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The short answer
Read the symbols, not the headline speed. U3 or V30 means it sustains 30MB/s writing, which is what 4K video needs. A1 or A2 relates to random access speed, which is what matters in a phone or a handheld console. High Endurance is a different product and the only sensible choice for a dash cam or a security camera.
The number printed largest on a memory card is the maximum read speed under ideal conditions, and it is the least useful figure on the package. What determines whether a card works for your purpose is the sustained write speed and, for some devices, random access performance. Both are indicated by small symbols most people ignore.
What the symbols mean
Speed class: U1, U3, V30, V60, V90
These specify minimum sustained write speed. U1 and V10 mean 10MB/s. U3 and V30 mean 30MB/s. V60 and V90 mean 60 and 90MB/s. For recording video, this is the specification that matters, because video writes continuously and a card that dips below the camera’s requirement causes a dropped recording.
- 1080p video: U1 or V10 is sufficient.
- 4K video: U3 or V30 minimum.
- High bitrate 4K or 6K: V60 or V90.
Application class: A1 and A2
These specify minimum random read and write operations per second, which matters when the card stores apps or a game library rather than large sequential files. A1 requires 1500 read and 500 write operations per second; A2 requires 4000 and 2000. For a phone, a tablet or a handheld console, A2 is worth having. For a camera, it is irrelevant.
Bus interface: UHS-I and UHS-II
UHS-II cards have a second row of contacts and are much faster, and only in devices that support them. In a UHS-I device a UHS-II card works at UHS-I speeds, so there is no point paying for it unless your camera supports it.
Card capacity has its own naming: SDHC covers 4 to 32GB, SDXC covers 64GB to 2TB, and SDUC goes beyond. Older devices often cannot read SDXC at all, so check the maximum capacity your device supports before buying a large card.
Cards worth considering
SanDisk 128GB Ultra microSD card with adapter
The right card for general phone and tablet storage, where the workload is many small reads rather than sustained video writing. It is not the card for 4K recording, and for photos, music, documents and app data it does everything asked of it without paying for headroom you will not use.
SanDisk 128GB Extreme microSD card with adapter
This is the class of card an action camera or a drone needs. The sustained write rating is what prevents a recording stopping mid-shot, and the higher application class means it also performs well if the same card later lives in a phone or a handheld console.
SanDisk 128GB High Endurance video microSD card with adapter
A dash cam or a security camera writes constantly and overwrites the oldest footage forever. That workload kills a standard card within months, usually silently, so you discover it when you need the footage. High endurance cards use different memory designed for this, and in that application they are not optional.
Amazon Basics microSDXC memory card with full size adapter, A2 U3
Worth considering when you want the right specifications without the brand premium. The ratings are what matter, and this carries the two that cover most use cases. Buy from a seller you trust, because counterfeit memory cards are common and the fakes copy branding convincingly.
SanDisk 128GB Extreme PRO SD UHS-I card
Included because many cameras use full size SD, and the same rules apply: the sustained write rating decides whether continuous shooting or video recording is reliable. The faster read speed also shortens the time spent offloading a day’s photos, which is worth more than it sounds.
What we liked
- Cheap way to add storage to phones, cameras and consoles
- Speed ratings are standardised and comparable
- High endurance cards solve the dash cam problem
- Full size adapters make one card usable everywhere
What we didn't
- Counterfeits are widespread
- Headline speeds are read speeds, not write
- Cards wear out and fail, often without warning
- Older devices cannot use larger capacities
Avoiding counterfeits
Buy from the brand's own storefront or a known retailer
Marketplace listings with unusually low prices for high capacities are the main source of fakes.
Test the card when it arrives
Free tools such as H2testw on Windows or F3 on Linux and macOS write and verify the full capacity. A fake reports a large size and silently discards data beyond its real capacity.
Check the printing quality
Genuine cards have crisp printing and consistent fonts. Blurry text or a misaligned logo is a strong signal.
Be suspicious of very large capacities at low prices
Memory has a cost floor. A 1TB card for the price of a takeaway is not real.
Making a card last
- Format in the device that will use it, not on a computer. Cameras and consoles create the file system they expect.
- Use the official SD Card Formatter tool when formatting on a computer, since it handles the alignment correctly.
- Eject properly rather than pulling the card out during writing, which is the most common cause of corruption.
- Replace dash cam cards annually, regardless of apparent health. They are consumable in that role.
- Do not fill a card completely. Leaving some free space helps wear levelling and keeps performance consistent.
- Keep a copy. Memory cards fail suddenly and without warning, so a card should never be the only home of anything irreplaceable.
What speed card do I need for 4K video?
U3 or V30 as a minimum, which guarantees 30MB/s sustained writing. High bitrate cameras may specify V60 or V90, and the camera’s manual will state its requirement.
Does A2 make my phone faster?
It improves random access, which is what an operating system and apps need. On a phone that runs apps from the card, A2 is noticeably better than A1. For media storage only, the difference is small.
Why do dash cam cards fail so quickly?
Because loop recording overwrites the card continuously, which exhausts ordinary memory quickly. High endurance cards use memory designed for that write pattern and last far longer.
Can I use a UHS-II card in a UHS-I device?
Yes, and it will run at UHS-I speeds. There is no benefit unless the device supports the second row of contacts.
How long do memory cards last?
In light use, many years. In continuous recording, months. Failure is usually sudden rather than gradual, which is why keeping a second copy of important footage matters.
How to choose
Match the card to the job. Phone or console: A2 with U1 or better. Camera or drone shooting 4K: U3 or V30 at minimum. Dash cam or security camera: high endurance, no exceptions. Buy from a reputable seller, test the card when it arrives, and format it in the device that will use it.
