SD cards have speed classes that indicate their minimum guaranteed speeds. The speed class gives you an idea of the minimum write and read speeds that the card is capable of. Faster cards allow you to record high-resolution videos and transfer large files quickly.
What are the different SD card speed classes?
There are several speed classes for SD cards defined by the SD Association:
- Class 2 – Minimum write speed of 2 MB/s
- Class 4 – Minimum write speed of 4 MB/s
- Class 6 – Minimum write speed of 6 MB/s
- Class 10 – Minimum write speed of 10 MB/s
- UHS Speed Class 1 – Minimum write speed of 10 MB/s
- UHS Speed Class 3 – Minimum write speed of 30 MB/s
- UHS Speed Class 1 – Minimum write speed of 60 MB/s
- UHS Speed Class 2 – Minimum write speed of 120 MB/s
- UHS Speed Class 3 – Minimum write speed of 150 MB/s
The regular speed classes (Class 2, 4, 6, 10) refer to the minimum write speeds, while UHS speed classes refer to minimum read speeds. UHS (Ultra High Speed) cards are designed for high-resolution photos and full HD video recording.
What are the read and write speeds for each class?
Here are the typical read and write speeds for each SD card speed class:
| Speed Class | Minimum Write Speed | Typical Write Speed | Minimum Read Speed | Typical Read Speed |
|---|---|---|---|---|
| Class 2 | 2 MB/s | 2-5 MB/s | – | 10-20 MB/s |
| Class 4 | 4 MB/s | 5-10 MB/s | – | 15-30 MB/s |
| Class 6 | 6 MB/s | 10-20 MB/s | – | 20-40 MB/s |
| Class 10 | 10 MB/s | 10-30 MB/s | – | 20-60 MB/s |
| UHS Speed Class 1 | 10 MB/s | 10-30 MB/s | 10 MB/s | 40-100 MB/s |
| UHS Speed Class 3 | – | – | 30 MB/s | 60-200 MB/s |
| UHS Speed Class 1 | – | – | 60 MB/s | 90-250 MB/s |
| UHS Speed Class 2 | – | – | 120 MB/s | 120-312 MB/s |
| UHS Speed Class 3 | – | – | 150 MB/s | 150-400 MB/s |
As you can see, the guaranteed minimum write speed is lower than the typical write speed for each class. And UHS cards have much faster read speeds compared to regular SD card classes.
Which speed class is best for what use?
The speed class you need depends on how you intend to use the SD card. Here are some recommendations:
- Class 2 or 4 – Occasional photos with point-and-shoot cameras
- Class 6 or 10 – Standard point-and-shoot cameras, some HD video recording
- UHS Class 1 – Full HD video recording
- UHS Class 3 – 4K video recording, burst photo shooting
- UHS Class 1/2/3 – Professional 4K/8K video, high-speed burst shooting
So if you just need an SD card for casual photos, Class 10 is sufficient. But for professional needs like shooting 4K video or fast action photography, you’ll need UHS Class 3 cards to handle the high data bandwidth.
How does card capacity affect speed?
An SD card’s speed rating applies to its theoretical maximum capability. But its real-world speed is also affected by:
- The camera or device it’s used with
- File size – Large files like 4K video require higher sustained speeds
- Capacity – Cards with higher capacity tend to have faster speeds
A Class 10 SD card may not achieve its rated speeds in an older camera with a slower interface. And when recording large files like 4K video, a high-capacity UHS-3 card is recommended to maintain the required write speed.
In general, higher capacity cards are optimized for better performance. A 64GB or 128GB card often has faster speeds than a lower capacity card of the same rating.
Do read and write speeds matter for real-world use?
Read and write speeds certainly make a difference when recording and transferring large files. Some typical scenarios:
- Full HD video – Needs minimum 10 MB/s write speed for smooth recording
- 4K video – Requires minimum 30 MB/s sustained write speed
- 8K video – Needs blazing fast 150 MB/s+ write speeds
- Burst mode photos – Fast read speed allows quick saving of burst image sequences
- Transferring files – Faster read/write cuts down file transfer times
So clearly, having higher sequential read and write speeds enables handling of more data-intensive tasks. However, for basic uses like taking casual photos, Class 10 cards are adequate. Pay for faster UHS-rated cards only if you need to frequently record high-res 4K or 8K video.
Does the SD interface matter?
SD cards use different interfaces to transfer data between card and device:
- SD (default speed up to 25 MB/s)
- SD High Capacity (SDHC) (up to 104 MB/s)
- SD Extended Capacity (SDXC) (up to 312 MB/s)
- UHS-I (up to 104 MB/s)
- UHS-II (up to 312 MB/s)
- UHS-III (up to 624 MB/s)
Newer interfaces allow higher maximum speeds. But you are limited by the device’s interface – a UHS-III card will still operate at UHS-I speeds in a camera with only UHS-I support. Check your device’s SD card interface for maximum interface speed.
How are SD card speeds measured?
SD card read/write speeds are tested under standardized test conditions using benchmark software. But real-world speeds depend on several factors:
- File size – Larger files allow cards to reach higher sustained speeds.
- Device interface – The connecting interface limits maximum possible speeds.
- Fragmented writes – Non-contiguous writes are slower than sequential writes.
- Read/write mix – Actual speeds vary between pure read, pure write or a mixed workload.
- Testing tools – Speed test software uses different methodologies that can affect results.
So benchmark numbers do not always equal real-world performance. Cards may perform very differently in cameras due to the camera’s interface, system architecture, how it handles files, etc.
Are expensive cards faster than cheaper models?
In general, expensive high-end cards boast better performance than cheaper models of the same speed class. This is because:
- They use higher grade NAND flash memory chips
- Have advanced firmware for error correction and wear leveling
- Enable high sustained write speeds across the full card capacity
- Allow you to record at the highest possible bitrates
So pro cards from brands like SanDisk Extreme Pro, Sony Tough and Lexar Professional outperform cheaper options. But you pay a significant price premium for that extra performance.
Are there fake or counterfeit SD cards?
There are definitely fake memory cards being sold by shady sellers, especially on marketplaces like eBay and Amazon. Some signs you may have a counterfeit card:
- No branding or fake branding printed on the card
- Much lower capacity than claimed
- Slower read/write speeds than expected
- Errors like corrupted files or unreadable sectors
- Cheap price that seems too good to be true
Always purchase from reputable retailers and be wary of products from unknown Chinese brands. Run software like F3X or h2testw to verify if the full claimed capacity is available on the card.
How can I test the speed of an SD card?
You can test the real-world read/write speeds of an SD card using benchmarking software on your computer. Some popular tools include:
- BlackMagic Disk Speed Test – Provides read/write speeds as well as verified capacity.
- CrystalDiskMark – Provides sequential and random read/write speeds for different file sizes.
- IOmeter – Tests sustained read/write performance under configurable workloads.
- SD Card Test – Utility to test SD card speeds within Mac OS environment.
Run these benchmarks on your system with the SD card inserted to get accurate measurements. Compare speeds of different cards to select the right one for your needs.
How do I improve SD card speed and performance?
Some tips to boost SD card speeds and performance:
- Get a card with faster speed class rating for your use-case
- Choose reputable brands like SanDisk, Samsung for reliable performance
- Update device firmware and drivers to latest available version
- Use a USB 3.1 Gen 2 or Thunderbolt 3 reader for max transfer speeds
- Reformat the card in the device before first use
- Use a high-quality card reader that matches the card interface
- Avoid low-quality no-name card readers
- Don’t use the card until it is completely full
Following these tips will give you the fastest, most stable speeds from your shiny new high-speed SD card!
Conclusion
SD cards offer a wide range of speed capabilities to match your particular needs. The speed class gives a minimum speed guarantee, while real-world performance depends on the camera/device used. For full HD and 4K video recording, UHS-3 cards deliver blazing fast speeds. But lesser speed grades like Class 10 are fine for casual photography. Carefully research card speeds and choose a reputable brand to get a card that performs reliably in your device.