MicroSD cards continue to be a relatively inexpensive way of augmenting your phone’s storage, but buying these portable storage chips or rather choosing the best one suited to your specific needs still remains tricky.

If you need to buy one for your phone and are looking for factors that you should consider before buying a microSD card, we have all the key pointers covered in this article.

So, let’s get started.

Note: By write speeds mentioned below, we mean sequential write speeds. Random write speeds will be significantly lower. 

Also Read: Decoding Smartphone Storage – All About Internal and External Memory

Card Formats and compatibility – Micro SD vs SDHC vs SDXC

To start with, you will have to choose between three card formats – SD, SDHC, and SDXC.

The card format will determine whether or not the card you are buying will be compatible with your phone. For instance, if the SD card slot on your phone only supports MicroSDHC card, a microSDXC card will not work on it.

So, here is what differentiates the three formats:

Format Storage capacity Compatible with
MicroSD Up to 2GB All microSD card slots
MicroSDHC 2GB to 32GB Slots that support MicroSDHC or MicroSDXC formats (almost all smartphones)
MicroSDXC 32GB to 2TB Only with devices that support MicroSDXC

As of today, almost all smartphones support microSDHC cards and quite a few out there also support MicroSDXC cards.

At the same time, if your phone supports SDXC format and supports expansion up to 128GB, an SDXC card of capacity higher than 128GB will not work on your phone.

Ultra High-Speed Cards: UHS-I, UHS-II, and UHS-III

The UHS that you see inscribed on SD cards stands for Ultra High Speed. This denotes the type of bus interface used (or faster circuitry) that facilitates faster data transfer.

The UHS -I and UHS-II standards are generally denoted by only I and II on microSD cards and support speeds of up to 104MBps and up to 312MBps respectively.  The UHS-III standard can go up to 600MBps in theory.

If you plug a UHS cards in a phone that doesn’t support UHS, the card will still work. But the speed will be reduced to 25 MBps.

As of today, no smartphone supports UHS-II or UHS-III cards, so there is no point looking for one.

MicroSD Card Speed: UHS Class Speed Vs Card Class Speed

You will notice two sorts of Class denoted on an SD card – in ‘C’ and in ‘U’.

The U1 and U3 are UHS Class speeds that show the minimum write speed. U1 cards support at least 10Mbps write speed and the U3 cards will deliver at least 30MBps write speed.

You must note that these are minimum write speed values. It is possible for a U1 card to give a write speed greater than 10MBps, say 15MBps, as well.

The Speed Class, that’s denoted numeric value within the letter ‘C’ on your microSD card also denotes the sequential write speed. You can see the minimum write speed for different classes in the table below.

You must also know, that Class 10 cards have a bus speed of 25MBps as against 12.5MBps on Class 2, 4, and 6, which is to say, they offer significantly better maximum write speeds.

Needless to say, if you need your SD card for demanding applications like recording 4K Videos, you will need high write speed.

UHS Speed Class

UHS Speed Class Minimum write speed
UHS Class Speed U3 At least 30MBps
UHS Class Speed U1 At least 10MBps


Card Speed Class

Card Speed Class Minimum write speed Bus Speed (Theoretical maximum)
Class 10 At least 10MBps 25MBps
Class 6 At least 6MBps 12.5MBps
Class 4 At least 4MBps 12.5MBps
Class 2 At least 2MBps 12.5MPbs


Video Speed Class

Though UHS speed class and card speed class are the most popular metric for judging an SD card, Video speed class denoted by V90, V60, V30, V10, and V6 is also a new standard that exists. And this one too overlaps with other metrics for judging card speeds.

Video Speed Class Minimun write speed (sequential) Application
V90 90MBps 8K videos
V60 60MPps 8K, 4K Videos
V30 30MBps 4K, Full HD Videos
V10 10MBps Full HD, Standard Videos
V6 6MBps Standard Videos

Any V60 or higher card will most likely be a UHS-II card.

MicroSD App performance Class – A1 and A2

Wonder what A1 is on your microSD card? well, it’s a new standard that signifies that your card is fast enough to be used by Apps.

A1 cards need to be at least Class 10, U1, V10 and have a sequential write speed of 10MBps. A1 cards also need read and write IOSP (Input and Output operations per seconds) of 1500 and 500 respectively.

A2 App performance class was also added later and this mandates read and write IOSP of 4000 and 2000 respectively.

Only recently have we begun to seen Class A1 cards in India from Sandisk. A2 cards are still not available.

Rated Speed and relative speed

Apart from the above-mentioned ways for estimate speeds, sometimes manufacturers directly write the maximum write speed on the card itself (eg. 40MBps, 30MBps), especially on affordable SD cards. This speed is based on tests conducted by manufacturers in ideal lab conditions and thus the real world performance that you get will not be the same.

Some manufacturers also mention relative speed, that shows as 1000X or 500X, which is basically a comparison with the transfer rate of CD drives (150KBps). This isn’t something we see very often on MicroSD cards in India these days.

Speed of MicroSD Card Speed Classes summed up

Still confused? Let’s combine all microSD card speed indicators in one table for better clarity.

Minimum sequential write speed Card Speed Class UHS Speed Class Video Speed Class App Performance Class
90MBps V90
60MBps V60
30MBps 3 V30
10MBps 10 1 V10 A1
6MBps 6 V6
4MBps 4
2MBps 2


The fastest MicroSD card available today (For Phones)

Now that you know different metrics for judging a card it’s natural to wonder which is the fastest card you can buy today. The fastest microSD cards you can buy today are UHS-I U3/ V30 cards. Yes, theoretically you can go a lot higher, but practically that’s the best available as of now.

You can buy them in up to 256GB of storage and maximum sequential write speed you can get is around 90MBps.

Since you are choosing an SD card for your phone, you must know that even the fastest SD cards available are way slower than your phone’s internal storage.

Most phones don’t allow you to keep apps on SD storage these days. After Android KitKat, the SD card experience has never been as seamless but it’s more secure.

Make sure what you need to use microSD card for

The UHS Class 3 (U3) and Class 10 cards with a minimum write speed of up to 30MBps are what are ideal for 4K video capture, but you can also make do with UHS Speed Class 1 (U1) and Class 10 cards as well. For full HD video recording, you can go with U1 and Class 10 or Class 6 cards as well.

While buying for your phone, you should rather aim for the maximum storage capacity as you will primarily be using the extra space to store media files and images.

Buying MicroSD cards for Phones – Things to consider

So, these are the points that you should keep in mind before buying a MicroSD card. We would advise you to opt for familiar brands and from reputed stores as the market is simply flooded with fake cards. Also, ensure that you periodically back up your SD card because irrespective of the brand you go with, there are chances that your card might fail you at some point in time.




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