Memory Cards Explained: Speed Classes and What You Need

Pick up an SD card and you’ll find a cluster of tiny symbols crowded onto one corner: a number, a letter or two, a couple of logos that look almost identical. It reads like a barcode for insiders, and it’s the reason buying a memory card feels harder than it should. The good news is that only a few of those markings matter, and once you know what they mean you can walk past most of the marketing. This guide decodes the label so you can buy the right card the first time.

Capacity and speed are two different things

The biggest number on the card — the one in gigabytes — is capacity: how much footage or how many photos it holds. It says nothing about how fastthe card is. A large card can be slow and a small card can be fast; the two are unrelated. People shopping on capacity alone often end up with a roomy card that can’t keep up with their camera, which is exactly the wrong way round. Decide how fast a card you need first, then pick a size that fits how much you shoot between offloads.

You’ll also see the family labels SDHC (up to 32GB) and SDXC(64GB and up). These just describe the capacity tier and the formatting standard behind it — useful to know your camera supports the tier you’re buying, but not a speed rating.

Bus type: UHS-I, UHS-II and the row of pins

The busis the pipe the card uses to move data. On modern cards you’ll see a Roman numeral — I or II— printed near the logos. That’s the UHS (Ultra High Speed) bus type, and the easiest way to spot the difference is to flip the card over: a UHS-II card has a second row of gold pins that a UHS-I card lacks. That extra row is a wider pipe, and it raises the ceiling on how fast the card can go.

Here’s the catch that trips people up: a UHS-II card only reaches those higher speeds in a camera with a UHS-II slot. Drop one into a UHS-I camera and it works fine — it simply falls back to UHS-I speeds, so you paid for a fast lane you can’t use. Before you spend up for UHS-II, confirm your camera has the matching slot. If it doesn’t, a good UHS-I card is the smarter buy.

The Video Speed Class is the number to check

If you remember one thing from this guide, make it this: for video, the marking that matters most is the Video Speed Class — the letter V followed by a number, such as V30, V60 or V90. Unlike the flashy peak-speed numbers, the Video Speed Class is a promise about the floor: it guarantees a minimum sustained write speed the card will hold for as long as you keep recording.

• V30— at least 30 MB/s, sustained.
• V60— at least 60 MB/s, sustained.
• V90— at least 90 MB/s, sustained.

Video is unforgiving in a way photos aren’t. A camera writing a continuous stream of footage needs the card to keep up everysecond; if the card dips below the bitrate even briefly, the recording can stutter or stop. That’s why a guaranteed sustained minimum beats a headline peak speed the card only touches in short bursts. On older cards you may also see a UHS Speed Class mark — a number inside a Ushape, usually U1 or U3 — which works the same way (U3 lines up with V30). When both are printed, the V rating is the clearer one to trust for video.

How much speed does 4K actually need?

More resolution means more data per second, so faster formats demand faster cards. As a practical rule of thumb, a reputable V30 card handles most consumer 4K recording without drama. Where you need to step up is the more demanding modes: higher bitrates (some cameras offer a high-quality 4K setting that writes far more data) and higher frame rates like 4K60 or 4K120 for slow motion. Those push you toward V60 or V90. If you shoot in a demanding codec or plan to grade the footage heavily, err on the faster side. Because higher resolution is what drives these requirements, it’s worth understanding the formats themselves — our video resolution explained guide covers how 1080p, 4K and beyond translate into the data your card has to swallow.

The safest move is to open your camera’s manual and find the class it recommends for the exact mode you shoot. Manufacturers list a minimum for a reason, and matching or beating it is cheap insurance against a corrupted take.

Read speed versus write speed

Cards advertise two speeds, and they do different jobs. Write speedis how fast the card records incoming data — this is the one that matters when you’re filming or firing off a burst of photos. Read speedis how fast the card hands data back, which matters when you’re offloadingfiles to your computer at the end of a shoot. Marketing almost always leads with the big read number because it’s the higher of the two; the write speed is usually printed smaller, if at all. For anyone recording video, the write speed — and the sustained floor the Video Speed Class guarantees — is what you’re really buying. A fast read speed is a nice bonus that shortens the wait when you copy everything off; once the footage is on your computer, a portable SSD is where it should live for editing and backup.

Which card do I actually need?

Strip away the jargon and it comes down to two common cases:

• Most creators and photographers — a reputable V30 UHS-Icard covers everyday photos and mainstream 4K video comfortably, without overpaying for speed you won’t use.
• Demanding video work— high-bitrate footage, 4K at high frame rates, or long continuous takes call for a UHS-II V60 or V90 card, paired with a camera that has the UHS-II slot to match.

That’s genuinely most of the decision. When you’re ready for specific picks sorted by exactly this logic, our best SD cards for cameras roundup lays them out by the speed class you actually need.

Buy reputable, and dodge the counterfeits

Memory cards are one of the most counterfeited products online. A fake will often reporta big capacity while only having a fraction of it, or claim a speed class it can’t hold — and you don’t find out until footage corrupts partway through a shoot. Protect yourself by buying from the brand’s own store or a trusted retailer rather than an unknown third-party seller, treating suspiciously cheap high-capacity cards as a red flag, and formatting a new card in-camerabefore you rely on it. Sticking to well-known names isn’t brand snobbery here; it genuinely lowers the odds of losing work.

Card care that saves your footage

A card is the least glamorous part of your kit and the one most capable of ruining a day, so a few habits go a long way:

• Format in-camera, not on your computer. Letting the camera format the card keeps the file structure exactly the way that body expects it, which prevents a lot of write errors.
• Back up before you erase. Treat a card as temporary storage, never the only copy. Offload to your computer and a second location before you reformat for the next shoot.
• Retire aging cards.Flash memory wears out over thousands of write cycles. If a card starts throwing errors, running slow, or dropping frames, replace it — a working card is far cheaper than lost footage.

The short version

Capacity tells you how much fits; it says nothing about speed. Flip the card to check the pins for UHS-I versus UHS-II, and only pay for UHS-II if your camera has the slot. For video, the Video Speed Class— V30, V60, V90 — is the number that matters, because it guarantees the sustained write speed your recording depends on. Most people are well served by a reputable V30 UHS-I card; demanding video wants UHS-II V60 or V90. Buy from a trusted source, format in-camera, back up, and swap out tired cards. Do that, and the little cluster of symbols stops being a mystery — and your footage stays safe.