SD cards have speed classes that indicate how fast they can read and write data. The fastest mainstream speed class available today is UHS Speed Class 3 (U3), which has a minimum sequential write speed of 30 MB/s.
What are the different SD card speed classes?
There are several speed classes for SD cards defined by the SD Association:
- Class 2 – Minimum sequential write speed of 2 MB/s
- Class 4 – Minimum sequential write speed of 4 MB/s
- Class 6 – Minimum sequential write speed of 6 MB/s
- Class 10 – Minimum sequential write speed of 10 MB/s
- UHS Speed Class 1 (U1) – Minimum sequential write speed of 10 MB/s
- UHS Speed Class 3 (U3) – Minimum sequential write speed of 30 MB/s
The class number represents the minimum sequential write speed in megabytes per second. So a Class 10 card has a minimum write speed of 10 MB/s, while a UHS Speed Class 3 card has a minimum write speed of 30 MB/s.
What is meant by Class 10?
The Class 10 specification was introduced in 2009 as an improvement over older and slower speed classes. It guarantees a minimum sequential write speed of 10 MB/s, making it suitable for full HD video recording.
To carry the Class 10 rating, an SD card must deliver the following minimum speeds:
- 10 MB/s sequential write speed
- 10 MB/s sequential read speed
- Random 4KB write IOPS of 500
- Random 4KB read IOPS of 1500
Class 10 cards provide a major boost in sequential write performance compared to older cards. For example, Class 6 only requires a minimum sequential write speed of 6 MB/s. So a Class 10 card is nearly twice as fast.
Is UHS Speed Class 3 the fastest SD card class?
UHS Speed Class 3, first defined in 2011, is technically the fastest speed class for SD cards today. To carry the U3 rating, a card must deliver a minimum sequential write speed of 30 MB/s.
That makes U3 cards three times faster than the original Class 10 specification, with triple the sequential write speed. U3 also requires higher random read/write speeds than Class 10:
- 30 MB/s minimum sequential write speed
- 30 MB/s minimum sequential read speed
- Random 4KB write IOPS of 1500
- Random 4KB read IOPS of 3000
The UHS rating was created for advanced 4K and 8K video recording, professional/broadcast quality video, and burst mode shooting on high-resolution DSLR and mirrorless cameras. For these use cases, the extra performance of U3 cards makes a big difference.
Comparing Class 10 and UHS Speed Class 3 cards
Here is a direct comparison of the Class 10 and U3 speed requirements:
| Specification | Class 10 Minimum | UHS Speed Class 3 Minimum |
|---|---|---|
| Sequential Write | 10 MB/s | 30 MB/s |
| Sequential Read | 10 MB/s | 30 MB/s |
| Random Write IOPS | 500 | 1500 |
| Random Read IOPS | 1500 | 3000 |
As you can see, UHS Speed Class 3 requires triple the sequential read/write speeds and double the random read/write speeds compared to Class 10 cards. So there is a significant performance gap between the two classes.
Real-world performance difference
In benchmark tests, the real-world difference between Class 10 and U3 cards is clear:
- A typical Class 10 card will have sequential read speeds of 20-30 MB/s and write speeds of 10-20 MB/s.
- A U3 card will have sequential read/write speeds in the range of 70-90 MB/s, over 3 times faster.
- For random access, U3 cards score in the 1500-2000 IOPS range, compared to 500-800 IOPS for Class 10.
Cards with UHS Speed Class 3 deliver noticeably faster load times when reading large files like 8K video footage. Their higher write speeds also result in much shorter save times when recording burst photos or high bitrate 4K video. For professional media creation with high resolution cameras, the performance difference is crucial.
When is a UHS Speed Class 3 card beneficial?
A U3 rated card provides major performance benefits for these use cases:
- 4K and 8K video recording – The massive file sizes require constant writes at 20-30 MB/s minimum to avoid dropped frames and errors.
- High framerate video – Slow motion 120fps or 240fps video consumes space rapidly, needing peak write speeds.
- Burst photo shooting – Capturing 10+ high resolution images per second generates huge write workloads.
- RAW image capture – 40MB RAW photos are 5-10x larger than JPEGs, requiring fast writes.
For basic photo shooting, Class 10 cards are typically sufficient. But professional media creators need the extra speed of U3. Overall, UHS Speed Class 3 represents the cutting edge of SD card performance today.
Is there anything faster than UHS Speed Class 3?
At the moment, U3 is the highest SD card speed class available to consumers. However, there are a few bleeding edge technologies that exist above U3:
UHS Speed Class 3 Video (V30)
The V30 standard guarantees 30MB/s of minimum sustained write speed for video recording. It essentially builds on top of the U3 spec, ensuring reliable long-term video writes. V30 is geared specifically for 4K, 8K, and high framerate video capture.
UHS Speed Class 3 Video (V60/V90)
V60 and V90 standards take the sustained video write speeds even higher, to 60MB/s and 90MB/s respectively. These ensure stability when recording very high resolution or high framerate video at long durations. V60 and V90 cards are difficult to find but exist for professional 4K/8K videography.
SD Express
SD Express is a new SD 7.0 standard that introduces PCIe and NVMe interfaces for blazing fast sequential speeds up to 985MB/s. However, SD Express devices are not yet widely available to consumers.
In summary, while technologies like V90, V60, and SD Express push the limits of speed, UHS Speed Class 3 remains the fastest readily available class of SD cards today.
Do faster speed classes always deliver better real-world speeds?
While higher speed classes theoretically promise faster performance, real-world speeds depend on these factors:
- Card brand and model – Speed can vary significantly between brands and specific products.
- File sizes – Small files don’t fully utilize card potential. Large contiguous files push limits.
- Testing conditions – Benchmarks with different methods produce varied results.
- Hardware – The camera or card reader can limit max speeds.
Therefore, it’s not guaranteed that a U3 card will be 3 times as fast as a Class 10 card in all scenarios. But with all else being equal, the speed class ratings provide a reliable way to rank SD card performance tiers.
Are there fakes or mislabeled SD cards?
There is a risk of counterfeit or deliberately mislabeled memory cards from some disreputable sellers. Tests by consumer watchdog groups have found products labeled with incorrect speeds. For example:
- A card labeled as Class 10 only reaching Class 4 speeds
- A card labeled as U3 not meeting the 30MB/s sequential write minimum
The best way to avoid fakes is to buy from reputable, major brands like SanDisk, Samsung, Lexar, Sony, etc. Check reviews and look for signs of sketchy mislabeling on lesser known brands. Perform your own benchmark testing with tools like CrystalDiskMark to verify speeds.
Legitimate brands meeting their claimed specifications will show consistent results across benchmarking, so speed ratings do provide meaningful performance guidance in most cases.
How are SD card speed classes labeled?
Speed classes and ratings are labelled on the SD cards themselves. Typically, you will find the class icon, rating, and the minimum speeds guaranteed:
- Class 10 card – Labelled as C10 with 10MB minimum speeds
- UHS Speed Class 1 – Labelled as U1 with 10MB minimum speeds
- UHS Speed Class 3 – Labelled as U3 with 30MB minimum speeds
Cards with the V30, V60, and V90 video speed designations will also clearly indicate those ratings on the label. Be aware that counterfeit cards may mimic the labeling without meeting specifications.
How do I check the true speed class of an SD card?
There are a few ways to validate the true speed class and performance of an SD card:
- Inspect the labeling and packaging for the claimed speed class – This should indicate if it meets Class 10, U3, V30, etc.
- Run benchmark tests on the card using utilities like CrystalDiskMark or ATTO Disk Benchmark for Windows. Compare the sequential and random read/write results to the claimed minimum speeds. Legitimate cards will at least meet their ratings.
- Test the card in real-world conditions in your camera or device. Try recording high resolution 4K/8K video to check for dropped frames or errors. Shoot burst photos to see if performance matches expectations. Evaluating real-world functionality matters most.
Conclusion
While Class 10 remains a common standard, UHS Speed Class 3 represents the fastest current SD card class available to consumers. For professional media recording and content creation, U3 cards provide markedly faster peak and sustained performance, especially for 4K/8K video. When buying SD cards, U3 is the premier speed class for the highest real-world speeds.