion. Bitrate in megabits per second, times seconds, divided by eight, gives bytes. A 150 Mbps codec running for one hour is 150 × 3,600 ÷ 8 = 67,500 megabits worth of bytes, or 67.5 GB in the decimal sense. Stills are simpler still: frame count times file size.
The complication is that gigabyte means two things. Card and drive makers use the decimal gigabyte, 1,000,000,000 bytes, which is why a 128 GB card shows up as roughly 119 GB in your file browser — the browser is dividing by 1,073,741,824 and calling the result GB. Neither is wrong and neither is a scam; they are two conventions with one name. This calculator prints both so the card arithmetic and the computer arithmetic can be reconciled. On top of that, formatting and the reserved area take a slice, which is what the usable-share field is for. The video file size calculator goes into the bitrate side in more detail.
Typical rates, and why yours will differ
Bitrate depends on codec, resolution, frame rate and how much the scene moves, and camera menus state it directly. The figures below are orientation only — read your own camera menu and, better, record two minutes and look at the file size.
| Roughly | Bitrate | Per hour | On a 128 GB card at 90% usable |
|---|---|---|---|
| Efficient long-GOP HD | 50 Mbps | 22.5 GB | about 5.1 h |
| Common 4K delivery codec | 150 Mbps | 67.5 GB | about 1.7 h |
| Higher-quality 4K acquisition | 400 Mbps | 180 GB | about 39 min |
| Light raw or high frame rate | 1,000 Mbps | 450 GB | about 15 min |
Two things skew the estimate in practice. Variable bitrate codecs record less on a static locked-off shot and more on a windy tree line, so a day of interviews and a day of handheld exteriors are not the same day. And camera-original audio, proxies and any second recorder add on top of the video figure without appearing in it.
A card is not a backup, and one copy is not a backup
The 3-2-1 rule is three copies of the data, on two different kinds of media, with one copy off site. Under that definition the card in the camera does not count. The card is the original, and it is the thing you are about to reuse. Once footage exists on a working drive and nowhere else, that is one copy, and one copy is the state most people are in when they think they have a backup.
Two failure modes cover nearly all real losses, and neither is dramatic. The first is a card formatted before the offload was verified, usually on the morning of day two, usually by someone confident the copy finished. The second is a single external drive holding the only copy, dropped, stolen, or simply failing. Both are prevented by the same habit: never format until a second copy exists somewhere else, and verify by checksum or at least by file count and total size rather than by the copy dialog closing.
The practical version on a multi-day shoot is: offload to a working drive, immediately clone to a second drive kept in a different bag, start the off-site or cloud copy as soon as there is a connection, and only then release cards for reuse. The cloud backup guide covers the off-site leg, which is the one people skip because it is the slowest.
Budget the offload as part of the shoot day
Copying is not instant and it is not free of attention. A terabyte over a reader that manages 300 MB per second is nearly an hour, and that hour is paid once per copy unless you can write two destinations in parallel. On a big shoot the offload becomes a scheduled task with a person attached to it, and treating it as something that happens over dinner is how cards get formatted early.
Card capacity is a related judgement rather than a pure convenience question. Bigger cards mean fewer swaps and less chance of missing a moment; they also mean more material riding on one component and a longer offload before anything is safe. For work that cannot be reshot, splitting the day across more than one card is inexpensive insurance. If a timelapse run is part of the shoot, the frame counts from the timelapse calculator feed straight into the stills mode here.
Questions people ask
Why does a 128 GB card show as about 119 GB?
Because two definitions of gigabyte are in use. The card is labelled with decimal gigabytes of 1,000,000,000 bytes each, and most operating systems divide by 1,073,741,824 while still writing GB. 128,000,000,000 ÷ 1,073,741,824 is 119.2, so the card is exactly the size claimed and is being reported in a different unit. Formatting and reserved blocks take a further small slice on top. The calculator prints both figures so the number on the box and the number on screen can be matched up.
Is a card in the camera one of my three copies?
No. The card is the original, not a copy, and its whole purpose is to be reused. Treating it as a backup is what produces the most common real-world loss: a card formatted on the second morning because the first day was assumed to have offloaded. Count only copies on separate devices, ideally in separate places, and format nothing until at least two of those exist and have been verified by file count and size rather than by watching a progress bar disappear.
How do I estimate bitrate if the camera does not state it?
Record two minutes in the exact mode you plan to shoot, then look at the file size. Size in megabytes divided by seconds, times eight, gives megabits per second. Do it in a scene with movement rather than pointing at a wall, because variable bitrate codecs record far less on static scenes and the quiet test will underestimate a real day badly. That measured figure beats any published table, including the one on this page.
How many spare cards should I bring?
The calculator gives the count the footage requires; the number to carry is higher. A card can fail, a card can be left in a laptop, and a shoot can run long. The common working practice is enough capacity for the planned day plus at least one spare per camera, kept separate from the used ones so a full card is never mistaken for an empty one. Some crews use a physical convention — cards face down when shot, face up when clear — precisely because the mistake is easy to make when tired.
Should the third copy be cloud or another drive?
What matters is that one copy is somewhere the others are not, so a fire, a theft or a flood cannot take everything. Cloud satisfies that automatically and costs time on the upload, which for large footage volumes can run for days on an ordinary connection. A drive carried to another building satisfies it too and is far faster, as long as it actually leaves the building rather than sitting in the same bag. Both work; the failure is having all three copies within arm reach of each other.