What do SD and microSD card speed ratings actually mean?

A confusing stack of overlapping rating systems, added over time and printed together on the same card — and the one that matters depends entirely on what you are doing with it.

The ratings, in the order they were introduced:

Speed Class (C2, C4, C6, C10), shown inside a C. Guarantees a minimum sustained write speed in megabytes per second — so Class 10 means at least 10 MB/s.

UHS Speed Class (U1, U3), shown inside a U. Minimum sustained write of 10 or 30 MB/s. U3 is the practical requirement for 4K video.

Video Speed Class (V6, V10, V30, V60, V90). Minimum sustained write at that figure, designed specifically for video capture and the clearest of the systems.

Application Performance Class (A1, A2). Not about sequential speed at all — it specifies minimum random read and write operations per second, which is what matters when a card runs apps rather than storing files. A2 cards need host support to deliver their advantage, and without it can perform no better than A1.

Bus interface (UHS-I, UHS-II, UHS-III, SD Express), shown by Roman numerals and by extra rows of pins. This sets the maximum the card can theoretically achieve. UHS-II cards work in UHS-I devices but only at UHS-I speeds.

Why the advertised big number misleads. The headline figure is peak sequential read under ideal conditions. What actually determines whether video recording fails is minimum sustained write, which is the class ratings — a card advertising a very high read speed can still drop frames.

How to choose:

Video: match the V rating to the bitrate your camera records.

Running apps or an operating system: A2 where supported, and random performance matters far more than the headline.

Photos: almost any modern card is adequate; burst shooting benefits from a faster bus.

Buy from reputable sellers. Counterfeit cards misreporting their capacity are widespread and fail once written past their real size.