SD UHS-I vs UHS-II: Speed Differences and Camera Compatibility
SD cards come in two bus speed standards that affect both how fast they write and whether your camera can use their full speed. The difference matters most for 4K video recording.
The Physical Difference
UHS-II cards have an extra row of electrical contacts on the back. A UHS-II card fits in a UHS-I slot and will work at UHS-I speeds. A UHS-I card fits in a UHS-II slot and will also work, again at UHS-I speeds. The physical form factor is the same — both are standard-size SD cards.
This backward compatibility means buying a UHS-II card is never wrong, even if your camera only has a UHS-I slot. You just will not get the extra speed.
Speed Comparison
| Standard | Theoretical max | Real-world sustained write | |---|---|---| | UHS-I | 104 MB/s | 70–95 MB/s (top cards) | | UHS-II | 312 MB/s | 200–280 MB/s (top cards) |
In practice, UHS-I tops out around 90 MB/s sustained on the best consumer cards. UHS-II cards from Sony, Lexar, Delkin, and Angelbird reach 250–280 MB/s sustained.
Video Speed Class on SD Cards
Both UHS-I and UHS-II cards carry a Video Speed Class rating (V30, V60, V90). The class rating tells you the guaranteed minimum sustained write speed regardless of bus speed.
Most UHS-I cards are rated V30. V60 and V90 ratings require the higher throughput of UHS-II — the physics of the UHS-I bus cap prevents a true V90 UHS-I card.
This means: if your camera requires V60 for a recording mode and your SD slot is UHS-II, you need a UHS-II V60 card. A UHS-I card, even a fast one, cannot guarantee 60 MB/s sustained — the V30 UHS-I maximum is 30 MB/s guaranteed, though many cards exceed this in practice.
When UHS-II Matters for Your Camera
UHS-II is required or strongly recommended when:
- Your camera has a dedicated UHS-II slot and you want to use high-bitrate 4K modes (Canon EOS R3, R5 Mark II, Nikon Z8 SD slot, Sony FX3 SD slot at higher bitrates, Fujifilm X-T5, X-H2S)
- You are shooting All-Intra 4K (Panasonic S5 IIX, Lumix G9 II)
- You want 4K 60fps on bodies with a UHS-II SD slot
UHS-I is sufficient when:
- Your camera only has a UHS-I slot (older mirrorless bodies, all action cameras with SD slots, budget mirrorless)
- You are recording 4K 30fps at standard bitrates (under 100 Mbps) on a camera that accepts UHS-I for that mode
- You are using the secondary SD slot on a dual-slot body for overflow or lower-bitrate recording
microSD Adapter Note
MicroSD cards (common in action cameras and smartphones) can fit into a full-size SD slot via an adapter. The card's own speed class still applies — a microSD V30 in an adapter still delivers V30 performance in a full-size SD slot. No adapter exists to make a full-size SD card fit a microSD-only slot.
Checking Your Camera
Every camera's memory card requirement is on its official specification page. When CompatKit evaluates an SD card against a camera body, we look up the camera's per-slot, per-mode requirement directly from that page — not from assumptions about the camera's category.