Which SD card do I need, and what do all the numbers mean?

Card packaging carries six or seven symbols, most of which are marketing. The useful ones tell you the minimum sustained write speed, and minimum sustained is the number that decides whether a camera stalls mid-recording.

Safety

Step by step

1. Check the capacity family your device supports.Older cameras and devices may accept SDHC to 32 GB and refuse anything larger, regardless of speed. The manual or the manufacturer's site will state the maximum. This is the check that prevents a card that simply does not work.
2. Read the speed markings that mean minimum.A number inside a stylised V is the video speed class in megabytes per second sustained — V30 means at least 30 MB/s. A number inside a U is the UHS speed class, either 1 or 3, meaning 10 or 30 MB/s sustained. A number inside a C is the older speed class. These are guarantees of a floor, which is what matters for recording.
3. Ignore the large number on the front.The headline figure is peak read speed under ideal conditions. It affects how quickly you copy files off and nothing else. Two cards with identical headline speeds can differ enormously in sustained write.
4. Match the class to the job.Photographs and ordinary use: any current card is fine. High-definition video: V30 or U3. High frame rate or very high resolution video: check the camera manual, which will name the class it requires. Dashcams and security cameras: a card marked as high endurance, because constant rewriting destroys ordinary cards quickly.
5. Check the A rating only if the card runs software.A1 and A2 describe performance at handling many small random operations, which matters when a card holds apps on a phone or a single-board computer. For a camera it is irrelevant.
6. Buy the size you will actually fill.One enormous card means one point of failure and everything lost at once. Several moderate cards limit the damage from a failure or a loss, and give a natural break to offload. For most people two or three mid-sized cards beat one huge one.
7. Buy from a reputable seller.Counterfeits are widespread and convincing. Buy from a shop or the manufacturer's own storefront, not a marketplace seller with an implausible price.
8. Test a new card before trusting it.Free tools will write across the card's full claimed capacity and read it back, exposing a fake immediately. Do this before the holiday, not after it.

Tips

• Format a card in the device that will use it, not on a computer. Cameras lay out the card in the way their firmware expects.
• Reformat rather than deleting individual files at the end of a shoot. It is cleaner and reduces file system problems over time.
• Cards wear out. A card used constantly for years is a consumable; retire it before it retires itself.
• A card reader on a computer is often faster than connecting the camera by cable, and it does not flatten the camera battery.

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