The short answer: 128GB is the right default for most people, and 64GB is enough if you mainly shoot stills. A camera recording 4K at 100Mbps — which is the common high setting on a beginner mirrorless body — writes about 45GB per hour, so 128GB holds roughly 2.8 hours of it and 64GB holds about 1.4 hours. Step to 256GB only if your camera records a genuinely high-bitrate mode: at 360Mbps, which is the Fujifilm X-M5's published maximum, an hour of footage is about 162GB and even a 256GB card holds well under two hours. Everything below is derived from published bitrates, so you can run it for your own camera.
This page is about capacity. The other half of choosing a card is speed — whether the card can keep up with the write — and that is a different question with a different answer, worked through in SD card speed classes explained. A card can be fast enough and far too small, or huge and too slow. You need both right.
The one calculation
Camera makers quote video bitrates in megabits per second (Mbps). Storage is sold in gigabytes (GB). The conversion is two steps and you only have to do it once per camera:
- Divide the bitrate by 8 to get megabytes per second. 100Mbps becomes 12.5MB/s.
- Multiply by 3,600 for an hour, then divide by 1,000 for gigabytes. 12.5MB/s is 45,000MB per hour, which is about 45GB per hour.
A shortcut worth memorising: divide the Mbps figure by 2.2 and you get GB per hour, near enough. 100Mbps is about 45GB/hr; 60Mbps is about 27GB/hr; 360Mbps is about 162GB/hr. From there, hours on a card is just capacity divided by that number.
Hours per card, by real published bitrate
Every bitrate in the left column is a published specification for a camera this site covers, not an estimate. The hours are the arithmetic above, rounded down, and they are a touch optimistic in practice because a card is never completely empty and files are never perfectly packed.
| Camera mode | Bitrate | Per hour | 64GB | 128GB | 256GB |
|---|
| Sony ZV-E10, 4K at the 60Mbps setting | 60Mbps | ~27GB | ~2h 20m | ~4h 45m | ~9h 30m |
| Sony ZV-E10, 4K at the 100Mbps setting | 100Mbps | ~45GB | ~1h 25m | ~2h 50m | ~5h 40m |
| Fujifilm X-M5, DCI 4K | 100Mbps | ~45GB | ~1h 25m | ~2h 50m | ~5h 40m |
| Fujifilm X-M5, 6.2K open gate / 4K at maximum bitrate | 360Mbps | ~162GB | ~23m | ~47m | ~1h 35m |
Two things jump out of that table. First, the 60Mbps and 100Mbps settings on the same camera differ by nearly a factor of two in storage, so the card you need depends on a menu choice as much as on the camera. Second, high-bitrate modes change the answer completely: the jump from 100Mbps to 360Mbps turns a comfortable 128GB card into a 47-minute card.
How many photos fit?
Stills are far less demanding than video and the honest method is to measure rather than trust a chart, because file size varies with the scene — a detailed forest compresses worse than a plain sky, and the same camera will produce noticeably different file sizes for the two.
The thirty-second version: put the card in, shoot ten frames of something typical in the format you actually use, then look at the card's remaining-shots counter or divide the used space by ten. Your camera also does this for you — every body displays an estimated remaining frame count, and it recalculates as it learns what your files weigh.
The useful general shape, though, is this: raw files are several times larger than JPEGs, so shooting raw+JPEG roughly adds the two together, and bracketing or burst shooting multiplies everything. For a 24MP APS-C camera like the Sony a6400 or ZV-E10 — 6000 x 4000 pixels — a 64GB card is a lot of stills and nowhere near enough video. If you shoot both, size the card for the video.
Why two 128GB cards beat one 256GB card
This is the recommendation most buying guides skip, and it is the one that actually matters. A memory card is the only component in your kit that holds data existing nowhere else. Cards fail — rarely, but they do — and they also get lost, left in a card reader, or accidentally formatted. Every one of those events costs you everything on the card.
Splitting the same capacity across two cards halves the size of the worst day of your photographic life, and it gives you a spare when one fills mid-shoot. Two 128GB cards usually cost about the same as one 256GB card, and most beginner bodies have a single slot, so you cannot rely on in-camera backup. The discipline that goes with it: swap the card when it fills rather than deleting from it in camera, and offload to a computer or an external SSD before you reformat.
The one exception is a camera with two card slots set to simultaneous recording, where the second slot is a live backup. On those bodies, matching the two cards matters more than splitting them.
The size recommendation, by what you shoot
Stills only, beginner: 64GB, and buy two. You will not fill it in a day and the second card is cheap insurance.
Mixed stills and 4K video, which is most people: 128GB is the default, in pairs. At the 100Mbps settings most beginner cameras use, that is a shoot day with headroom.
Long-form video, events, or a camera with a high-bitrate mode:256GB, and check your camera's published maximum bitrate before you decide one is enough. At 360Mbps it is not.
Anything beyond that: the answer stops being a bigger card and becomes offloading in the field. A 512GB card is a large amount of unduplicated data to carry around, and a portable SSD is cheaper per gigabyte and safer. The full ranking is on best portable SSDs.
Capacity classes: SDHC, SDXC and SDUC
The capacity label on a card is also a format standard, and it is the reason a very large card sometimes does not work in an older camera. The SD Association defines SDHC up to 32GB, SDXC from 64GB to 2TB, and SDUC above that. A camera that supports SDHC only will not read an SDXC card at all, which is why old bodies cap out at 32GB. Every current mirrorless camera supports SDXC, so any card from 64GB to 1TB is readable — but if you are buying for an older second body, check which standard its manual lists.
The other formatting point: cards 64GB and above ship formatted as exFAT rather than FAT32, which is what removes the old 4GB per-file limit. Always format a new card in the camera rather than on a computer, so the camera writes its own directory structure and cluster size. It takes five seconds and it prevents a specific class of write errors.
Does a bigger card make the camera slower?
No — capacity and speed are independent specifications. What is true, and confusing, is that the same modelof card is often faster in larger capacities: SanDisk's own data sheet quotes up to 140MB/s write for the 256GB Extreme PRO and up to 90MB/s for the 64GB and 128GB versions of the identical card. So buying bigger sometimes buys faster, but the guaranteed floor — the V-rating — is the same across the family. That distinction is unpacked in speed classes explained.
Should I fill a card completely?
Try not to. Write performance commonly degrades as a card approaches full, which is exactly when a long video clip is most likely to stop recording unexpectedly. Treat the last 10% as reserve and swap cards instead — which is another argument for two smaller cards over one large one.
How long do SD cards last?
Longer than most people's cameras, if they are treated as consumables rather than archives. Flash memory wears with write cycles, but ordinary photographic use is nowhere near the endurance limits of a modern card. The practical failure modes are physical — a bent contact, a cracked plastic shell, a card left in a hot car — which is why the ruggedised cards exist and why a card is never a backup. The full ranking with live prices is on best SD cards for 4K video.
