If it's been a while since you've bought portable flash memory, you might be surprised by the broad availability and affordability of high speed, high capacity microSD cards. Commonly used to expand the storage in devices ranging from smartphones to drones, microSD cards are becoming more frequently purchased than any other SD form factors, although full sized cards remain popular among digital camera owners.

Here's a quick overview of the different SD card sizes:

  • Standard SD cards: SD (SDSC), SDHC, SDXC, SDIO -- 32 x 24 x 2.1-1.4mm
  • miniSD cards: miniSD, miniSDHC, miniSDIO -- 21.5 x 20 x 1.4mm
  • microSD cards: microSD, microSDHC, microSDXC -- 15 x 11 x 1mm

To be clear, this guide will specifically be recommending microSD cards, but much of the information leading up to that will apply to the other form factors if that's what you're interested in.

The SD Association approved the final microSD spec in July 2005 and those early cards only supported up to 128MB of storage -- an early limit that was expanded later by the SDHC and SDXC specifications.

  • microSD: Max storage of 2GB, transfer rate of 25MB/s, uses FAT12, FAT16 or FAT16B file systems
  • microSDHC: 4GB to 32GB of storage, transfer rates from 50MB/s to 150MB/s, typically FAT32 file system
  • microSDXC: 32GB to 200GB of storage, transfer rates from 50MB/s to 312MB/s, uses exFAT file system

As noted, a significant portion of the SD cards available today are of the microSD form factor, and a great majority of those are of the microSDXC specification, which is to say that this is more than likely what you're looking for or will probably wind up purchasing. MicroSDXC cards can be further broken down into various different speed classifications, which ultimately apply to all of the SD card families mentioned above.

Speed Class Min. Seq. Writes UHS Speed Class Video Speed Class Ideal Workload
Class 2 (C2) 2MB/s     SD recording and playback
Class 4 (C4) 4MB/s     720p/1080p video
Class 6 (C6) 6MB/s   Video Class 6 (V6) 720p/1080p, some 4K video
Class 10 (C10) 10MB/s UHS Class 1 (U1) Video Class 10 (V10) 720p/1080p/4K video
  30MB/s UHS Class 3 (U3) Video Class 30 (V30) 1080p/4K video @ 60/120fps
  60MB/s   Video Class 60 (V60) 8K video @ 60/120fps
  90MB/s   Video Class 90 (V90) 8K video @ 60/120fps

The SD Association has come up with several different speed classification systems to help differentiate what cards are best suited to what purposes. The plainly stated "Class" number is the most immediate indicator to the speed of an SD card, with "Class 2" (2MB/s) cards being toward the bottom of the barrel and best geared toward standard definition video work or less demanding loads, and "Class 10" (10MB/s) cards being capable of recording or playing up to 4K video.

Further, some SDHC and SDXC cards tout the Ultra High Speed (UHS) classification, indicating support for one of three UHS specifications (UHS-I, UHS-II and UHS-III), which offer improved data transfer rates by various advancements. UHS-III v6.0 for instance was released in February 2017 and added two new full-duplex specifications to the standard.

The organization's "Video Speed" classification is more succinct in conveying its information, with "Video Class 10" for instance applying to cards that have a minimum sequential write speed of 10MB/s, and these go up to Video Class 90 (V90 -- 90MB/s) which can handle 8K video at 60 or 120fps.

Application Performance Class

Excluded from the first table above, there is a relatively new "Application Class" for SD cards -- Class 1 and 2 (A1, A2) -- which outline minimum IOP performance. A1 cards are good for a random read performance of 1500 IOPS and random writes of 500 IOPS, while A2 steps that up to 4000 IOPS and 2000 IOPS.

Additionally, if you see an SD card stamped with either A1 or A2, you can rest assured that it offers a sustained sequential write speed of at least 10MB/s.

Class Min. Seq. Writes Min. Random Read Min. Random Write Ideal Workload
A1 10MB/s 1500 IOPS 500 IOPS Editing and updating app data, not just storage
A2 10MB/s 4000 IOPS 2000 IOPS Higher performance, special uses

Of course, the faster the card, the more you can expect to pay, so it makes sense to know what your needs are and buy accordingly. Pro photographers will inevitably want the quickest card they can buy, but there's little justification for that when expanding the storage of a budget smartphone for example.

Although our guide should have equipped you with the information you need to pick your own microSD card (or standard SD card for that matter), we went ahead and chose three that stood out as good all-around choices for a few different shopping categories:

Best Overall microSD Card

Samsung Evo Select 64GB MB-ME64GA/AM - $23 on Amazon

Runner up: SanDisk Ultra 64GB SDSQUAR-064G-GN6MA

You may have noticed Samsung's dominance in the flash drive sector if you've upgraded your desktop with solid state drives over the years and there's a good chance that the company's memory chips are in your smartphone. That being the case, it shouldn't come as too much of a surprise to see a Samsung-branded microSD card listed here.

For only $20, the 64GB Evo Select offers 100MB/s reads and 60MB/s writes, ships with an SD card adapter and has become the best-selling microSD card on Amazon over the last year. The 64GB SanDisk Ultra advertises the same read speeds for the same price on Amazon, but its write speed isn't readily available (even on the company's website). Assuming it provides the same specs, choosing between the two (Evo and Ultra) would come down to pricing and availability, and Samsung seems to have an overall edge there.

Best High Performance microSD Card

Samsung Evo Select 128GB MB-ME128GA/AM - $45 on Amazon

Runner up: SanDisk Extreme SDSQXAF-128G-GN6MA

Without running any benchmarks and judging today's cards purely on paper, this category also seems to be cinched up by the Samsung Evo Select. The 128GB Evo Select touts read speeds of up to 100MB/s and write speeds of 90MB/s (60MB/s for the 64GB model), and that lines up with the fastest competition we could find -- namely the SanDisk Extreme 128GB, which lists the same read and write rates.

However, even if the SanDisk Extreme has an edge on the Evo Select in performance, the latter would still likely come out ahead in terms of value when accounting for cost seeing as the 128GB Evo Select is only $40, while a SanDisk Extreme of the same capacity is going for $65. Of note, there is also a non-Select Samsung Evo available that touts 100MB/s transfer speeds and UHS-III compliance but has lesser write speeds and isn't any more affordable than the Evo Select.

Of note, if you want top performance on a device that uses full size SD cards, you're better off buying said full size cards instead of microSD cards that come with an SD adapter. Top performing SD cards such as those from Sony and Lexar can reach 300MB/s reads, whereas the fastest cards here top out at 100MB/s.

Best High Capacity microSD Card

SanDisk Ultra 400GB SDSQUAR-400G-GN6MA - $200 on Amazon

SanDisk takes the prize if you simply want all the gigabytes you can get in a microSD card. The company's Ultra series offers seven different capacities and stretches on up to 400GB, which is well beyond what you'd find from any other flash card provider, which typically top out at 256GB, as is the case for Samsung's Evo Select for instance.

Priced at $200, the 400GB SanDisk Ultra touts similarly speedy read rates as the other cards (95/100MB/s) but as already mentioned, we haven't been able to nail down the card's write performance. Fortunately, it's not being chosen here for its speed so we can look passed the missing specification. For what it's worth, you aren't paying much of a premium per gigabyte for jumping from the ~200GB drives to the 400GB Ultra, which is going for about $0.49/GB, around the same price per gig as the lower capacity microSD cards.