What the markings (Classifications) on an SD Card mean and their relation with Data Transfer Speed

SD Cards are a handy storage device, which are multi-purpose, multi-faceted and portable. They are used in a variety of applications such as digital cameras, smartphones etc. But, while buying an SDCard there are too many parameters written on the SDCard which can confuse the buyer. What are they and which ones matter the most. Let’s take a look.

Classifications

Storage (SD*C)

It is roughly in increasing order of capacity:

1. SD — 0 to 2 GB cards
2. SDHC — 2 to 32 GB cards
3. SDXC — 32 to 2048 GB (or 2 TB) cards
4. SDUC — 2048 to 131072 GB (2 to 128 TB) cards

Marking: Usually Size in GB is written.

Physical Classification

SD Cards come in three formats (based on physical dimensions):

1. Normal — for digital cameras etc.
2. Mini
3. Micro — for smartphones etc.

Marking: Can be seen explicitly as one type of card would be having more dimensions than the other.

Read-Write Speed (C*)

The SD Cards have further classification according to the SD Association, based on Read-Write Speeds called Class (roughly, they dont say it, but they mean it) :

1. Class-10 — roughly 10–15 MBps (Fastest)
2. Class-6
3. Class-4
4. Class-2
5. Class-0

The Class 10 is the best in terms of speeds and performance. However, if you’re using a smartphone, a Class 6 would work equally well.

Marking: The letter C, with class number written inside it.

Bus Speed (U*)

There is another form of classifications called HS meaning ‘High Speed’. This is also based on Speed, but on the ‘Bus Speed’ instead of Read-Write Speeds.

1. UHS 3 — upto 624 MBps (Fastest)

2. UHS 2 — upto 312 MBps

3. UHS 1 — upto 104 MBps

4. HS

5. Normal

Marking: The english letter ‘U’ with the number denoting the type.

Which Classification is the Most Accurate Indicator of Speed?

As an experiment, I tested a Transcend MicroSD Card having the following specifications:

Storage — 16 GB (SDHC)

Class — 10 (C10)

Bus — UHS1 (U1)

I transferred a Folder of roughly 1 GB in size to the card using various devices. The details of which can be seen below.

via Card Reader (Zebronics) — USB2.0

via old iPod -MiniUSB to USB2.0

I also transferred the same files after inserting the card into a SAMSUNG phone USB-C, which yielded speeds of roughly 19–20 MBps.

What this essentially means that a microSD Card having ‘Class 10’ and ‘UHS-1’ , both marked on the surface of the card, may have an actual average speed of 10–15 MBps (i.e it depends mostly on the Speed Class) which is Class-10, in this case.

Conclusion

As can be seen in the above experiment, the Speed of an SDCard also depends on the type of data-transfer mechanisms and USB Ports you’re using.

But by and large, the Class Speed i.e Read-Write Speed is what you should look for while buying an SDCard and it is the most accurate indicator of data-transfer speed.

References

1. https://chasingheartbeats.com/uhs-1-vs-class-10/
2. https://www.quora.com/What-is-difference-between-micro-SD-and-micro-SDHC-and-micro-SDXC-cards
3. https://www.sdcard.org/consumers/choices/file_system/index.html
4. https://www.sdcard.org/consumers/choices/speed_class/index.html
5. https://www.sdcard.org/consumers/choices/bus_interface/index.html

Feedback

If you have some suggestions or you find something missing or incorrect in this article. Feel free to reach out to me via e-mail :

ishaan.kalorano@gmail.com