A complete reference for SD card types, speed classes, bus interfaces, and performance ratings.
Secure Digital (SD) cards are widely used in cameras, phones, embedded systems, and portable electronics. Their performance depends on three major factors:
- Capacity class (SD / SDHC / SDXC / SDUC)
- Speed class (C, U, V, E)
- Bus interface (Standard, High‑Speed, UHS‑I/II/III, Express)
This guide explains each classification and how to choose the right card for your device.
Size
Storage Capacity Classes SD Card Capacity Classes
| Type | Capacity Range |
|---|---|
| SD (Secure Digital) | Up to 2 GB |
| SDHC (High Capacity) | >2 GB to 32 GB |
| SDXC (Extended Capacity) | >32 GB to 2 TB |
| SDUC (Ultra Capacity) | >2 TB to 128 TB |
Compatibility rule:
- Devices that support SDXC also support SDHC and SD.
- Devices that support SDUC support all previous types.
- Older devices may NOT support SDXC or SDUC.
Write speed ratings
Bus Speed Ratings (Data Transfer Speeds)
| Bus Type | Max Speed | Notes |
|---|---|---|
| Default Speed | 12.5 MB/s | Legacy |
| High‑Speed | 25 MB/s | Widely supported |
| UHS‑I | 50 MB/s or 104 MB/s | Single‑row contacts |
| UHS‑II | 156 MB/s (full‑duplex) / 312 MB/s (half‑duplex) | Second row of pins |
| UHS‑III | 312 MB/s (full‑duplex) / 624 MB/s (half‑duplex) | Rare |
| SD Express | 985 MB/s+ | PCIe/NVMe‑based |
Important: Bus speed ≠ real‑world write speed. Bus speed is the maximum interface capability, not guaranteed performance.
Speed Class Ratings (C, U, V, E)
Original Speed Class (C)
| Class | Minimum Write Speed |
|---|---|
| C2 | 2 MB/s |
| C4 | 4 MB/s |
| C6 | 6 MB/s |
| C10 | 10 MB/s |
UHS Speed Class (U)
| Class | Minimum Write Speed |
|---|---|
| U1 | 10 MB/s |
| U3 | 30 MB/s |
Video Speed Class (V)
| Class | Minimum Write Speed |
|---|---|
| V6 | 6 MB/s |
| V10 | 10 MB/s |
| V30 | 30 MB/s |
| V60 | 60 MB/s |
| V90 | 90 MB/s |
Express Speed Class (E)
| Class | Minimum Write Speed |
|---|---|
| E150 | 150 MB/s |
| E300 | 300 MB/s |
| E450 | 450 MB/s |
| E600 | 600 MB/s |
These are used for high‑end cameras, 4K/8K video, and SD Express devices.
Application Performance Class rating
Designates the minimum number of Input/Output Operations per Second (IOPS)
| Class | Read IOPS | Write IOPS |
|---|---|---|
| A1 | 1,500 | 500 |
| A2 | 4,000 | 2,000 |
These ratings matter for:
- Smartphones
- Tablets
- Single‑board computers (Raspberry Pi)
- App storage and random access workloads
A2 cards require host support to reach full performance.
Read speed rating
The read speed rating of an SD card is usually indicated by the number and data unit, i.e. MB/s, that is printed on the front of the card (e.g., “100 MB/s”).
Important:
- Read speed is usually higher than write speed.
- Manufacturers advertise read speed because it looks better.
- Write speed is what matters for video recording.
Choosing the Right SD Card
For photography
- U3 / V30 minimum
- UHS‑I or UHS‑II depending on camera
For 4K video
- V30 or V60
- Avoid C10/U1 cards
For 8K video
- V60 or V90
- Prefer UHS‑II or SD Express
For smartphones / app storage
- A2 recommended
For Raspberry Pi
- A1 or A2
- Avoid cheap unbranded cards
Leave a Reply