Knowledge Base

Micro SD Card Buying Guide: Adding Storage to Your Phone

grand master



If you’re looking to add more storage to your smartphone, a Micro SD card can be a great investment. In this article, we look at Micro SD cards: how do you compare them, what do speed ratings mean and how much storage do you really need?


Adding Storage to Your Phone with an SD Card


Micro SD.jpgOver the past few years, our smartphones have become true multimedia hubs. We use them to take photos, to record videos, to store our entire collection of music and to watch high-definition TV: either on the phone itself or through a connected television. This means we’re demanding more and more storage space than before.


One of the cheapest ways of adding storage to your smartphone is to insert a Micro SD card. Many smartphones have a slot for this and you’ll gain additional storage space for your photos, music, videos, apps, documents and games. It’s also possible to use a SD card for transferring files between your laptop, camera and smartphone with SD cards being swappable depending on the data you wish to access.


Before buying a Micro SD card for your phone, you should first check that it supports Micro SD cards. Many Android handsets support Micro SD (including the Galaxy S II & Galaxy S III) but there are some notable exceptions such as the HTC One X and Google Nexus 4. Micro SD expansion is also not supported on the Apple iPhone and on many Windows Phone devices (the Samsung ATIV S is an exception here).


If your phone doesn’t support Micro SD expansion, you’ll either need to use cloud storage services or you’ll need to purchase a new phone with more storage.


SD, Mini SD and Micro SD: Which Size?


SD cards are sold in three different form factors: the standard full-sized SD card, the mini-sized SD card and the micro-sized SD card. Except from the physical dimensions of the card, all three types of card work in the same way and all are compatible with each other.


Form Factor Comparison.jpg

Left to right: SD card, mini-SD card, micro-SD card. Adapted from Wikipedia (CC-licensed).



Height (mm)

Width (mm)

Depth (mm)

Used in…

SD Card




Most PCs & digital cameras

Mini SD





Micro SD




Smartphones, tablets

SD cards, Mini SD cards and Micro SD cards differ in their physical dimensions.


Micro SD Adapter.jpgIf you have a fairly modern smartphone or tablet, the chances are that it’ll require the micro-sized version of an SD card. In contrast, many PCs, laptops and digital cameras still use full-sized SD cards.


To ensure that your SD card can be used in as many devices as possible, we recommend buying a Micro SD card: the smallest of the three. This will usually come with a free adapter that transforms your Micro SD card into a full-sized SD card when required.


Class Ratings & SD Card Performance


When comparing SD cards, most people jump right in and look for the cheapest card with the largest amount of storage. This is generally a bad idea as SD cards can differ substantially in the performance that they offer. Using a slow SD card in your smartphone will lead to performance bottlenecks: this means your phone could lag when loading applications, recording videos or browsing photos.

Speed Ratings.png

SD cards are given a class rating. Ratings include class 2, class 4, class 6, class 10 and UHS-I (given in order of slowest to fastest).


The easiest way of comparing SD card performance is to look at its “class rating”. The class rating tells you the minimum write speed of the card, measured in megabytes per second. Higher speeds will give better performance so you should ideally look for higher-class SD cards. For the best performance on a smartphone, class 10 micro SD cards are recommended.


Class Rating

Minimum Write Speed


Class 2


Often cheap, but poor performance & not suitable for HD video

Class 4


Minimum required for HD video recording

Class 6



Class 10


Recommended for best smartphone performance



Ultra High Speed I. Requires UHS-capable hardware.

Comparison of SD card “class” ratings and minimum guaranteed write speed.


Note that the SD card class rating only gives you the minimum write speed that is guaranteed by the manufacturer. It doesn’t tell you anything about real-world speeds. In reality, data write speeds tend to be higher than the stated speed, as are data read speeds. Nevertheless, higher class ratings tend to correlate with better performance and so opting for a class 10 SD card will normally give you the best performance.


SD Tools.jpgOnce you’ve got an SD card, you may be curious to know how well it performs. Android owners can download the free SD Tools application to measure the read and write speed of their SD card. The application is fairly straightforward and easy-to-use, although owners of the Galaxy S III and Galaxy Note II will need to manually configure the SD card path. To do this, tap on the Menu button followed by Advanced Path and enter /mnt/extSdCard.


The image on the right shows an example SD card benchmark test that we performed on a Class 10 Micro SD card. The test tells us that the card can write date at 13.2MB/s and can read data at 17.1MB/s. This is above the minimum speed of 10MB/s that is guaranteed by the manufacturer.


An alternative application for benchmarking your SD card is AndroBench. Note that this does not work correctly on Samsung devices.


Storage: How many GBs?


The next most important thing to consider when buying an SD card is the amount of storage that you need. You should look for an SD card with ample room for your files, whilst ensuring that your smartphone can support that amount of storage. Most people will be looking at a micro-SDHC card up to 32GB storage.


Card Type




Up to 2GB


SDHC (SD High Capacity)

2GB – 32GB

Most smartphones support SDHC, usually up to 32GB storage. May be lower on some handsets.

SDXC (SD eXtra Capacity)

More than 32GB

The Galaxy S III & Galaxy Note II support SDXC but only up to 64GB storage.


On most modern smartphones and tablets, SDHC-type cards are supported at capacities of up to 32GB. However, you should double-check your phone’s specifications to find out what you can use.


Depending on the amount of storage that your new SD card provides, it should be enough for the following:













800 songs

1,600 songs

3,200 songs

6,400 songs

SD Video

3 hours

5 hours

11 hours

21 hours

HD Video

40 minutes

80 minutes

3 hours

5 hours

Full HD Video

30 minutes

1 hour

2 hours

4 hours

These estimates are based on average file sizes of 2.5MB for an 8 megapixel photograph, 5MB for a MP3 music file, 25MB/minute for an SD video recording, 100MB/minute for a HD video recording and 135MB/minute for full HD video recording. Bitrate of video recordings tested on Galaxy Note II. File sizes may differ depending on the photo/video content and the handset used to record it.


Buying your SD Card: How to Avoid Fakes


There have been many reports of counterfeit SD cards being sold on the internet: these are often poor quality products that advertise higher speeds and higher capacities than they can actually offer.


Using a counterfeit SD card is likely to lead to data loss so it’s important to buy your SD card from a trusted source. This could mean buying an SD card on the high street or from a trusted online retailer such as Mobile Fun or Expansys. If you’re buying an SD card from Amazon, stick to products that are sold directly by Amazon rather than those which are provided by third-party sellers.


Finally, once your new SD card has arrived, it’s a good idea to test its authenticity. The industry standard application for this is called h2testw – it’s a Windows application that will test your SD card for the capacity and speed that is promised.  SD Tools for Android can also perform some basic checks on your SD card.


Moving Apps to an SD Card


Android Logo.jpgIf you're running short of internal memory on your Android device, you may be able to free up some space by moving some apps to an SD card. To do this, navigate to Settings > Applications manager and tap on the application that you wish to move. If the application is able to run from an SD card, there will be a "Move to SD card" option.


Note that as of Android 4.1 Jelly Bean, you'll need to root your device before you can move apps to an SD card. It's also possible to move a greater variety of applications to an SD card if your phone is rooted. Certain apps won't work from SD however: keyboards, widgets and services included. You should double-check the Android guidelines for running apps from SD first.


Your Thoughts…


Providing your smartphone supports it, a new Micro SD card can be a great way to upgrade the storage on your device without breaking the bank. Micro SD is great for storing photos, music and videos and can also come in handy when you want to transfer files between your laptop and smartphone.


Do you use a Micro SD card in your smartphone? Is Micro SD expansion an important feature to you, or do you prefer Apple’s approach of not supporting it? Do you think cloud storage is a suitable alternative to Micro SD? We’d love to hear your thoughts: please drop us a comment below and let us know what you think!


Ken Lo writes about mobile technology and the mobile industry at Ken's Tech Tips.

Interesting informative and well written as usual!

Interesting read. I put a 32Gb in my Smartphone and my Mifi. Class 10 Samsung as they were a good price.


surprised you have too root with jellybean to move apps to sd.

all very informative, thanks


Great article. More of this stuff is needed. Smiley Very Happy

grand master

I've just done a test on my sd card currently in my phone and its a 2GB Sandisk and the app when run reports a 8mb/s write speed.....which doesn't seem to be in the list as it jumps from class 6 to class 10? There's no class number printed on the card. Can't remember getting the card as i've got so many of different sizes and speeds. Seems to do the job in my cheap Chinese phone though


Nice mention on cloud storage, I like the idea of cloud storage more than actually using it to be honest and I've been trying out lots recently. Ubuntu One music is my newest experiment.


One realisation I've made is that if you want to use the cloud properly and make it as seamless (local seeming) as possible you have to cache data so you really need a good SD card with the cloud too. Otherwise you'll be re-downloading the same thing over and over again.


I buy mine off ebay, be very careful because most of them are fakes, I bought a load of them before I found anyone selling genuine cards on ebay for a cheap price but I always pay with paypal so always got my money back on the fake ones.


After buying a load of fakes before I found this seller called low price memory they have an elephant as a logo, I have had 3 cards off them and my sister has had one too and none were fakes so I suppose they can be trusted.  

I was sceptical, but download a program called h2testw and you can see if a card is the real thing and also the actual speed of it.  I got a 32GB class 6 card for less than £15 delivered and the actual speed I have tested it at is over 14mb/s so good enough to be called a class 10!


Great and useful guide well written Smiley Happy


No, cloud storage is not a suitable alternative to Micro SD.  Well, it may be in certain areas but many parts of this country still get less than 2Mps broadband.  Until everyone can have high-speed internet SD cards will be a necessity.

Thanks for a very useful and informative post.