What you have to know about SD card speed

Hi guys,

I just want to discuss something related to my SD Card Test app. Sometimes I got negative feedback (low rating) with the following content:

“this app is wrong because it reports slow sd card speed (only 20 MB/s) and I have a Class 10 sd card subtitled up to 100 MB/s”

In fact, the app is good and the sd card is slow, that’s the reality.

Class 10 means the minimum sequential write speed is 10 MB/s. That’s all and nothing more. Anything above this value is just “up to”, and in fact that means nothing.

Let me tell you an example:

I have an Samsung EVO 64 GB Sd card, the type is Class 10/U1/XC1.  The spec mentions “up to” 48 MB/s.

But is this true? The trick is the next: the highlighted value is the read speed and not the write.

The reality is the next:

  • write speed:  23 MB/s
  • read speed: 45 MB/s

I tested it with my app, and with my computer too, the result is the same.

My app will assign the class type based on the write speed and you can compare the read and write speed.

 

 

 

 

 

 

 

 

 

The specification is the next:

  • class 10/UHS-1: the minimum sequential write speed is 10 MB/s
  • UHS-3/V30: the minimum sequential write speed is 30 MB/s
  • V60: the minimum sequential write speed is 60 MB/s
  • V90: the minimum sequential write speed is 90 MB/s

So when you find the measured speed too slow, don’t think my app is wrong, just check the class type and don’t believe for the marketing texts (“up to”).

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