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

Additionally, the SD Association has announced that SD specification 7.0 and 7.1 will introduce two new card types: an SD Ultra Capacity (SDUC) classification which will bring support for cards with up to 128TB of storage, and SD Express, a joint effort with PCI-SIG to create SD cards that are compatible with PCIe 3.0/NVMe v1.3 protocols and will offer peak theoretical speeds of 985MB/s.

Imagine if every electronic device you owned with storage replaced its slow interfaces with NVMe solid state drives. Offering speeds equivalent to many SSDs, SD Express cards will be functionally similar to removable SSDs. Utilizing PCI Express and NVMe will allow faster data rates and still be backwards compatibility at the expense of speed.

As mobile device cameras continue to improve, games get bigger, and more complex desktop apps are able to run on tablets or large phones, the need for fast, large capacity storage is clear. Future microSD Express cards will use the same or less power than current options available. Low power sub-states ensure that maximum energy consumption is at or below what previous generations have achieved.

Getting to the microSD cards...

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 specification in July 2005 and those early cards only supported up to 128MB of storage -- a limitation that was expanded later by the SDHC and SDXC specs.

  • 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 -- typically uses FAT32
  • microSDXC: 64GB to 2TB of storage, transfer rates from 50MB/s -- uses exFAT

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 have 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 through various advancements.

UHS-III v6.0 for instance was released in February 2017 and added two new full duplex specifications to the standard (data can be transferred to and from the card at the same time to reach speeds of 156MB/s for UHS-II using half duplex or 312MB/s with full duplex, while UHS-I is limited to 104MB/s and UHS-III has maximum theoretical throughput of 624MB/s using full duplex).

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 this goes 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:

What to Buy

Best Value microSD Card

Samsung Evo Select 64GB U3 - $11 on Amazon

Runner up: Samsung Evo Select 128GB U3 - $21 on Amazon (same card, more storage)

You may have noticed Samsung's dominance in the flash drive market if you've upgraded your computers with a solid state drive 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 $11, the 64GB Evo Select (U3) 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. If you need more storage, the Evo Select 128GB touts the same read speeds but faster 90MB/s writes, and it's even better value all up for just $20.

There are plenty of other alternatives at this price point, but beware many are U1 cards that advertise the same read speeds as the Samsung cards, but slower writes of only 20-30MB/s.

Best High Performance microSD Card

For Smartphones/Tablet use: SanDisk Extreme 128GB U3/V30 A2 - $30 on Amazon

For Cameras/Video recording: Delkin 1900x 64GB UHS-II U3/V60 - $85 on Amazon

Value alternative: Samsung Evo Select 128GB U3 - $21 on Amazon

For a wide majority of users, the best value cards will be fast enough and will offer plenty of storage. However, for more specialized use and if high-end performance is required, make sure you are buying a microSD card that is right for the task and that your device can take full advantage of the card's rating.

If you want fast storage for a smartphone or tablet, you should be more concerned about fast random access and reading small files simultaneously. The $30 SanDisk Extreme 128GB (SDSQXA1-128G-GN6MA) is rated for faster A2 application performance (4000 read and 2000 write IOPS) and lists 160MB/s sequential reads with 90MB/s write speeds. Although benchmarks suggest that you shouldn't necessarily expect those speeds at all times, it's not the peak sequential speeds what you're after here, but the faster random reads and writes.

For video recording on drones and video equipment, you want a card with the highest rating (UHS-II V90) but those are not available in the microSD form factor. The next best thing, UHS-II V60 gets you sustained 60 MB/s read speeds. Delkin's microSDXC UHS-II cards offer read speeds up to 300 MB/s and write speeds up to 100 MB/s. The Lexar Professional 1800x and 1000x series are also pretty good. To take full advantage of these cards make sure your card slot is UHS-II rated.

Finally, if you want the best performance on a device that uses full size SD cards, you're better off buying a full size SD card 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 and better sustained writes in general in this form factor.

Best High Capacity microSD Card

More storage: Kingston Digital Canvas React 512GB U3/V30 A1 - $150 on Amazon

Speed and Storage: SanDisk Extreme 400GB U3/V30 A2 - $110 on Amazon

Kingston, Micron and Sandisk offer the highest capacity microSD cards available, with cards set to hit 1TB before 2H 2019. Kingston's "Digital Canvas React" series goes up to 512GB and it's no longer as expensive as it used to be only months ago at $150. PNY also has a 512GB microSD card, though that model is more expensive at $190.

SanDisk cards max out at 400GB for the moment, while other brands rarely exceed 256GB. Both 512GB and 1TB SanDisk Extreme UHS-I microSD cards will become available April 2019 with MSRP set at $200 for 512GB and $450 for 1TB. Meanwhile, the 400GB card's pricing has been cut in half and now it's $110.

In terms of pricing and read/write performance (100MB/s reads and 80MB/s writes), Kingston's card is positioned in-between the alternative recommendations, with the SanDisk Extreme 400GB being a better pick if speed is of essence. It's the same card as above but in a higher capacity, touting peak reads and writes of 160MB/s and 90MB/s and that unique A2 rating.