The short version: the V number on an SD or microSD card is its guaranteed minimum sustained write speed in megabytes per second — V30 means at least 30 MB/s, V60 at least 60, V90 at least 90. V30 covers most 4K recording; V60 and V90 are for high-bitrate video and long bursts. The older U3/U1 and Class 10 symbols are earlier versions of the same idea, and when a card carries a V rating you can ignore them. microSD cards use exactly the same ratings.
SD cards and their packaging come with a bunch of different codes, and while they’re meant to make choosing easier, they often create more confusion than clarity. I get a lot of questions about which code matters for which camera, so here’s every symbol you’ll see, what it means, and which one you actually need.
Related: SDHC or SDXC? They’re capacity formats, not speed ratings — here’s the difference between SDHC, SDXC, and SDUC.
Every SD card speed symbol, decoded
| Symbol | What it guarantees | In plain terms |
|---|---|---|
| V30 / V60 / V90 (Video Speed Class) | Minimum sustained write of 30 / 60 / 90 MB/s | The one that matters for video and bursts today. V6 and V10 exist but you’ll rarely see them |
| U1 / U3 (UHS Speed Class) | Minimum sustained write of 10 / 30 MB/s | Older system; U3 = V30 in practice. Ignore if a V rating is present |
| Class 2 / 4 / 6 / 10 (Speed Class) | Minimum sustained write of 2 / 4 / 6 / 10 MB/s | The original system; Class 10 = U1 = V10. Obsolete for cameras |
| I / II / III (UHS bus interface) | Bus ceiling of about 104 / 312 / 624 MB/s | How fast data can move, not a guarantee. Your camera’s slot must match to benefit |
| A1 / A2 (Application Performance Class) | Random read/write for running apps | Matters for phones and tablets, not cameras |
| SDHC / SDXC / SDUC | Capacity range and file system | Not a speed rating at all; most camera cards today are SDXC |
How to read SD card codes: Practical examples
So here are a couple of practical examples.
In this example, using one of my Prograde Digital SD cards, the codes work like this. At left is the SDXC logo. From that, you can tell that the storage capacity is greater than 32GB and the file system is formatted in exFAT. A key implication of that is that it will work in newer cameras with high-bitrate recording that save video in one continuous stream rather than chaptering into 4GB chunks (which is the filesize limit of the FAT32 filesystem used on SDHC cards). Next to that is the Video Speed Rating; in this case it’s V60, so is capable of minimum sustained write speeds of 60MB/s. Under that, the “II” refers to a UHS Bus Interface of UHS-II. Next to that are the older speed classes, the U3 UHS Speed Class and the Class 10 Speed class symbols. The R at the right indicates that it’s compatible with proprietary Prograde Digital software known as Refresh Pro, which helps keep the memory card storage healthy.
Here’s another example, using a Manfrotto Card.
In this case, there’s the SDXC symbol at left (partially obscured in this shot), it has a Video Speed Class rating of V90, and the “II” and UHS-II are duplicative and both refer to the UHS Bus Interface. I’ve included this card because it’s somewhat unusual in that it doesn’t include the older speed ratings of U3 and Class 10. Photo by David Coleman / Have Camera Will Travel
Which Speed Class do you actually need?
Match the card to what your camera writes. Cameras quote video bitrates in megabits per second; divide by eight to get the megabytes per second the card has to sustain (or use my Mbps to MB/s converter).
| What you shoot | Card write speed involved | Class to buy |
|---|---|---|
| Stills, occasional 1080p, modest bursts | Under 20 MB/s | V30 (or U3) is plenty |
| 4K up to about 200 Mbps | Up to 25 MB/s | V30 |
| 4K at higher frame rates or bitrates (roughly 200–400 Mbps) | 25–50 MB/s | V60 |
| Very high bitrates, 6K/8K, RAW video on SD, long RAW bursts | 50 MB/s and up | V90 (UHS-II slot required to get it) |
Two things to check before spending more: whether your camera’s slot is UHS-I or UHS-II (a UHS-II V90 card in a UHS-I slot runs at UHS-I speed), and whether the manual specifies a minimum class for a particular video mode — if it does, that’s the answer. I go through the slot question in detail in what memory card speed your camera actually needs, and the bus interfaces in UHS-I vs UHS-II.
The same ratings apply to microSD
Everything on this page applies equally to microSD cards: the same V, U, and Class symbols, the same UHS bus markings, the same meanings. The only practical difference is that microSD cards tend to top out lower in real-world speed than full-size SD at the same rating, which is why I test them separately — see my microSD speed tests if you’re shopping for an action camera, drone, or phone.
The Video Speed Class system in detail
The two types of shooting that are most demanding of an SD card’s speed are video and photo burst modes (i.e., a long, rapid sequence of photos).
The SD Association, which is the organization that designs and controls the SD card specs, has latched onto video shooting to devise speed ratings. The underlying reason is that if your SD card isn’t fast enough for the camera to save its video capture in real time, you can end up with dropped frames, lockups, or other errors. That, obviously, would create a bad user experience.
Key point: A faster SD card won’t magically give you better quality video or photos, but having an SD card that’s fast enough for the camera will help prevent problems like dropped frames, lockups, and related errors.
So the SD Association has come up with rating systems that indicate the minimum sustained write speed of the SD card.
Over the years, as video capture has gotten better and larger, the SD Association has responded with different rating systems.
I’ll start with the latest one, which is most relevant to cameras shooting larger resolutions such as 4K, 5K, and 8K video.1
Video Speed Class (aka V speed rating system)
The current SD card speed rating system for recording video and similar activities is known simply as Video Speed Class.
Most (but not all) newer cards come with this rating system. The Video Speed Class was first introduced in 2016, but it has taken time for SD card manufacturers to roll it out into their products.
It’s designed to indicate SD cards suitable for recording 4K, 8K, 3D, HDR, and 360-degree video recordings.
It is denoted by a ‘V’ followed by the speed class number. For example: V90 or V30. It starts at V6 and currently maxes out at V90. The most common you’ll see are V30, V60, and V90. I have rarely seen one with V10 (and you won’t, because it duplicates the existing Class 10 rating that was already widely adopted), and I’ve never seen one with V6.
Key takeaway: The ‘V’ in the speed class rating indicates the minimum sustained write speed for video recording in megabytes per second (MB/s).
This table shows the V rating and its corresponding minimum sustained write speed. As you can see, the pattern is pretty clear and self-explanatory.
| Video Speed Class | Minimum Sustained Write Speed |
| V6 | 6 MB/s |
| V10 | 10 MB/s |
| V30 | 30 MB/s |
| V60 | 60 MB/s |
| V90 | 90 MB/s |
What this fundamentally means is: if you’re recording a video that writes data to the SD card at 30MB/s (which is 240 Mb/s (megabits per second)), you need an SD card with a Video Speed Class of V30 or higher. Using a card with a lower might result in the recording stopping or frames dropping.
Older SD card speed rating systems
Before the current V rating system, there have been two earlier rating systems. While these were designed for slower cards, they’re still relevant because the codes still appear on current SD cards.
UHS Speed Classes (aka U speed rating system)
And older system of speed ratings is known as UHS Speed Class. UHS stands for “Ultra High Speed”.
I mention it here in some detail because it’s not just some historical relic. Even SD cards that have the newer V rating system will also have the older U rating system as well. For example, you’ll might see both V30 and U3 on a card.
Key point: When looking at the speed ratings on SD cards, you can focus on the V rating and ignore the older U and C ratings.
When it was first announced in 2009, consumer cameras capable of shooting 4K and above were rare. At the time, 1080p was the cutting edge, along with much lower video bitrates that modern cameras can shoot at.
But even then, an SD card that was too slow to support 1080p video could create problems. That’s something I ran into quite a bit with GoPro cameras, which were among the devices pushing the envelope of SD card speeds at the time. And it’s something that really pushed me into a years-long deep dive into memory card speeds and speed tests.
Key point: The speed class represents the minimum transfer rate, meaning your card is rated never to write slower than the specified rate.
With the UHS Speed Class, there are two main classes:
- UHS Speed Class 1 (U1)
- UHS Speed Class 3 (U3)
Again, they’re tied to minimum sustained write speeds. In this case, the connection isn’t quite as self-evident, but it’s still straightforward.
| UHS Speed Class | Minimum Sustained Write Speed |
| U1 | 10 MB/s |
| U3 | 30 MB/s |
Speed Class
Finally, there’s an even older rating system simply known as Speed Class. And, again, it’s still used on modern SD cards.
Frankly, I wish they’d just drop this one now on cards where it’s not relevant. Including the Class 10 marking on a card that also has V90 marked is just a recipe for unnecessary confusion.
This rating system began with Classes 2, 4, and 6. Class 10 was later added when it was clear that faster cards were needed for Full HD video.
| Speed Class | Minimum Sustained Write Speed |
| Class 2 | 2 MB/s |
| Class 4 | 4 MB/s |
| Class 6 | 6 MB/s |
| Class 10 | 10 MB/s |
UHS Speed Class vs UHS Bus Interface
There’s yet another area for easy confusion: the difference between UHS Speed Class and UHS Bus Interface. Using UHS in both makes them sound much the same, but they’re actually referring to quite different things.
I won’t go into much technical detail here, but the gist is this:
- UHS Speed Class is the rating system for sustained minimum write speeds, designed for measuring suitability for recording video. It’s marked with U1 or U3.
- UHS Bus Interface refers to the actual physical connection between your SD card and the device it’s inserted into. There are two primary versions of this: UHS-I and UHS-II.
Speed is still relevant here, but it concerns hardware capabilities, not a rating system.
A UHS-I interface can reach speeds of up to 104MB/sec, while the more advanced UHS-II can achieve up to 312MB/sec. There’s also a newer version, UHS-III, which has a maximum transfer rate of up to 624MB/sec, but it’s not widely available in consumer devices, and the reality is that we probably won’t ever have to worry about it because it hasn’t been adopted by camera manufacturers.
Tip: For a quick visual way to tell the difference between UHS-I and UHS-II, look on the back of the card. A UHS-II card will have two rows of contact pins. A UHS-I card will have one row of pins.
Video Speed Class vs Application Speed Class
As if that wasn’t enough, there’s yet another speed rating system code that’s packed onto many newer SD cards. You might have seen a code like A1 or A2.
That’s known an Application Speed Class. The short version is that this isn’t really relevant to cameras, so if you’re buying an SD card to use in a camera, you can safely ignore it.
It’s designed for handheld devices that can use SD cards for computing storage. Rather than saving data in a long, fast sequential stream, as when shooting video, these computer devices use small, choppy bits of data that are measured in what is known as random read/write speeds.
Common questions
Is Class 10 the same as U1 and V10?
Effectively, yes — all three guarantee a minimum sustained write speed of 10 MB/s. They’re the same threshold expressed by three generations of rating systems, which is why many cards carry all three symbols.
Does a higher speed class help with photos, not just video?
For single shots, no — any current card keeps up. For long bursts it does: a faster card clears the camera’s buffer sooner, so you can keep shooting. The ratings were designed around video, but the sustained write speed they guarantee is exactly what a burst of RAW files needs.
Can I use a V90 card in a camera that only needs V30?
Yes. Faster cards are backward compatible, and the camera simply won’t ask for more than it needs. You’re just paying for speed you won’t use in that camera — though the faster card can still speed up offloading to your computer with a UHS-II reader.
Wrap up
Looking ahead, newer rating systems will probably come out that factor in high-bitrate capture, such as 3D, virtual reality, and augmented reality cameras. But odds are, we will have moved onto newer and faster generations of memory cards for those.
For now, the V rating system is the newest and most relevant for the current generation of cameras.
When looking at the speed ratings on SD cards, you can focus on the V rating and ignore the older U and C ratings.
References & Further Reading
- SD Association: Speed Classes
- SD Association Press Release, “New SD Association Video Speed Class Supports 8K and Multi-File Video Recording”(February 25, 2016)
- David Coleman (Have Camera Will Travel): Fastest SD Card Speed Tests
- SD Association White Paper, “Video Speed Class: The New Capture Protocol of SD 5.0” (February 2016)
- Technical Committee, SD Association, “SD Specifications: Part 1: Physical Layer Simplified Specification,” (Version 6.00; August 29, 2018)
- SD Association Press Release, “SD Association Defines New High-Speed Performance Options for SDXC and SDHC Memory Cards and Devices” (June 23, 2010)
- SD Association, “Understanding the New UHS-III” (2017)
- SD Association Press Release, “SD Association Double BUS Interface Speeds with UHS-III” (February 22, 2017)
- SD Association, “Application Performance Class: The New Class of Performance for Applications on SD Memory Cards (SD 5.1)” (November 2016)
Notes & References:
- Although, in reality, the speed demands have less to do with resolution than they do video bitrate. Bitrate is the volume of data over time. It’s conventionally measured in megabits per second (which, confusingly, is not the megabytes per second that SD card speeds are measured in; I have an explanation of MB/s vs Mb/s separately). [↩︎]
