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SD speed class marks state a card’s minimum sequential write speed under conditions defined by the SD Association. They are not a peak-speed claim. They matter most when a device must keep writing without interruption, as in video recording. The two ratings people most often compare, U3 and V30, both guarantee 30 MB/s. They come from different class families, though, and apply in different bus modes.
The four speed class families
The SD Association defines four kinds of speed class: Speed Class (C), UHS Speed Class (U), Video Speed Class (V) and SD Express Speed Class (E). Each mark is a minimum sequential write rate. The official standards are on the SD Association speed-class standards page and the Speed Class overview.
| Marking | Minimum sequential write speed |
|---|---|
| C2 | 2 MB/s |
| C4 | 4 MB/s |
| C6 / V6 | 6 MB/s |
| C10 / U1 / V10 | 10 MB/s |
| U3 / V30 | 30 MB/s |
| V60 | 60 MB/s |
| V90 | 90 MB/s |
| E150 / E300 / E450 / E600 | 150 / 300 / 450 / 600 MB/s respectively |
These are guaranteed floors, not maximum or typical file-transfer speeds. A card marked V30 may read and write much faster than 30 MB/s in bursts. The mark only promises that sustained sequential writes will not fall below that level under the standard’s test conditions.
Why different symbols share the same number
C10, U1 and V10 all mean 10 MB/s. U3 and V30 both mean 30 MB/s. The families differ in the bus mode in which the mark applies:
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#1 Best Overall
- CAPTURE LARGER THAN LIFE. Unlock 4K UHD(3) brilliance and pristine high res stills with video speed class ratings of U3 and V30(4).
- SPEED BARRIERS SHATTERED. Save precious moments with blazing read speeds up to 250MB/s(2) and write speeds up to 170MB/s(2) [256GB-1TB capacities(1)].
- MAXIMIZE WITH MASSIVE CAPACITY. Capture for longer and store more with up to 2TB(1) of storage that can hold up to 2,808 minutes of 4K UHD video recorded at 30 fps (641MB/minute)(9).
- DEFY THE ELEMENTS. Unrelentingly resilient, Sandisk SD memory cards are engineered to perform in extreme conditions, despite rough handling and constant use.(6)
- CONTENT MANAGEMENT, SIMPLIFIED. Back up, organize, and transfer everything easily with the Sandisk Memory Zone desktop app,(7) whether you use an SD card slot or a card reader.
- C10 is used in High Speed mode or faster.
- U1 and U3 are used in SDR50/DDR50 mode or faster.
- V60 and V90 are used in UHS-II mode or faster.
- E-classes are used in SD Express mode.
So the symbols should not be treated as interchangeable in every device. Check which marks your device asks for.
What speed class does not tell you
Bus speed (UHS-I, UHS-II)
Speed class describes the card’s minimum sequential performance. The bus mode describes the connection between card and host, and it can limit the speed you actually get. The host must support the mode, and the SD Association notes the bus must be fast enough not to affect memory write speed (Speed Class overview). A V90 card gains nothing in a device that cannot use UHS-II.
Rank #2
- CAPTURE LARGER THAN LIFE. Unlock 4K UHD(3) brilliance and pristine high res stills with video speed class ratings of U3 and V30(4).
- SPEED BARRIERS SHATTERED. Save precious moments with blazing read speeds up to 250MB/s(2) and write speeds up to 170MB/s(2) [256GB-1TB capacities(1)].
- MAXIMIZE WITH MASSIVE CAPACITY. Capture for longer and store more with up to 2TB(1) of storage that can hold up to 2,808 minutes of 4K UHD video recorded at 30 fps (641MB/minute)(9).
- DEFY THE ELEMENTS. Unrelentingly resilient, Sandisk SD memory cards are engineered to perform in extreme conditions, despite rough handling and constant use.(6)
- CONTENT MANAGEMENT, SIMPLIFIED. Back up, organize, and transfer everything easily with the Sandisk Memory Zone desktop app,(7) whether you use an SD card slot or a card reader.
Capacity family
Capacity is a separate property. The SD Association lists these families (source):
| Family | Capacity |
|---|---|
| SD | Up to 2 GB |
| SDHC | Over 2 GB to 32 GB |
| SDXC | Over 32 GB to 2 TB |
| SDUC | Over 2 TB to 128 TB |
Application Performance Class (A1, A2)
A1 and A2 are for cards that run applications and hold application data, such as in smartphones. They cover random as well as sequential performance, so they answer a different question from V30 or U3. See the SD Association’s pages on Application Performance Class and running smartphone apps.
Rank #3
- CAPTURE LARGER THAN LIFE. Unlock 4K UHD(3) brilliance and pristine high res stills with video speed class ratings of U3 and V30(4).
- SPEED BARRIERS SHATTERED. Save precious moments with blazing read speeds up to 250MB/s(2) and write speeds up to 170MB/s(2) [256GB-1TB capacities(1)].
- MAXIMIZE WITH MASSIVE CAPACITY. Capture for longer and store more with up to 2TB(1) of storage that can hold up to 2,808 minutes of 4K UHD video recorded at 30 fps (641MB/minute)(9).
- DEFY THE ELEMENTS. Unrelentingly resilient, Sandisk SD memory cards are engineered to perform in extreme conditions, despite rough handling and constant use.(6)
- CONTENT MANAGEMENT, SIMPLIFIED. Back up, organize, and transfer everything easily with the Sandisk Memory Zone desktop app,(7) whether you use an SD card slot or a card reader.
Choosing a class for video recording
The SD Association FAQ gives general examples: C10, U1 or V10 for HD to Full HD recording, and U3 or V30 for Full HD to 4K. These are guidelines, not a guarantee for every device. Required speed varies with the camera’s bit rate, recording mode, interface and the video format itself. The camera maker’s stated requirement takes priority.
- Find the card requirement in the device manual for your recording mode (for example 4K at a given frame rate or bit rate).
- Confirm which interface the device supports, such as UHS-I, UHS-II or SD Express.
- Confirm the card format it accepts (SDHC, SDXC and so on).
- Choose a card whose class meets or exceeds the requirement. Higher V-classes only help if the host supports the faster bus.
In the Association’s words, the speed class standards “guarantee minimum writing speeds and deliver the best performance” for both cards and devices.
Quick Recap
Best Value
- Save time with card offload speeds of up to 200MB/s powered by SanDisk QuickFlow Technology (Up to 200MB/s read speeds, engineered with proprietary technology to reach speeds beyond UHS-I 104MB/s, require compatible devices capable of reaching such speeds. Based on internal testing; performance may be lower depending upon host device, interface, usage conditions and other factors. 1MB=1,000,000 bytes. X = 150KB/sec. SanDisk QuickFlow Technology is only available for 64GB, 128GB, 256GB, 512GB and 1TB capacities. 1GB=1,000,000,000 bytes. 1TB=1,000,000,000,000 bytes. Actual user storage less.)
- Pair with the SanDisk Professional PRO-READER SD and microSD to achieve maximum speeds (sold separately)
- Shot speeds up to 90MB/s (Write speed up to 90MB/s. Based on internal testing; performance may be lower depending upon host device. 1MB=1,000,000 bytes. X = 150KB/sec.)
- Perfect for shooting 4K UHD video and sequential burst mode photography (Full HD (1920x1080) and 4K UHD (3840 x 2160) video support may vary based upon host device, file attributes and other factors. See HD page on SanDisk site.)
- UHS Speed Class 3 (U3) and Video Speed Class 30 (V30) (UHS Speed Class 3 designates a performance option designed to support 4K UHD video recording with enabled UHS host devices. UHS Video Speed Class 30 (V30), sustained video capture rate of 30MB/s, designates a performance option designed to support real-time video recording with UHS enabled host devices. See the SD Association’s official website.)
Rank #4
- Great choice for compact to mid-range point-and-shoot cameras
- Quick transfer speeds up to 140MB/s (Up to 140MB/s read speed engineered with proprietary technology to reach speeds beyond UHS-I 104MB/s, requires compatible devices capable of reaching such speeds. Based on internal testing; performance may be lower depending on host device, interface, usage conditions and other factors. 1MB=1,000,000 bytes.)
- Up to 128GB to store tons of pictures (1GB=1,000,000,000 bytes. Actual user storage less.)
- Exceptional video recording performance with UHS Speed Class 1 (U1) Class 10 rating for Full HD video (1080p) (UHS Speed Class 1 (U1) designates a performance option designed to support real time video recording with UHS enabled host devices. See consumers speed page on SanDisk site. Full HD (1920x1080) video support may vary based upon host device, file attributes, and other factors. Visit the SanDisk Video Knowledge Base for more information.)
- Compatible with SanDisk SD UHS-I card reader (sold separately)
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