microSD Card Buying Guide

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: