SDHC vs SDXC: What Is the Difference?

SDHC (Secure Digital High Capacity) and SDXC (Secure Digital eXtended Capacity) are two of the capacity classes defined by the SD Association for SD and microSD cards. The class tells you how much the card can hold, which file system it is formatted with and which host devices can read it. It says nothing about speed: an SDHC card and an SDXC card can use exactly the same bus and reach the same transfer rates.

The classes exist because each step in capacity needed a new way of addressing the card. Original SD cards topped out at 2 GB. SDHC, introduced with version 2.0 of the SD specification in 2006, raised the ceiling to 32 GB. SDXC, from version 3.0 in 2009, raised it to 2 TB, and SDUC, from version 7.0 in 2018, allows up to 128 TB.

All four types share the same physical sizes, full-size SD and microSD, and the same contacts for the basic bus. What changes is the capacity range and the default file system.

The file system matters for embedded use. FAT32 limits single files to 4 GB, which is one reason video and logging devices moved to SDXC and exFAT. Some embedded operating systems and bootloaders still expect FAT32, so check before choosing a card above 32 GB.

SD compatibility runs one way. A host built for a newer class reads the older ones, so an SDXC slot accepts SDHC and standard SD cards. A host built only for SDHC cannot be relied on to read an SDXC card, even though the card fits the slot. Some older SDHC hosts will work once the card is reformatted to FAT32, but the SD Association does not guarantee it and the 4 GB file limit returns.

The same rules apply to microSD: microSDHC and microSDXC follow SDHC and SDXC exactly, and an adapter changes only the shape. When a datasheet lists an SDXC or microSDXC slot, it will also take SDHC cards.

Since the capacity class does not set speed, cards carry separate markings for the bus they use and the minimum speed they sustain.

Start from the slot, not the card. Check the device datasheet for the highest class it supports (SDHC, SDXC or SDUC), the bus it runs (High Speed, UHS-I or UHS-II) and whether its bootloader needs FAT32. A UHS-II card in a UHS-I slot simply runs at UHS-I speed, so there is no gain in paying for a faster bus than the host has.

Then size the card to the job. For recording, pick a video speed class at least equal to the stream’s bit rate in MB/s, with headroom. For an operating system, look for A1 or A2. For logging, compare the card’s endurance rating with the data written per day. Finally, buy from a traceable source: counterfeit cards that report a false capacity are common in consumer channels.

SD and microSD slots appear on many embedded devices. Many of our Arm SBCs have a microSD slot, often used to load or recover a system image, and most of our rugged tablets have a microSD slot for maps, photos and forms. Several rugged Jetson computers carry an SD slot beside their eMMC for removable data.

Consumer cards are a weak point in these systems. They are usually rated 0 to 70 °C or narrower, use TLC or QLC flash with modest endurance, and may lose data if power is cut mid-write. Industrial SD cards are built for −40 to 85 °C, use SLC or pSLC flash, add power-fail protection and health reporting, and keep a fixed bill of materials. For anything more than occasional transfers, specify an industrial card and keep constant logging on an SSD.

We fit industrial SD and microSD cards as standard where a system needs removable media, chosen for temperature range and endurance, and imaged and validated with the rest of the build.