How to Buy an SD Card: Speed Classes, Sizes, and Capacities Explained

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Secure Digital (SD) cards are used in digital cameras, music players, smartphones, tablets, and even laptops. But not all SD cards are created equal—you’ll find different speed classes, physical sizes, and capacities to consider.

Some devices—like cameras—may require an SD card for their primary storage area. Other devices—like smartphones, tablets, and even computers—may offer the ability to use an SD card to increase storage or make it mobile. However, different devices require different types of SD cards. Here are the differences you’ll need to keep in mind when picking out the right SD card for your device.

Speed Class

Not all SD cards offer the same speeds. This matters for some tasks more than others. For example, if you’re a professional photographer taking photos in rapid succession on a DSLR camera and saving them in high-resolution RAW format, you’ll want the fastest SD card you can get so your camera can save them as quickly as possible. A fast SD card is also important if you want to record high-resolution video and save it directly to the SD card. If you’re just taking a few photos on a typical consumer camera or using an SD card to store some media files on your smartphone, the speed isn’t as important.

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It’s only $35 for a blazing fast class 10 64 GB card.

Manufacturers use “speed classes” to measure an SD card’s speed. The SD Association that defines the SD card standard doesn’t actually define the exact speeds associated with these classes, but they do provide guidelines.

There are four different speed classes—10, 6, 4, and 2. Class 10 is the fastest, suitable for “full HD video recording” and “HD still consecutive recording.” Class 2 is the slowest, suitable for standard definition video recording. Classes 4 and 6 are both deemed suitable for high-definition video recording.

There are also two Ultra High Speed (UHS) speed classes–1 and 3—but they’re more expensive and are designed for professional use. UHS cards are designed for devices that support UHS.

Here are the associated SD class speed logos, in order from slowest (class 2) to fastest (UHS class 3):

sd card speed class 2 sd card speed class 4 sd card speed class 6 sd card speed class 10 sd card uhs class 1 sd card uhs class 3

You’ll probably be okay with a class 4 or 6 card for typical use in a digital camera, smartphone, or tablet. Class 10 cards are ideal if you’re shooting high-resolution videos or RAW photos. Class 2 cards are a bit on the slow side these days, so you may want to avoid them for all but the cheapest digital cameras. Even a cheap smartphone can record HD video, after all.

An SD card’s speed class is identified on the SD card itself—just look for the logo. You’ll also see the speed class on the online store listing or on the card’s packaging when purchasing it. For example, in the below photo, the middle SD card is speed class 4, while the two other cards are speed class 6.

If you see no speed class symbol, you have a class 0 SD card. These cards were designed and produced before the speed class rating system was introduced. They may be slower than even a class 2 card.

Physical Size

SD cards also come in different sizes. You’ll find standard SD cards, miniSD cards, and microSD cards.

Standard SD cards are the largest, although they’re still pretty small. They measure 32x24x2.1 mm and weigh just two grams. Most consumer digital cameras for sale today still use standard SD cards. They have the familiar “cut corner”  design.

miniSD cards are smaller than standard SD cards, measuring 21.5x20x1.4 mm and weighing about 0.8 grams. This is the least common size today. miniSD cards were designed to be especially small for mobile phones, but now that we have an even smaller size—microSD—miniSD cards aren’t too common.

microSD cards are the smallest size of SD card, measuring 15x11x1 mm and weighing just 0.25 grams. These cards are used in most cell phones and smartphones that support SD cards. They’re also used in many other devices, such as tablets.

Choosing a size is really just about what fits into the device you have. SD cards will only fit into matching slots. You can’t plug a microSD card into a standard SD card slot. However, you can purchase adapters that allow you to plug a smaller SD card into a larger SD card’s form and fit it into the appropriate slot. Below, you can see an adapter that lets you use a microSD card in a standard SD card slot.

Capacity

Like USB flash drives, hard drives, solid-state drives, and other storage media, different SD cards can have different amounts of storage.

But the differences between SD card capacities don’t stop there. SD Standard Capacity (SDSC) cards range in size from 1 MB to 2 GB (and sometimes even 4 GB—although that’s not standard). The SD High Capacity (SDHC) standard was created later, and allows cards 2 GB to 32 GB in size. An even more recent standard, SD Extended Capacity (SDXC) that allows cards 32 GB to 2 TB in size.

To use an SDHC or SDXC card, you’ll need to make sure your device supports those standards. At this point, the vast majority of devices should support SDHC. In fact, the SD cards you have are probably SDHC cards. SDXC is newer and less common.


When buying an SD card, you’ll need to buy the right speed class, size, and capacity for your needs. Be sure to check what your device supports and consider what speed and capacity you’ll actually need.

Image Credit: Ryosuke SEKIDO on Flickr, Clive Darra on Flickr, Steven Depolo on Flickr

Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.