How to Identify Your Micro SD Card’s Class & Speed Rating

Look at the back of any SD or microSD card and you’ll see a cluster of small symbols — a number inside a circle, maybe a “U” shape with a digit in it, sometimes a roman numeral. Those symbols are the card’s class and speed rating, and they tell you the slowest that card is allowed to write — not how fast it usually runs. Here’s how to read them and match a card to what you’re actually shooting.

What Do Micro SD Card Class and Speed Ratings Mean? How to Read the Symbols on Your Card

Every SD and microSD card prints its speed rating directly on the label — you just need to know which symbol means what. There are four symbol families in use, and a single card can carry more than one at once, since newer standards were added on top of older ones rather than replacing them.

• Number in a circle (2, 4, 6, or 10) — the original Speed Class rating, in MB/s minimum sustained write speed.
• “U” shape with 1 or 3 inside — the UHS Speed Class: U1 guarantees 10 MB/s minimum, U3 guarantees 30 MB/s minimum.
• Roman numeral I or II — the UHS bus interface generation. UHS-II cards have a second row of contacts on the back and need a UHS-II-compatible reader or slot to hit their top speed.
• A small “V” with a number (V6, V10, V30, V60, V90) — the newer Video Speed Class, aimed squarely at 4K and 8K recording.
• A rounded rectangle with “E” and a number (E150, E300, E450, E600) — SD Express Speed Class, the newest standard (added in spec SD 9.1, 2023), built for the PCIe/NVMe-based SD Express bus.

If you see a plain “10” in a circle with no U or V symbol, that’s an older Class 10 card — still fine for most phones and basic HD video, but not built for 4K burst recording the way a U3 or V30 card is.

What Speed Class 2/4/6/10 Actually Means

The original Speed Class system has four tiers, each naming its guaranteed minimum sustained write speed in megabytes per second: Class 2 writes at least 2 MB/s, Class 4 at least 4 MB/s, Class 6 at least 6 MB/s, and Class 10 at least 10 MB/s. Class 10 has been the practical floor for years, since anything slower struggles with modern camera files.

These numbers describe the worst case, not the card’s ceiling. A Class 10 card can easily run at 40–90 MB/s in real use — the class rating only promises it won’t drop below 10 MB/s even in a bad moment, which matters for continuous video recording where a stall means a dropped frame.

UHS-I vs UHS-II — What’s the Difference?

UHS (Ultra High Speed) is a faster bus standard layered on top of the original SD spec, and it comes in two generations. UHS-I uses the same single row of pins as a standard card and tops out around 104 MB/s. UHS-II adds a second row of contacts on the back of the card and can reach 300 MB/s or more — but only in a device and reader that both support UHS-II.

Put a UHS-II card in a UHS-I-only phone or camera and it still works fine — it just runs at UHS-I speed, because the extra contacts sit unused. That’s why matching the card to your device matters as much as matching it to your footage: paying for UHS-II speed you can’t access is money wasted.

Matching a Card’s Class to Your Use Case

The fastest, biggest-capacity card on the shelf isn’t automatically the right one — match the speed class to what you’re actually recording. Capacity and speed class are two separate specs, and a huge card with a low speed class will still choke during continuous 4K recording or rapid burst photography.

• Basic photos & casual phone use: Class 4 or Class 10 is plenty — standard photo files write in well under a second regardless.
• 1080p / HD video: Class 10 or UHS-I U1 (10 MB/s minimum) handles smooth continuous recording.
• 4K video: UHS-I U3 or V30 (30 MB/s minimum) is the realistic floor; many camera manuals require it outright.
• 4K60 / 8K & high-bitrate video: V60 or V90 rated cards, usually paired with UHS-II hardware.
• Burst-mode continuous shooting: U3 or faster — a slow card forces the camera’s buffer to fill and stalls the burst mid-sequence.

Dash cams and security cameras deserve a special note: they write continuously for hours, which is a harder endurance test than the write-speed rating alone. Look for cards explicitly rated for “high endurance” or continuous recording, not just a high speed class. Major card makers — SanDisk, Samsung, Lexar, and Kingston — all print these ratings the same standardized way, so the symbols mean the same thing regardless of brand.

Why the Rating Is a Minimum, Not an Average

Every class and UHS symbol on a card describes a guaranteed floor, never an average or a peak. Manufacturers test to the worst realistic case — random small writes, a nearly full card, warm operating temperature — and the printed number is what survives that stress test. Real-world sequential speed is almost always higher.

That gap is exactly why two cards with the same class rating can feel different in daily use: one might average well above its floor, another might barely clear it. If your card seems slow for its class, that alone doesn’t mean anything is wrong — it's still meeting its guarantee. It's a separate question from whether the card has actually failed.

Read Speed vs. Write Speed

Every class, U, V, and E rating on this page describes guaranteed minimum write speed — not the big number printed on the packaging. Retail boxes almost always advertise read speed instead (“up to 170MB/s!”), because it’s the larger, more marketable figure. Read speed matters for offloading footage to a computer; write speed is what your camera or phone depends on while it’s actually recording.

This is the single most common point of confusion when buying a card. A card can legitimately advertise 170 MB/s read while only carrying a U1 (10 MB/s minimum write) rating — both numbers are true, they just measure different directions of data flow. If a camera manual specifies “requires V30,” checking only the read-speed number on the packaging won’t tell you whether the card actually qualifies; look specifically for the class/U/V/E symbol itself.

Capacity Class vs. Speed Class — SD, SDHC, SDXC & SDUC

A card’s storage capacity and its write-speed rating are two completely independent specifications, even though buyers frequently assume a bigger card is automatically a faster one. Capacity is labeled separately: SD (up to 2GB), SDHC (2GB–32GB), SDXC (32GB–2TB), and SDUC (2TB–128TB). A card can be SDXC-capacity and still carry only a Class 4 speed rating, or SDHC-capacity with a V90 rating — the two labels combine independently.

This matters at the point of purchase: a 512GB card is not a faster card just because it holds more data. Always check the capacity letters (SDHC/SDXC/SDUC) AND the speed symbol (circle/U/V/E) separately — a high-capacity, low-speed-class card will still stutter recording 4K video no matter how much room is left on it.

Fake & Counterfeit Cards — What Happens to Your Data

A counterfeit card has a reprogrammed controller reporting a false capacity — it might claim 256GB while only holding a fraction of that in real NAND flash. It looks completely normal (correct capacity shown, files save fine) until the real physical limit is reached, at which point it silently overwrites your earliest files instead of throwing an error. Warning signs: an unusually low price, blurry packaging, no serial number, or an unauthorized third-party seller. The only reliable check is a full-write-and-verify test — H2testw (Windows) or F3 (Mac/Linux) writes data across the entire claimed capacity and reports exactly where real storage ends.

This is a genuinely different failure mode from a legitimately broken card: the NAND that exists usually still works, but files corrupt or vanish silently once the real ceiling is crossed. If you suspect a card is counterfeit and files are already missing, stop writing to it immediately — every new file risks overwriting data still sitting in the card’s real, smaller-than-advertised capacity. If this has already happened to you, our lab regularly recovers files from cards affected by counterfeit-controller overwrite — start a free evaluation and we’ll tell you what’s still there before you do anything else.

What If the Card Has Actually Failed?

A card that runs slower than expected or shows an uncertain rating is not the same problem as a card that has actually failed — corrupted, unreadable, cracked, or no longer recognized by any device. If you’re just trying to decode the speed symbols, nothing here needs fixing. If the card genuinely won’t mount or is throwing errors, stop using it immediately; every additional write risks overwriting data that could still be recovered.

For a card that’s physically damaged — snapped, cracked, or crushed — see our guide on recovering a micro SD card broken in half. For any failed microSD card, our lab offers dedicated micro SD card data recovery using chip-off and direct NAND extraction — even snapped and severely corrupted cards are often recoverable. The same lab also handles failed SSDs from Samsung, Crucial, Western Digital, Seagate, and Toshiba, plus retired formats like Sony’s Memory Stick and old CompactFlash cards still holding decades-old photo archives. Our team can evaluate it free — No Data, No Data Recovery Fee, since 1999.

About eProvided — Data Recovery Specialists Since 1999: our data recovery lab has spent over 27 years reading the minimum write speed a card was rated for against what it actually does under a microscope, and recovering the ones that stop working altogether.

What Our Customers Say

Rated 4.9 / 5 from 67 verified reviews on Trustpilot

Frequently Asked Questions About SD Card Speed Ratings

Recovery Services for Every Card Failure

If your card is unreadable, cracked, or the wrong speed class isn't the problem — it has actually failed — these are the services that get the data back: