Memory Card Speed Classes Explained: U3, V30, V90, A2, UHS-II and CFexpress

What U3, V30, V60, V90, A1/A2, UHS-I, UHS-II and CFexpress VPG markings actually guarantee, which card to buy for 4K cameras, action cams, phones and drones, and how to spot fake cards.

By Gadget Soch·16 Oct 2026·8 min read

A memory card’s label is crowded with symbols: a U inside a bucket, a V with a number, A1 or A2, Roman numerals and a speed in MB/s. The wrong card can mean a camera that stops recording mid-clip or a phone that feels sluggish. This guide explains what each marking guarantees, which card suits 4K cameras, action cams, phones and drones, and how to avoid fake cards.

Quick answer

For 4K video in most cameras, action cams and drones, a card marked U3 or V30 is the usual starting point. Always confirm with your device manual.
V60 and V90 are for high-bitrate video and are only found on UHS-II (and faster) SD cards.
A1 and A2 refer to app performance (random read and write), which matters for phones and handheld consoles, not cameras.
UHS-II cards only reach their higher speeds in UHS-II devices. In a UHS-I slot they run at UHS-I speeds.
CFexpress is a separate, faster card family used in higher-end cameras. Look for the VPG rating if your camera’s video modes require it.
Buy from authorised sellers and test every new card’s real capacity.

Two kinds of numbers: guaranteed minimums and marketing maximums

The most important thing to understand is that speed class symbols describe a guaranteed minimum sustained write speed. That is what video recording needs, because the camera writes data continuously and cannot pause if the card slows down. Big “up to” MB/s figures on packaging are usually peak read speeds, useful for file transfers but not a recording promise.

The SD Association advises choosing a card with the same or higher class symbol than your device asks for.

Speed Class, UHS Speed Class and Video Speed Class

Kingston’s guide summarises the three generations of minimum sustained write speeds:

Speed Class: C2 (2MB/s), C4 (4MB/s), C6 (6MB/s), C10 (10MB/s).
UHS Speed Class: U1 (10MB/s) and U3 (30MB/s).
Video Speed Class: V6 (6MB/s), V10 (10MB/s), V30 (30MB/s), V60 (60MB/s), V90 (90MB/s).

The SD Association confirms that Class 10, U1 and V10 all indicate the same 10MB/s write speed, and that Video Speed Class was defined to meet demand for high-resolution 4K and 8K recording. It also notes that V30 applies to UHS bus cards, while V60 and V90 apply only to UHS-II and UHS-III cards.

Why the bitrate matters more than the resolution

A camera’s need depends on its video bitrate, not just “4K”. Bitrate is often quoted in megabits per second (Mbps), while card speeds are in megabytes per second (MB/s). Divide Mbps by 8 to compare: a 100Mbps mode needs about 12.5MB/s, comfortably within V30. Higher-bitrate modes, especially all-intra codecs, which the SD Association notes are usually larger in size, may call for V60 or V90.

A1 and A2: application performance for phones

Apps and games read and write many small files at random locations, unlike a long video recording. According to the SDA, A1 cards deliver at least 1,500 random read IOPS (input/output operations per second) and 500 random write IOPS, while A2 cards deliver 4,000 read IOPS and 2,000 write IOPS. Both classes also require a sustained sequential write of 10MB/s.

There is a catch. A2’s extra performance relies on features such as command queuing and cache, which the host device must support. In a phone or handheld that does not support A2, an A2 card may perform no better than A1. Many recent flagship phones have no microSD slot at all.

UHS-I vs UHS-II (and beyond): the bus interface

Roman numerals describe the bus interface, which caps how fast data can move between the card and the device. The SD Association lists these maximum bus speeds:

UHS-I: up to 104MB/s (SDR104), using a single row of pins.
UHS-II: up to 312MB/s half duplex or 156MB/s full duplex, using a second row of pins.
UHS-III: up to 624MB/s full duplex, with two rows of pins.
SD Express: PCIe-based, from 985MB/s up to 3,940MB/s depending on the PCIe generation and lanes.

A UHS-II card has a second row of contacts on the back. The SDA notes that actual card performance varies and differs from bus speed. In a UHS-I camera or reader, a UHS-II card falls back to UHS-I speeds, so you pay for speed you cannot use.

Capacity labels: SDHC, SDXC, SDUC

These labels describe capacity, not speed. The SDA defines SDHC as above 2GB up to 32GB (FAT32), SDXC as above 32GB up to 2TB (exFAT) and SDUC as above 2TB up to 128TB.

CFexpress: the faster card family

The CompactFlash Association’s CFexpress standard uses the PCIe interface with the NVMe protocol, the same technology found in fast SSDs. Versions 1.0, 2.0 and 4.0 have been released, with 4.0 arriving in 2023. There are three sizes:

Type A: the smallest, with one PCIe lane.
Type B: two PCIe lanes, for double the maximum speed of Type A. It shares the physical size and connector of XQD cards, and some XQD cameras accept it after a firmware update.
Type C: the largest. The CFA states that no cameras or cards currently use it.

Lexar lists the theoretical maximums as 1,000MB/s for Type A and 2,000MB/s for Type B under CFexpress 2.0, doubling to 2,000MB/s and 4,000MB/s under CFexpress 4.0. Type A and Type B are not interchangeable, so buy the type your camera’s slot takes.

VPG ratings

The CFA’s Video Performance Guarantee (VPG) certifies a minimum sustained write speed, similar in spirit to V30 or V90 for SD. VPG 200 and VPG 400 guarantee 200MB/s and 400MB/s. Newer VPG800 and VPG1600 levels guarantee 800MB/s and 1,600MB/s. The CFA notes that the newer profile is not backward compatible with the older one, so a VPG800 card is not automatically accepted by a camera mode that asks for VPG200 or VPG400. Some cameras block certain high-bitrate modes on cards without the required VPG level, so check the manual.

Which card for which device

Mirrorless cameras shooting 4K: U3/V30 is the usual minimum. Choose V60 or V90 UHS-II only if your camera has a UHS-II slot and uses high-bitrate or all-intra modes.
Cameras with CFexpress slots: buy the correct Type (A or B) and the VPG level your camera specifies for its video modes.
Action cameras: requirements vary by model. Kingston lists V30 or UHS-3 for the GoPro HERO10 Black, for example.
Drones: DJI’s Mini 4 Pro spec sheet lists recommended cards that are all U3 and V30 or V90 rated. Follow your model’s recommended list.
Phones and handheld consoles: an A1 or A2 microSD card from a known brand. A2 only helps if the device supports it.
Dashcams and security cameras: consider endurance-focused cards, as they write constantly.

How to spot fake memory cards

Counterfeit cards are a real problem, and TP-Link’s VIGI support team describes the most dangerous type: cards that report a larger capacity than they actually have. When you exceed the real capacity, data is silently corrupted or lost. Warning signs include:

Prices far below the brand’s usual market price.
Capacities that do not exist in the brand’s official lineup.
Packaging missing specifications, warranty information or a proper barcode.
Real-world speeds far below the printed class, such as a “V30” card writing at 5-10MB/s.
Unknown sellers on marketplaces, even if the listing uses a famous brand name.

How to test a new card

Buy from the brand’s official store or authorised retailers, and keep the invoice for warranty.
Before using it, run a full write-and-verify capacity test: H2testw on Windows or F3 on macOS and Linux.
Fill all available space during the test. Any errors mean the card is fake or faulty.
Format the card in the device you will use it in after testing.

Buying checklist

Check your device manual for its required card type, speed class and maximum capacity.
Match the minimum write class (U3, V30, V60, V90 or VPG) to your highest-bitrate video mode.
Only pay for UHS-II if your device has a UHS-II slot or you want faster transfers with a UHS-II reader.
For phones, prefer A1 or A2, and check whether the phone even has a card slot.
Buy from authorised sellers and test every card’s capacity.

Frequently asked questions

Is V30 enough for 4K video?

For many consumer cameras, drones and action cams, yes. High-bitrate or all-intra modes on advanced cameras may need V60 or V90, so check the manual.

Will a UHS-II card work in a UHS-I camera?

Usually yes, since the cards are physically compatible, but it will run at UHS-I speeds. You only get the extra speed in a UHS-II device or reader.

Can I use a CFexpress Type A card in a Type B slot?

No. They are different physical sizes, and the CFA describes them as separate form factors. Buy the type your camera specifies.

Why does my card show less space than advertised?

A small difference is normal because of how capacity is counted and formatted. A large gap, or errors in a capacity test, points to a fake card.