SD Speed Classes
Memory Cards · 6 specs
Direct Answer
SD Speed Classes are standardized performance metrics defined by the SD Association to guarantee minimum sustained write speeds for uninterrupted video recording and random IOPS execution for applications. They encompass legacy Speed Class (C2–C10), UHS Speed Class (U1/U3), Video Speed Class (V6–V90), and Application Performance Class (A1/A2).
Source: SD Association (SDA) Speed Class Specification · Last reviewed Aug 28, 2026
Specs
| Speed Class (Legacy C-Class) | Class 2 (2 MB/s), Class 4 (4 MB/s), Class 6 (6 MB/s), Class 10 (10 MB/s) |
| UHS Speed Class (U-Class) | U1 (10 MB/s min sustained write), U3 (30 MB/s min sustained write) |
| Video Speed Class (V-Class) | V6 (6 MB/s), V10 (10 MB/s), V30 (30 MB/s), V60 (60 MB/s), V90 (90 MB/s) |
| App Performance Class A1 | 10 MB/s sequential, Min 1,500 Read IOPS, Min 500 Write IOPS |
| App Performance Class A2 | 10 MB/s sequential, Min 4,000 Read IOPS, Min 2,000 Write IOPS (requires Command Queue & Host Cache) |
| Guaranteed Metric | Minimum sustained sequential write speed on fragmented NAND (never peak burst speed) |
Sequential Write Classes for Video Capture
To prevent dropped frames during continuous video capture, the SDA introduced guaranteed minimum sequential write metrics across three historical branding waves:
- Speed Class (C2, C4, C6, C10): Guarantees 2, 4, 6, or 10 MB/s minimum write speed (standard for 1080p Full HD).
- UHS Speed Class (U1, U3): U1 certifies 10 MB/s; U3 certifies 30 MB/s, required for baseline 4K video recording.
- Video Speed Class (V6 to V90): Designed for high-framerate 4K, 8K, 360-degree, and RAW video. V60 (60 MB/s) and V90 (90 MB/s) cards generally mandate dual-row UHS-II bus hardware.
Application Performance Class (A1 vs A2)
Traditional SD cards were optimized purely for large sequential video blocks, resulting in poor random access performance when running smartphone apps or operating systems. App Performance Classes mandate minimum Random Read/Write IOPS:
- A1: Requires 1,500 Read IOPS and 500 Write IOPS.
- A2: Requires 4,000 Read IOPS and 2,000 Write IOPS. A2 requires host device driver support for Command Queuing (CQ) and Host Memory Cache; in unsupported legacy readers, an A2 card operates with standard A1 performance.