The short answer
Storage need equals bitrate multiplied by recording time, for every channel. Most buyers start from a capacity, 64GB or 128GB, and want the hours of footage it holds. The method is to take the camera's video bitrate in megabits per second, convert it to gigabytes per hour, and divide the card capacity by that rate. The result is a rough estimate. It depends on the bitrate the camera actually uses and on the codec, and it should be treated as planning arithmetic, not as a test result. Read the bitrate from the spec sheet, and ask the supplier to confirm it in writing.
The method, step by step
Video bitrate is the number of bits written per second. A bit is one eighth of a byte, and an hour has 3,600 seconds, so gigabytes per hour equal megabits per second multiplied by 0.45. That follows from 3,600 divided by 8 divided by 1,000. Minutes per gigabyte are the reverse: 60 divided by the gigabytes per hour, or about 133 divided by the megabit rate. Here is the procedure:
- Find the bitrate for the resolution and codec you plan to use, in Mbps.
- Convert to GB per hour by multiplying by 0.45.
- Add the channels: a front and rear camera writes two streams, and a three-channel unit three.
- Divide capacity by the combined hourly rate to get hours before overwrite.
- Allow headroom because locked event clips and file system overhead take space too.
The frame rate and bitrate note explains why bitrate varies even at the same resolution.
Worked examples with assumed bitrates
The table uses assumed example bitrates to show the method. They are illustrations, not measurements, and real cameras differ by model, scene and setting. Replace them with the figures on your own spec sheet.
| Setup (assumed bitrate) | GB per hour | Minutes per GB | Hours on 64GB | Hours on 128GB |
|---|---|---|---|---|
| 1080P, 8 Mbps | 3.6 | about 17 | about 17.8 | about 35.6 |
| 1440P, 12 Mbps | 5.4 | about 11 | about 11.9 | about 23.7 |
| 4K, 20 Mbps | 9.0 | about 6.7 | about 7.1 | about 14.2 |
| 4K front + 1080P rear, 20 + 8 Mbps | 12.6 | about 4.8 | about 5.1 | about 10.2 |
Real usable capacity is a little less than the label, because cards are sold in decimal gigabytes and the file system takes some space. Treat the hours above as upper estimates.
The codec effect
H.265 compresses video more efficiently than H.264. At similar image quality it needs about half the bitrate, so a given card holds about twice as much footage. That makes H.265 attractive for 4K and dual-channel models, where storage fills quickly. The trade-offs are that some older players and editing tools handle H.265 less smoothly, and that the actual bitrate a camera uses depends on its firmware settings. The full comparison is in the H.265 versus H.264 note. When comparing two cameras, compare the bitrate and codec together, not the resolution alone.
What size card for which use
The right size depends on how long you want footage to survive before it is overwritten. A daily commuter who reviews footage only after an incident needs a few hours of loop. A fleet that reviews weekly needs more, and a parking-mode user needs space for long time-lapse files. The table below gives a practical starting point; adjust it to your camera's bitrate.
| Use | Typical starting card | Reason |
|---|---|---|
| Single-channel 1080P, daily commute | 64GB | Many hours of loop at a moderate bitrate |
| Single-channel 4K | 128GB | Higher hourly rate; keeps a useful loop |
| Front and rear dual-channel | 128GB | Two streams share the card |
| Rideshare, front plus cabin | 128GB | Long daily hours and locked clips to keep |
| Parking mode with time-lapse | 128GB | Long recording periods at low frame rates |
Always check the maximum card size the camera accepts, as covered in the card selection note.
Event clips and the locked folder
The loop is not the only user of space. When the G-sensor locks a clip, that file moves out of the loop and stays until someone deletes it. Many cameras reserve a fixed share of the card for locked files, and once that share is full, further events may overwrite older locked ones or fail to lock. In a busy fleet, a few false triggers a day from potholes can fill the locked area within weeks. Two remedies work together: lower the G-sensor sensitivity for rough-road vehicles, and download and clear locked clips on a schedule. The download note covers the routine.
Parking mode and long recordings
Parking mode changes the arithmetic. A time-lapse parking mode writes one frame every second or so, which is a tiny fraction of a normal stream, so hours of parking add little to the storage load. Motion-triggered parking is different: the camera may record full-rate clips whenever something moves in view, and a car parked on a busy street can accumulate far more footage in an evening than it did driving. Buffered parking mode, which keeps the seconds before an impact, also writes continuously at a low rate and stores a rolling buffer. If your customers will use parking mode, size the card for the motion-triggered case, not only for driving, and check that the parking files have their own folder so they do not crowd out driving footage. The mechanics of each mode are in the parking mode note.
Trips, settings and trimming the need
Two practical adjustments change the result without changing the card. The first is the resolution setting itself: a camera that can record 4K can also record 1440P, and the difference in hours per card is large. Some buyers ship cameras set to a lower resolution by default and let users raise it. The second is trimming what is recorded: shorter segments do not reduce the total, but lowering the bitrate setting, where the menu allows it, does, at some cost in image detail. For long trips without a chance to download, work the numbers backwards: hours wanted multiplied by the hourly rate gives the capacity needed, then round up a size. A two-week trip in a rental fleet, for example, calls for a different answer than a daily commute.
Planning a program
For a program, convert the planning arithmetic into a specification. State the camera bitrate, the codec, the card capacity, and the number of hours of loop you want. If the numbers do not meet, change one input: pick a larger card, switch to H.265, or lower the bitrate setting. The desk confirms bitrate and card capacity with partner factories in Shenzhen and records them in the program sheet, with the final figures confirmed in writing. For related ranges such as lead time, see MOQ and lead time, and to get a card sized into a quotation, use the quote form.