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Buyer's guide

The speed-class markings, decoded

A single microSD card can wear five different speed markings, and manufacturers count on you not knowing which one to read. Strip away the marketing and there are really only two questions that matter: how fast can it write video stuttering, and how fast can it juggle lots of tiny reads and writes for apps.

The two things speed actually means

Sustained write is a card's guaranteed floor — the slowest it will ever write once it's busy. It's what keeps a 4K recording from stuttering. Every class marking based on a number (Class 10, U3, V30) is describing this one thing in different eras of branding.

Random I/O is how quickly the card handles thousands of small, scattered operations — the pattern you get when a card is running apps or an operating system rather than streaming one big file. This is a completely separate measurement, and it's the one that decides whether a Steam Deck or Raspberry Pi feels snappy or sluggish.

Reading the write-speed markings

They all mean the same kind of thing, just from different generations of the spec:

  • Speed Class (C10): the old guard. A "10" in a C-shape means 10 MB/s minimum sustained write. Effectively the entry floor today.
  • UHS Speed Class (U1 / U3): a number inside a U. U1 is 10 MB/s, U3 is 30 MB/s.
  • Video Speed Class (V6 – V90): the current standard, and the clearest. The number is the minimum write in MB/s. V30 = 30 MB/s, V60 = 60, V90 = 90.

In practice: V30 is the real floor for 4K video, and it's plenty for most people. V60 and V90 are for high-bitrate 4K, 6K/8K, and high-frame-rate shooting — worth it if your camera demands it, wasted money if it doesn't. U3 and V30 describe the same 30 MB/s floor, so don't pay twice for both being printed.

The Application Performance Class (A1 / A2)

This is the Application Performance Class — the random-I/O rating, and the one buyers most often miss. It's measured in IOPS — operations per second — not MB/s:

  • A1: 1,500 read / 500 write IOPS minimum.
  • A2: 4,000 read / 2,000 write IOPS minimum.

If a card will run software — a handheld PC, a retro emulation device, a Pi, a phone loading apps — this matters far more than the headline write speed. For pure app and OS work, an A2 card with mediocre sequential speed will feel faster than a V90 card with no A-rating at all.

The bus: UHS-I, UHS-II, and SD Express

The bus interface is the pipe everything flows through, and it sets the ceiling no class rating can exceed:

  • UHS-I tops out around 104 MB/s. The vast majority of cards on the market — and it's enough for almost everything.
  • UHS-II adds a second row of pins on the back and roughly triples the ceiling to ~312 MB/s. Only useful if your device also has that second pin row.
  • SD Express is a different animal: it runs on PCIe/NVMe — the same interface as a modern SSD — and reaches up to ~985 MB/s. This is what the Nintendo Switch 2 requires, marked with an "EX" on the card. A normal microSD, however fast, won't work for Switch 2 game storage.

Ignore the giant number on the front

That bold "200 MB/s" is peak sequential read under ideal conditions — the best case, achieved once, in a lab. It tells you almost nothing about how the card behaves when it's working. The class markings are guaranteed minimums, which is exactly why they're the numbers worth trusting. Read the little symbols, not the big font.

One more label: capacity families

SDHC covers cards up to 32 GB, SDXC spans 64 GB to 2 TB, and the newer SDUC goes beyond that. Your device has to understand the family — an old camera that only speaks SDHC won't read a 256 GB SDXC card no matter how you format it.

Match the card to the job

This is exactly what the "For:" chips on our cards do — we read the spec columns and tag each card good, ok, or avoid for common uses, using these same thresholds: