SD and microSD Card Markings Explained
A quick reference for decoding the symbols and labels printed on SD and microSD cards.
Card size
SD: the full-size card (32 × 24 mm), used in cameras and card readers.
microSD (often written "SD Micro"): the small version used in phones, drones, dashcams, action cameras and handheld consoles such as the Switch. An adapter lets it fit a full-size SD slot.
Capacity type
SD: up to 2 GB
SDHC (High Capacity): over 2 GB up to 32 GB
SDXC (Extended Capacity): over 32 GB up to 2 TB
SDUC: over 2 TB, up to 128 TB (rare)
Older devices may only support SD or SDHC, so check device compatibility before buying a large card.
Speed markings
Speed Class (C in a circle, e.g. C10): the minimum sustained write speed. C10 means 10 MB/s.
UHS Speed Class (U in a bucket shape): U1 means 10 MB/s minimum, U3 means 30 MB/s minimum. U3 is generally needed for 4K video.
Video Speed Class (V6, V10, V30, V60, V90): the minimum write speed in MB/s. V30 is a common choice for 4K, while V60 and V90 suit high-bitrate or 8K video.
UHS bus (Roman numerals I, II, III): the interface type. UHS-I tops out around 104 MB/s. UHS-II (a second row of pins) reaches about 312 MB/s.
A1 / A2 (App Performance): for microSD cards used to run apps or games on Android phones and handhelds. Both guarantee minimum random read/write performance, and A2 is faster.
Large MB/s figures (e.g. "170MB/s"): these are maximum read speeds. Write speed is usually lower, and is what the classes above actually guarantee.
Detailed specifications
Physical size
Format Dimensions (mm) Pins
SD 32 × 24 × 2.1 9 (UHS-II adds a second row, 17 total)
microSD 15 × 11 × 1.0 8 (UHS-II adds a second row, 16 total)
Capacity and file system
Type/Capacity /Default file system
SD up to 2 GB FAT12 / FAT16
SDHC 2 GB to 32 GB FAT32
SDXC 32 GB to 2 TB exFAT
SDUC 2 TB to 128 TB exFAT
A card reformatted to a different file system may not work in a device that expects the default. Many cameras and consoles require exFAT for SDXC.
Speed class minimum sustained write speeds
C2 - 2 MB/s
C4 - 4 MB/s
C6 - 6 MB/s
C10 - 10 MB/s
U1 - 10 MB/s
U3 - 30 MB/s
V6 - 6 MB/s/
V10 - 10 MB/s
V30 - 30 MB/s
V60 - 60 MB/s
V90 - 90 MB/s
The UHS (U) and Video (V) classes only apply to cards using the UHS bus.
Bus interface maximum theoretical speeds
Bus - Maximum speed Notes
Default Speed - 12.5 MB/s Original SD
High Speed - 25 MB/s Basic SDHC
UHS-I - 104 MB/s 1.8 V signalling
UHS-II - 312 MB/s Second pin row, full-duplex
UHS-III - 624 MB/s Rare in practice
SD Express - 985 MB/s (SD 7.x) PCIe / NVMe interface, needs a compatible host
SD Express - 3,940 MB/s (SD 8.0) PCIe / NVMe interface, needs a compatible host
Real-world speeds are lower than these maximums. UHS-II and SD Express cards fall back to UHS-I speeds in a device that doesn't support them.
Application Performance classes (microSD)
Class - Random read / Random write / Sustained write
A1 - 1,500 IOPS / 500 IOPS / 10 MB/s
A2 - 4,000 IOPS / 2,000 IOPS / 10 MB/s
A2 performance depends on host support for command queuing, so an A2 card in an older phone may perform like A1.
Video recording requirements
Recording - Typical bitrate / Minimum class
1080p at 30 fps - 20–50 Mbps (about 3–6 MB/s) C10 / U1
4K at 30 fps - 100 Mbps (about 12.5 MB/s) U3 / V30
4K at 60 fps, 10-bit - 200–400 Mbps (25–50 MB/s) V60
6K / 8K or RAW - 600 Mbps and up (75+ MB/s) V90 (UHS-II)
Check your camera's manual, as required bitrate varies by model and recording mode.
Voltage and power
Operating voltage is 2.7–3.6 V (3.3 V nominal).
UHS cards switch to 1.8 V signalling when the host supports it.
UHS-II and SD Express cards draw more power and run warmer, which matters in small devices such as drones and action cameras.