What kind of microSD card do I need for a dashcam, and why do cheap cards fail?
A dashcam needs high endurance, not high speed and not high capacity. It writes continuously whenever the vehicle is on, often in a hot windshield-mounted enclosure, and it loops — overwriting the oldest footage forever. That combination is the hardest workload in consumer flash: constant writes, high temperature, and no idle time. Buy cards rated for continuous recording and sized so the loop period matches the footage you want to keep, and expect a card that works fine in a phone to fail in a dashcam within months.
Why the workload is unusually harsh
Three things happen at once in a dashcam that rarely happen together elsewhere.
- Continuous writing while the vehicle is running — a large share of the day for a commercial vehicle.
- High temperature — the unit sits against glass in direct sun, and a parked car in summer can exceed the ambient temperature by a wide margin.
- Endless looping — the same cells are erased and rewritten repeatedly, because old footage is overwritten in order.
What that means for the three specifications
Most buyers choose on speed class and capacity. Both are the wrong primary criteria here.
| Specification | Common assumption | What a dashcam actually needs |
|---|---|---|
| Speed class | Higher number is better | Adequate is enough; endurance matters far more |
| Capacity | Bigger is safer | Big enough for the loop period, and no bigger than needed |
| Endurance | Rarely considered | The single most important rating |
How to size capacity for the loop, not for the maximum
A dashcam overwrites the oldest file when the card fills, so capacity sets how far back the footage goes. Work out your loop period backwards from that.
Suppose the camera records at a bitrate that produces roughly 4GB per hour (illustrative). A 128GB card then holds about 30 hours, which at two hours of driving a day is roughly two weeks of history. A 32GB card would hold about a week.
The question to answer is "how much history do I need" — for a private car, days; for a commercial fleet reviewing incidents, weeks. Buying capacity you do not need means paying more and spreading writes across more cells (which helps endurance), but it also means the card spends longer before wear levelling cycles matter. There is no single right answer; there is a right answer for your loop period.
Why cheap cards die in months
A card built for a phone is optimised for storage that is written occasionally and read often. Its cells may be perfectly good and still not survive a year of continuous looping at 60°C.
The failure is usually not a total stop. The card starts dropping frames, then refusing to record at all, then becoming unreadable. By the time the user notices, the footage they needed is often already gone — which is the worst possible failure mode for a device whose entire purpose is evidence.
If you are specifying cards for a dashcam product or buying them for a fleet, ask for the endurance rating and the operating temperature range in writing, and test a sample in the actual enclosure on a hot day rather than on a desk.
Sources
- SD Association — speed classes and application performance
- SD Association — endurance and reliability guidance
Figures quoted on this page are from the published reporting listed above. Where a number is an estimate rather than a published figure, it is described as approximate.
Frequently asked questions
Can I just use a high-end phone card in a dashcam?
It will usually work at first and often fail within a year under continuous recording. A high-end consumer card is optimised for speed and capacity, which a dashcam does not need, rather than for endurance at temperature, which it does. The specification you want is continuous-recording endurance across the operating temperature range, not the fastest sequential write in a cool room.
Does a bigger card make a dashcam more reliable?
Not by itself, but it does reduce the write frequency per cell, because the loop takes longer to come round and wear levelling has more room. It also extends how far back your footage goes. If reliability is the goal, choose an endurance-rated card sized for a loop period you actually need — both matter, and neither substitutes for the other.
Why does the card get hot, and does it matter?
Writing flash consumes energy, and a dashcam enclosure is small, sealed, mounted against glass and often in direct sun. Heat reduces endurance and can trigger throttling, so the write rate drops exactly when the device is trying to keep up. That interaction is why an in-enclosure test on a hot day tells you more than any datasheet.
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