MicroSD cards – Everything you need to know

A microSD card is a very small proprietary flash memory card that is used to save data.

MicroSD cards are a type of Secure Digital card that are designed to store data on smartphones, eBooks, cameras, camcorders, music players, etc.

Brief background

It was developed by the SD association. SD cards in general were invented in 1999 by SanDisk, Panasonic, and Toshiba, all three of which are the pioneers of flash memory technology. They formed the SD association which has now grown to encompass the whole tech industry. At the time of writing this, the SD association has over 1000 members.

The microSD card was originally named T-Flash, but T-Mobile took the SD association to court over the use of the alphabet T. T-Mobile had a copyright claim over the alphabet T and wouldn’t permit anyone else to use it.

MicroSD cards were invented because Motorola complained that the old SD cards were simply too big for their mobile phones. Thus the SD association had to find a way to slim it down and slim it down, and they did. Thus the microSD cards were born.

Types of microSD cards

All microSD cards may look the same on the outside but they are very different on the inside.

microSD cards
A High capacity, Class 10 MicroSD card

MicroSD cards are rated based on capacity and (data transfer) speeds.

Data Capacity

There are four types of microSD cards based on capacity or the amount of data that each card can hold. They are:

  • SDSC – Secure Digital standard capacity
  • SDHC – Secure Digital high-capacity
  • SDXC – Secure Digital extended capacity
  • SDUC – Secure Digital ultra capacity

SDSC cards can only hold a maximum capacity of 2GB. They were invented in the year 2000, so that should explain why the capacity is so low. Once upon a time though, it was huge, now they are mostly extinct.

SDHC cards have a maximum capacity of 32GB. They were the first real microSD cards and were released in 2006. They usually come preformatted with FAT32.

SDXC cards are a massive improvement on the SDHC cards. SDXC cards can reach sizes of 2TB. They were released in 2009 and come preformatted with exFAT.

SDUC cards are currently the best microSDs that you can find around. Think of them as the flagship of microSD cards. These cards can reach capacities of 128TB. They were released to the market in 2019.


Several speed classes are used to rank microSD cards.

Original speed class

This is the most common speed class that you’ll see. On many SD cards, it is simply denoted with a C and a number. These numbers tell how fast the card is in Megabytes per second (MB/s). There are 4 speed classes:

  • Class 2 – 2MB/s
  • Class 4 – 4MB/s
  • Class 6 – 6MB/s
  • Class 10 – <10MB/s (10 to 90MB/s)

microSD cards
A 128GB Extended Capacity (SDXC) UHS class I card

Ultra high speed (UHS) Class

The UHS speed classes are an improvement over the older speed class. A UHS microSD card usually has a number written inside a U. There are two classes here that indicate the speed at which a microSD card can read/write data for sustained periods. These are:

  • Class I – 10MB/s
  • Class III – 30MB/s

Video speed

This is a new speed class. It is designed to take advantage of the latest speed improvements in microSD cards. These cards are usually designed to match MLC NAND flash and support 4K and 8K video. This is especially very important for cameras. Cards that support this speed standard have a V symbol.

MicroSD cards
A 32GB microSD card that supports class 10, UHS class 1, and Video speed (V10) speeds
  • Class 6 (V6) – 6MB/s (SD, HD & 4K video)
  • Class 10 (V10) – 10MB/s (SD, HD & 4K video)
  • Class 30 (V30) – 30MB/s (SD, HD, 4K & 8K video)
  • Class 60 (V60) – 60MB/s (SD, HD, 4K & 8K video)
  • Class 90 (V90) – 90MB/s (SD, HD, 4K & 8K video)

Not everyone uses the original speed class, others use the high speed class. This is usually signified by a high speed (HS), ultra high speed (UHS), or SD Express (sdEx) being written on the SD card.

Below is an infographic that should simplify things a bit.

  • Default – 12.5MB/s (Class 2, Class 4 and Class 6)
  • High Speed – 25MB/s (Class 10)
  • UHS-I – 104MB/s (UHS Class I)
  • UHS-II – 312MB/s
  • UHS-III – 624MB/S
  • SD Express – 3938MB/s

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