Best SD Card for Your CPAP (2026 Guide)

The best SD card for a CPAP machine is a full-size 32GB SDHC card formatted FAT32. This applies to every major CPAP brand: ResMed, BMC, React Health Luna G3, and Lowenstein Prisma. Any name-brand card from SanDisk, Samsung, or Kingston in that size works. The card that ships with your machine is fine to keep using, but if you need a replacement or a reader to connect it to your computer, this guide covers exactly what to buy and what to avoid.

What Card Your Machine Uses

Every CPAP machine that CPAP Clarity supports uses a standard, full-size SD card. That's the one that's 32mm x 24mm, the same size that was in digital cameras before everything went mirrorless. Not micro SD, not mini SD. The full-size card.

Here's the breakdown by brand:

ResMed (AirSense 10, AirSense 11, AirCurve 10, AirCurve 11 in all variants): the slot is behind a small cover on the left side of the machine, on the end opposite the humidifier water tub, when you face the screen (no need to remove the water chamber). ResMed ships a 2GB card (AirSense 10) or 4GB card (AirSense 11). Both record to a DATALOG folder structure with EDF signal files.

BMC (E-20A, G3 A20): on the E-20A the slot is on the side opposite the humidifier, on the same panel as the air filter. The G3 A20 is different: its slot sits on the humidifier side, so you slide off the water chamber to reach it. BMC ships a small card (typically 2GB). The machine writes proprietary binary files in a BMC folder. For a deeper look at the BMC data format, see our BMC E-20A review or BMC G3 A20 review.

React Health Luna G3: the slot is on the back panel. The Luna G3 uses the same physical SD card type but writes its own binary format. Three different firmware versions exist with slightly different data layouts, but they all use the same card spec. See our Luna G3 review for details.

Lowenstein Prisma Smart: the slot is on the side of the machine. The Prisma writes JSON configuration, XML event files, and WMEDF signal files.

The card spec is universal across all of them: full-size SDHC, FAT32, 32GB maximum.

What Size to Get

Two constraints matter here: capacity and format.

Maximum size: 32GB. Cards larger than 32GB use the exFAT filesystem by default, and CPAP firmware across all brands cannot read exFAT. A 64GB card will not work. Stay at 32GB or under.

Required format: FAT32. This is the filesystem format, separate from the card's storage capacity. Cards 32GB and under ship from the factory pre-formatted as FAT32, so most people never need to think about this. More on formatting below.

The practical sweet spot: 8GB or above. Here's roughly how much data each machine writes per night:

• ResMed AirSense 11: approximately 2 to 5 MB per night
• ResMed AirSense 10: approximately 5 to 15 MB per night (stores more detailed waveform data)
• BMC E-20A / G3 A20: approximately 1 to 3 MB per night
• React Health Luna G3: approximately 1 to 4 MB per night
• Lowenstein Prisma Smart: approximately 2 to 6 MB per night

A 4GB card holds roughly 1 to 3 years of nightly data on any machine. An 8GB card doubles that. A 32GB card is essentially unlimited for practical purposes.

Do You Need a Manufacturer-Branded Card?

No. ResMed sells SD cards as branded accessories at a noticeable markup over generic equivalents. The ResMed-branded cards are standard SanDisk or Kingston SD cards repackaged with ResMed branding. The same applies to any other CPAP brand that sells branded media. The machine doesn't check who made the card.

A generic 32GB SD card from SanDisk, Kingston, or Samsung is widely available at standard SD-card pricing. It's the same hardware.

One thing worth knowing: you don't need to create any folders manually before inserting a new card. The machine creates its own directory structure the first time it runs with a new card. ResMed creates a DATALOG folder, BMC creates a BMC folder, and so on. Just format it FAT32, insert it, and use the machine as normal. After your first night of therapy, the folders will be there.

How to Format Your Card

Most cards 32GB and under are already formatted FAT32 when you buy them. You only need to format a card yourself if:

• You're reusing an old card that was formatted differently
• The machine is throwing errors reading the card
• You want to clear old data and start fresh

On Windows: Insert the card, open File Explorer, right-click the drive, choose Format. In the File System dropdown, select FAT32. Make sure "Quick Format" is checked and click Start.

On Mac: Open Disk Utility (Applications > Utilities > Disk Utility). Select the SD card in the left sidebar. Click Erase. In the Format dropdown, choose MS-DOS (FAT). Give it a name (anything works) and click Erase.

SD Association Formatter (any platform): The SD Association's official SD Memory Card Formatter (opens in new tab) is a free tool designed specifically for formatting SD cards. It handles edge cases (like cards with non-standard partition tables) that the built-in OS formatters sometimes struggle with. If Windows or Mac formatting doesn't fix a stubborn card, try this tool before giving up on the card.

One rule: don't manually create folders. The machine expects to create its own folder structure. If you create a DATALOG or BMC folder yourself, you may cause a conflict. Start with an empty, formatted card.

What Does NOT Work

A few things people try that don't work:

Cards larger than 32GB. The exFAT format these cards use is not compatible with any CPAP firmware we've tested. A 64GB or 128GB card will not be recognized by the machine.

Micro SD with an adapter. Physically, a micro SD card with a full-size adapter can fit in the slot. The adapter introduces an extra contact point between the micro card and the machine's slot that a native full-size SDHC card does not have. ResMed specifies a standard full-size SD card in their user guide, not a micro-with-adapter. An adapter can work, but native full-size cards are inexpensive enough that there's no reason to risk intermittent contact issues.

WiFi SD cards. Wireless SD cards (Eye-Fi, Toshiba FlashAir, and similar products) had built-in WiFi chips and appeared to be regular SD cards. Most of these product lines have been discontinued, but some are still available secondhand. They are not compatible with CPAP firmware. The machine either won't recognize them or will write corrupted data.

Our Top Picks: SD Cards

All three cards below are native full-size SDHC cards that fit directly into your CPAP's SD slot. No adapters needed. Any of them will hold years of nightly therapy data.

Full-size 32GB SDHC cards for CPAP machines

Editor's Pick

SanDisk Ultra 32GB SDHC UHS-I

The most widely available full-size 32GB SDHC card. 100MB/s read speed, UHS-I U1. Ships pre-formatted FAT32.

Compare on Amazon→

Prime Pick

Gigastone 32GB SDHC UHS-I

Full-size 32GB SDHC alternative with Prime shipping. UHS-I A1 V10, single card. Reformat to FAT32 if your machine needs it.

Compare on Amazon→

Most Reviewed

SanDisk Ultra 32GB Class 10 SDHC UHS-I

A second SanDisk full-size 32GB SDHC model, the highest-volume listing in this class. Class 10 UHS-I, native full-size.

Compare on Amazon→

All three cards are faster than what CPAP data transfer requires. The machine writes a few megabytes per night; even the slowest UHS-I card transfers that in under a second. Pick whichever is available at the best price.

Our Top Picks: SD Card Readers

To import your CPAP data into CPAP Clarity (or any analysis software), you need to connect the card to your computer. If your laptop has a built-in SD card slot, use that. Otherwise, pick the reader that matches your ports.

SD card readers by connection type

Best for Modern Laptops

USB-C SD Card Reader

Plugs into modern MacBooks, iPads (USB-C), and most Android phones. Supports full-size SD and microSD.

Compare on Amazon→

Best for Older Laptops

USB-A SD Card Reader

Plugs into older laptops and desktops with USB-A ports. Same functionality, typically a touch cheaper.

Compare on Amazon→

Best for Older iPhones

Apple Lightning SD Card Reader

Apple's official Lightning to SD Card Camera Reader. For iPhones and iPads with Lightning ports (iPhone 14 and earlier).

Compare on Amazon→

Android phone via USB OTG. Android phones with USB OTG support can connect to a USB-C card reader and access the SD card directly. Combined with a mobile browser, you can import your data from your phone. CPAP Clarity's folder import works in Chrome on Android.

iPhone and iPad. Current iPhones (iPhone 15 and later) use USB-C, so a USB-C SD card reader connects directly. Older iPhones with Lightning ports need the Apple Lightning reader listed above. CPAP Clarity's folder import works in Safari on iOS 17 and later. For a walkthrough of importing from cloud storage instead of a physical card, see our cloud storage import guide.

Once You Have the Card

The workflow is simple. Insert the card into your machine before going to sleep. Use your machine as normal. In the morning, remove the card and insert it into your computer using whichever reader applies to you.

Open CPAP Clarity and select the whole SD card folder when prompted. The import reads the entire directory structure and all session files automatically. You don't need to pick individual files. This works the same way regardless of whether your machine is a ResMed, BMC, Luna G3, or Lowenstein.

For a full walkthrough of the import process, see the CPAP SD card data guide. If you're new to CPAP Clarity, How to Use CPAP Clarity is the place to start. Once your data is in, How to Read CPAP Data explains what all the numbers mean. If the card isn't working, see our SD card troubleshooting guide.

Common Questions

Do all CPAP brands use the same SD card?

Every brand that CPAP Clarity supports (ResMed, BMC, React Health Luna G3, and Lowenstein Prisma) uses the same physical card: a standard full-size SDHC card, formatted FAT32, 32GB maximum. The data formats written to the card are different between brands, but the card itself is interchangeable. A SanDisk 32GB SDHC that works in a ResMed will work in a BMC or Luna G3.

Can I use the same card in OSCAR and CPAP Clarity?

Yes. Both tools read the same files from the same SD card. You can import into CPAP Clarity, then plug the same card into OSCAR (or vice versa) without any issues. The files are not modified during import. If you're curious how CPAP Clarity compares to OSCAR, see our OSCAR alternative guide.

Does removing the card affect my therapy?

No. The SD card records data but plays no role in delivering therapy. Removing it (while the machine is off) does not change pressure, humidification, or any other setting. The machine will simply not record data on nights when the card isn't present.

Can I use a micro SD card with a full-size adapter?

We don't recommend it. The adapter adds a contact point between the micro SD card and the CPAP's SD slot that a native full-size card does not have. ResMed specifies a standard full-size SD card in their user guide. A native full-size 32GB SDHC card is inexpensive and avoids the risk of intermittent read errors from poor adapter contact. If you already have a micro SD with adapter and it's been working, there's no urgent reason to switch, but for a new purchase, go with a native full-size card.

What if my machine won't recognize the card?

First, make sure the card is 32GB or smaller. Cards above 32GB are formatted exFAT by default and won't work. Second, try reformatting it as FAT32 using the steps in the formatting section above. If the OS formatter doesn't help, try the SD Association's free formatting tool (opens in new tab). Third, check that the card is fully seated in the slot. If none of that works, the card itself may be faulty: try a different card. For more help, see our SD card troubleshooting guide.

How often should I check my data?

That depends on where you are in your therapy. In the first few weeks, checking weekly gives you a sense of how your body is adjusting. Once therapy is stable, monthly is plenty for most people. If you're troubleshooting a problem (high leak rate, poor sleep quality), daily check-ins are worthwhile until the issue is resolved.

What do I do if the card is full?

Two options. First, you can reformat the card (FAT32, as described above) to wipe it and start fresh. Second, you can replace it with a larger card, up to 32GB. If you want to keep your historical data, copy the entire card contents to your computer before reformatting or swapping cards.

Can I leave the card in the machine all the time?

Yes, and most people do. The card is only accessed when the machine is writing data at the end of a session. Leaving it in is fine. Just remember to remove it when you want to import your data.

Primary Sources

• ResMed AirSense 11 product page (opens in new tab): manufacturer specifications; the AirSense 11 user guide (document 403974/2) specifies standard SD memory card, FAT16 or FAT32 format, maximum 32GB capacity
• SD Association: SD/SDHC/SDXC/SDUC Capacity (opens in new tab): defines SDHC (SD High Capacity) as 2GB to 32GB with FAT32 filesystem
• SD Association: SD Memory Card Formatter (opens in new tab): official formatting tool for SD/SDHC/SDXC cards, recommended over OS-native formatters