Memory card packaging is a stack of overlapping symbols — a C with a number, a U with a number, a V with a number, roman numerals, “up to 300 MB/s” in large type. Each symbol measures something different, and the one that matters for a given camera is rarely the one printed largest. This guide decodes each label, explains which guarantees apply to burst stills versus video recording, and shows how to match a card to what a camera actually demands.
Four labels, one card: a decoder
Every modern SD card carries labels from four independent rating systems, defined by the SD Association. Speed Class (the C symbol) and UHS Speed Class (the U symbol) certify minimum sustained write speeds. Video Speed Class (the V symbol) certifies minimum sustained writes under video-specific test conditions. The bus interface — UHS-I, UHS-II — is the physical data pipeline itself, with its own ceiling independent of any of the classes.
Capacity families are the fifth system, and they say nothing about speed: SDHC covers 4–32 GB, SDXC covers above 32 GB up to 2 TB, and SDUC extends beyond that. Confusing the systems is how buyers overpay — a “UHS-II” card is a bus classification, not a performance promise, while a “V30” card is a performance promise whose delivery depends on the bus behind it.
Speed Class and UHS Speed Class: the write-speed floors
The original Speed Class symbol — C2, C4, C6, C10 — guarantees a minimum sustained write rate of 2, 4, 6 or 10 MB/s. The UHS Speed Class supersedes it: U1 guarantees 10 MB/s and U3 guarantees 30 MB/s of sustained writing. That is the entire meaning of the symbol — a floor under the worst-case write, not an average, not a peak.
These floors were designed for older workloads. 10 MB/s handles Full HD recording and standard burst photography; 30 MB/s covers heavy 1080p bitrates and entry 4K. The ratings still matter as sanity checks — a card with no class symbol beyond “Class 4” has no business in any current camera — but modern devices have moved past what U1 and U3 can express, which is why the V-class exists.
Video Speed Class: the rating built for 4K and 8K
Video Speed Class, introduced by the SD Association for high-bitrate recording, sets the number of the class equal to the guaranteed minimum sustained write speed in MB/s. V6 means 6 MB/s, V10 means 10, V30 means 30, V60 means 60, and V90 means 90. The test conditions are video-specific — sustained large-block writes — which is what recording actually does, as opposed to the small random writes phone apps perform.
The mapping to use cases is direct. V30 sustains 4K recording up to roughly 240 Mbps, the top bitrate of most current consumer cameras; DJI’s own recommended-card list for the Osmo Action 5 Pro, which records 4K/120 at up to 120 Mbps, consists of U3/V30 microSD cards. V60 and V90 exist for the cameras that exceed that: 8K recording, RAW video, multi-stream 360° capture. Kingston’s guidance is blunt on the pairing — only UHS-II cards meet the standardized definitions of V60 and V90, so a camera requiring those classes for a mode is implicitly asking for the faster bus too.
Bus types: UHS-I, UHS-II and the two rows of pins
The bus is the road, the classes are the speed limit. UHS-I defines bus modes up to 104 MB/s; UHS-II adds extra data channels and raises the ceiling to 312 MB/s. You can identify a UHS-II card physically: it carries a second row of contact pins on its back. A UHS-II card in a UHS-I slot works fine but runs at UHS-I speeds; the card never exceeds the slot.
This is why the roman numeral is the first thing to check, before any class symbol. A camera with a UHS-I slot derives no benefit from a V90 card, and a camera whose manual demands V60 for a given mode will not record that mode reliably on a V30 card no matter how good its marketing reads. The guarantee travels in one direction only: the class certifies the card, the manual certifies what the camera needs.
Why “up to 300 MB/s” is irrelevant for video
The big number on the front of the package is almost always a peak read speed — how fast the card empties into a computer, sometimes only in a vendor’s proprietary reader mode. Several UHS-I cards advertise read speeds well above the standard’s 104 MB/s ceiling using proprietary double-data-rate modes that require a matching reader; in a standard UHS-I host, those speeds never materialize.
Video recording, meanwhile, is a pure write workload. Peak read speed contributes nothing to whether a 4K/60 take survives; the sustained write floor does. When a recorder outruns a card, the camera’s buffer fills and recording stops or drops frames — the failure no speed class above the requirement ever rescues, and no read speed ever prevents. Read speeds matter for your import workflow after the shoot, which is a real but separate purchase decision from the card that goes in the camera. The same logic applies to drones and action cameras: a card certified at V30 comfortably serves the 120 Mbps ceiling of the action cameras in our action camera comparison, while a camera body that records 8K — see the entry-level picks in our budget mirrorless guide — tops out where V30 still suffices.
The counterfeit problem
Speed-class symbols are printed on the label, and labels can be faked. Counterfeit cards — often rebranded small-capacity or slow flash sold as large fast cards — remain a persistent problem on marketplace platforms, and their failure mode is nasty: a card that reports 128 GB but corrupts data past its real 16 GB of flash, or one that stalls during sustained writes and kills a recording session. The symptoms often appear only after the footage matters.
Mitigation is behavioral, not technical: buy from the manufacturer’s own channels or established retailers rather than third-party marketplace sellers, be suspicious of capacity-to-price ratios far below the brand’s own listings, and test any card from a new source with a write-verification utility — free tools such as h2testw write a pattern across the full capacity and read it back, exposing reprogrammed controllers immediately. A card that passes a full-capacity verification and sustains its class rating in practice is almost always genuine.
Matching the card to the camera
The camera manual outranks every general rule. Manufacturers publish per-mode card requirements — a given recording mode may demand V60 specifically, while others accept V30 — and the manual’s table is the authoritative list. Three practical steps cover most buyers: find the highest-bitrate mode you will actually use, buy a card one class above the manual’s stated minimum for headroom, and verify the bus type your camera’s slot supports before paying for UHS-II.
Two closing notes on economics. First, cards exceed cameras in lifespan: the card you buy for today’s camera will likely live through the next two, so paying for a class above the current requirement is rarely wasted. Second, endurance matters more than speed for continuous workloads — dashcams, trail cameras and interval work benefit from the high-endurance product lines vendors build for sustained writes, since consumer cards optimized for burst speed are not rated for hours of uninterrupted recording.