What is an SD card?

By: The SSD Lab Team · Team

Published: Updated:

Short answer

An SD (Secure Digital) memory card is a removable flash-storage card whose electrical, mechanical and file-system behaviour is defined by the SD Association. Cards are used in cameras, phones, drones, handhelds and many other devices. The SDA describes three defined form factors historically — full-size SD, miniSD and microSD — with microSD having replaced miniSD in everyday use (SD Standard Overview).

This page is the “what is it” guide. Naming confusion between TF and microSD is covered in TF card vs microSD.

Form factors

Form factor Role Notes from the SDA
SD (full size) Cameras, laptops with full-size slots Write-protect switch present; 9 pins on High Speed / UHS-I (overview)
miniSD Older phones Largely superseded by microSD in the market
microSD Phones, action cams, handhelds Smaller; no write-protect switch; 8 pins on High Speed / UHS-I

The SDA FAQ states that miniSD / microSD differ from full-size SD in form factor, size, pin count and write-protect switch; the logic and electronics are identical (SDA FAQ).

Capacity families

Manufacturers ship cards under capacity-family logos. From the SDA consumer FAQ:

Always follow the device manual for the largest family and Speed Class it accepts (SDA FAQ — right size).

Speed Class (minimum write marks)

Flash write speed varies; video recording needs a constant minimum write rate. The SDA defines four kinds of marks — Speed Class, UHS Speed Class, Video Speed Class and SD Express Speed Class — each with a number that indicates minimum writing speed under the standard’s conditions (Speed Class page, FAQ).

Family Marks SDA notes
Speed Class C2, C4, C6, C10 C10 applies to High Speed bus products
UHS Speed Class U1, U3 Apply to UHS bus families (UHS-I / II / III)
Video Speed Class V6, V10, V30, V60, V90 V60 / V90 need UHS-II / UHS-III product family
SD Express Speed Class E150, E300, E450, E600 For SD Express bus; NVMe-based access per SDA

Example application mapping from the SDA FAQ (illustrative of what the Association associates with each mark, not a promise for every camera): Class 10 / U1 / V10 — HD to Full HD; U3 / V30 — Full HD to 4K; V60 — 4K UHD to 8K; V90 — 8K UHD (SDA FAQ — applications).

Match symbol families. The SDA warns that expected write speed is not available when host and card use different class symbols that happen to share the same MB/s number (e.g. host Class 10 with card U1) (Speed Class page). Choose the same or higher mark within the family the host documents.

Bus modes (UHS and SD Express)

UHS (Ultra High Speed) is the bus-interface family. The SDA FAQ lists classifications up to 624 MB/s: UHS-I (104 MB/s), UHS-II (324 MB/s), UHS-III (624 MB/s) on SDHC/SDXC cards and devices (FAQ — What is UHS?).

SD Express cards use PCIe with the NVMe protocol while keeping a UHS-I fallback path; the SDA describes them as removable SSDs with rates up to 3940 MB/s depending on generation (FAQ — What is SD Express?).

FAQ

What is a microSD card? The small form factor of the SD family — same standards family, smaller package (SDA FAQ).

How long does an SD card last? The SDA says lifespan depends on manufacturing and that current technology with normal usage typically gives 10 years or more; it does not publish a single TBW figure for all cards (SDA FAQ — service life).

Which brand does the SDA recommend? None — the Association creates standards and does not recommend manufacturers (SDA FAQ).