Whilst testing a new ProGrade Digital SD card recently, it really struck me how much of a mess the SD card association has made with card speed classifications and SD card nomenclature. There are so many symbols written on SD cards these days that it’s really hard to understand what you need for your camera, and what the card is actually going to give you. In this article, I’m going to explain things in as simple terms as possible!

General Note on Speed Ratings

All memory cards will perform at slightly different speeds when data is being written to them, and when data is being read from them. In 99% of cases, the read speed of the card will be much faster than the write speed. Of course, card manufacturers want to put the biggest number on the card, so if you see something like 100MB/s (megabytes per second) written on the card, it’s referring to the read speed and not the write speed, unless stated otherwise. Not only that, but they will also most likely be quoting a peak maximum read speed, rather than a sustainable speed. Sustained write speeds tend to be a few percent lower, but I have encountered some cards in the past that reach nowhere near their quoted read speeds. In other words, that number is the first indicator of a card’s performance, but it’s not the only thing you should be looking at.

xxxx Speed Vs. MB/s

Confusingly, there are some major SD card manufacturers that don’t quote their card speeds in MB/s at all, and instead use a speed factor such as 1000X. Lexar is probably the most likely brand that you’ll come across who does this, and it doesn’t help the consumer at all because it means that in the spur of the moment, you can only really compare Lexar cards to other Lexar cards on the shelf of your local store.

This speed naming scheme is a leftover from the days of CD-ROM drives, where a standard speed drive read things at 150KB/s.  Therefore 1X is equal to 150KB/s. It’s a ridiculously arbitrary way to label SD cards in the modern age! Below is a table of some common speed ratings so that you can easily convert things to the much more useful MB/s rating.

X SpeedMB/s


Megabytes Vs. Megabits

Another point of confusion that often arises while discussing or researching memory card (or hard drive) speeds, is the bits and bytes. MB/s means megabytes per second. Mb/s means megabits per second. Sometimes these are also written as MBps and Mbps just to confuse things even further.

There are 8 bits in a byte, so to get from megabytes to megabits you just multiply by 8. Conversely, if you have a speed that’s quoted in megabits per second (Mb/s), simply divide it by 8 to get to megabytes per second. There are far too many possible and regularly used values of these to create a handy table as I did in the previous section, so I’ll refer you to this online bits/bytes calculator instead.

You won’t often see SD card speed quoted in megabits per second, most brands do stick to megabytes per second, but you will often see megabits used in camera specifications when talking about video data rates. For example, in the specifications for the Panasonic GH5s, it lists the following data rate for 4k video at 60fps: 4K 50/60fps internal recording in 4:2:0 8-bit at 150Mbps in IPB. 

If you aren’t paying attention, this can be very confusing! The data rate for this particular video format is 150Mbps – that’s 150 megabits per second, which equates to 18.5 MB/s (Math: 150 divided by 8).  It would be quite easy to find a memory card that is capable of a continuous write speed of 18.5 megabytes per second, but not so easy to find one that would be capable of 150 megabytes per second write speed, had you got your bits and bytes confused!


SD – Secure Digital – Card capacities: 128MB to 2GB

SDHC – Secure Digital High Capacity – Card capacities: 4GB to 32GB

SDXC – Secure Digital Extended Capacity – Card capacities: 64GB to 2TB

SDUC – Secure Digital Ultra Capacity – Card capacities: up to 128TB

These different types of SD cards all have the same form factor, although the pin layout on the back of SDXC and SDUC cards is different from the earlier SD and SDHC cards. Essentially what you see here is like a timeline of SD card evolution. Most devices built after 2010 will have a card slot that is compatible with SDXC cards, but if you’re not sure, you should consult the manual for your camera because an SDXC card will not work in a slot that was only designed for older SD or SDHC cards. However, older SD and SDHC cards will work in an SDXC or SDUC slot. SDUC cards are based on the newer SD Express interface which uses PCIe and NVMe transfer protocols to allow a huge increase in read and write speeds compared to all previous versions (up to 958MB/s). At the time of writing this guide, the SD Express format has been announced, but there has yet to be a camera that supports it, or a manufacturer making SDUC cards that use is.

Now, this part might seem complicated, but let me simplify things for you. As SD cards evolved, these new types of cards essentially just allowed higher capacities and higher speeds. All you need to do is concentrate on getting an SD card that has the right capacity for your needs (16GB, 32GB, 64GB, 128GB etc.) and then possibly using the information in the next sections to make sure the card also has the speed that you need. If your camera is from after 2010 then it won’t matter what SD format the card is. Out of all of the little logos that are stamped on an SD card, this one is perhaps the least important. Just get the capacity that’s right for you, and double-check that your camera will support it. The answer will be ‘yes’ for 99% of you.