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SD card types and symbols explained

An SD card has a dozen tiny marks printed on it, and each one tells you something about its size or speed. I have drawn every card type and every mark below, so you can match them to the card in your hand.

SD, miniSD and microSD, to scale

An SD card, a miniSD card and a microSD card drawn in 3D at the same scaleSD: 32 x 24 x 2.1 mmminiSD: 21.5 x 20 x 1.4 mmmicroSD: 15 x 11 x 1 mm

Parts of an SD card

The front and back of a full-size SD card: the cut corner, the lock switch on the side and nine gold contacts on the backCut corner shows which way inLock switch9 gold contacts on the backSlide down to make it read-onlyGrip ridges

The lock switch only asks your device to treat the card as read-only, so some readers ignore it.

microSD and the SD adapter

A microSD card from the front and back, twice its real size, with eight contacts, and a microSD sliding into an SD adapter at real sizeA step on one side: fits one way8 contactsSlides into an SD adapterThe adapter's own 9 contacts fit any SD slot
The two cards on the left are shown at twice their real size.

SD vs SDHC vs SDXC vs SDUC

LogoCapacityFile system
SDUp to 2 GBFAT12, FAT16
SDHC2 GB to 32 GBFAT32
SDXC32 GB to 2 TBexFAT
SDUC2 TB to 128 TBexFAT

A device that reads SDXC also reads SDHC and SD, but an older SDHC-only camera cannot read an SDXC card.

What the symbols on an SD card mean

A full-size SD card with its marks labelled: SDXC, the UHS-II numeral, Speed Class 10, UHS Speed Class 3 and Video Speed Class 90, and a microSD Express card with the EX and A2 marksCapacity typeBus: UHS-IISpeed Class 10UHS Speed Class 3Video Speed Class 90SD ExpressApp Performance A2
The microSD card is shown at twice its real size.

The speed marks are the slowest a card is allowed to write, so the real speed is usually higher.

Speed Class

MarkMinimum write
Speed Class 22 MB/s
Speed Class 44 MB/s
Speed Class 66 MB/s
Speed Class 1010 MB/s

Video Speed Class

MarkMinimum write
Video Speed Class 66 MB/s
Video Speed Class 1010 MB/s
Video Speed Class 3030 MB/s
Video Speed Class 6060 MB/s
Video Speed Class 9090 MB/s

UHS Speed Class

MarkMinimum write
UHS Speed Class 110 MB/s
UHS Speed Class 330 MB/s

App Performance Class

MarkRandom IOPSMinimum write
App Performance Class 11,500 read, 500 write10 MB/s
App Performance Class 24,000 read, 2,000 write10 MB/s

Bus interface

MarkBusTop bus speed
UHS-IUHS-I104 MB/s
UHS-IIUHS-II312 MB/s
UHS-IIIUHS-III624 MB/s
SD ExpressSD Express985 to 3,940 MB/s

UHS-I vs UHS-II

The backs of two SD cards: a UHS-I card with one row of nine contacts and a UHS-II card with a second row, seventeen in allUHS-I: one row, 9 contactsUHS-II: a second row, 17 in all

The second row only helps in a UHS-II slot; anywhere else, a UHS-II card runs at UHS-I speed.

Which SD card should you buy?

Look forForWhy
App Performance Class 2Phone or tabletmicroSDXC with A2 runs apps from the card faster
Video Speed Class 304K videoV30 (or U3) keeps up with most 4K cameras
Video Speed Class 908K or high-bitrate 4KV60 or V90, in a UHS-II camera
SD ExpressNintendo Switch 2Only microSD Express cards can save games
UHS-INintendo SwitchA UHS-I microSDXC card

SD card sizes compared

FormatSizeWhere you see it
SD32 x 24 x 2.1 mmCameras, laptops, card readers
miniSD21.5 x 20 x 1.4 mmOld phones; replaced by microSD
microSD15 x 11 x 1 mmPhones, drones, action cams, Switch

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