How to compare memory cards with different speed labels

One memory card says 100 MB/s, another says V30, and a third has U3 printed on it. Those labels do not all measure the same thing, so the biggest number does not automatically mean the best card for your camera, laptop, or other device.

Compare how the cards actually read and write in the same reader, then match those results to what each label promises. The USB & SD Speed Test measures real read and write speed for a memory card or flash drive.

Compare the cards fairly

1. Back up important files on each card. Open the USB & SD Speed Test and follow the on-screen prompts to select the card and run its read and write tests.
2. Test each card in the same card reader, adapter, computer port, and browser. A different reader can change the result, even when the cards are the only thing you changed.
3. Use the same test options for both cards, if the tool offers options. Avoid copying or downloading other large files during a run, and keep both cards at a similar level of free space.
4. Record the read and write results separately. Repeat the tests a few times and compare the typical results, not just the highest run.
5. Check each card's printed speed marks and packaging. Note whether a figure is a maximum read speed, a video speed class, a UHS speed class, or an application performance rating.

Write down the card model and reader you used with the results. A result such as 75 MB/s read and 24 MB/s write is more useful than saying only that one card “felt faster.” For more on why read and write results can differ, see how sequential and small-write speed differ.

What the labels and results mean

A card has a V30, U3, or C10 mark

These are speed classes, not promises that every device will transfer files at exactly that speed. They describe minimum sustained write performance under specified conditions. For example, V30 and U3 indicate a 30 MB/s minimum write class, while C10 indicates a 10 MB/s class. The card's actual performance depends on the card and the equipment reading it.

These classes can help you decide whether a card meets a camera's stated video recording requirement. They do not mean that every computer will read the card at 30 MB/s, or that its write speed will always equal the class number. See what a V30 card rating does not tell you for more about that distinction.

The packaging says “up to” a high speed

An “up to” figure is a maximum advertised result, often for reading under favorable conditions. It may not describe write speed, sustained recording speed, or performance in your reader. Compare it only with a result measuring the same direction and type of transfer.

One card reads faster, but the other writes faster

That is a normal result. Read speed matters when copying photos or videos from a card to a computer. Write speed matters when saving files to the card, such as during a burst of photos or video recording. Choose based on the task that matters to you, and check the device maker's minimum requirements for recording.

The cards perform nearly the same

Your reader or port may be limiting both cards, or their real-world speeds may simply be close in this test. The tool measures performance through the setup you used, not a card's maximum capability in every device. Try a reader that supports the card's interface before concluding that the cards are equal.

Results change a lot between runs

Other file activity, available free space, heat, or background work can affect a run. A single unusually fast or slow result is weak evidence. Repeat under the same conditions and compare the usual results. If the score changes sharply or the card disconnects, check the reader and connection as well as the card.

Get a more useful result

1. Repeat the comparison in one setup. Use one reader and one port for all cards. If you swap adapters or use a built-in reader for one card and a USB reader for another, you are comparing the whole setup, not only the cards.
2. Check the reader's limits. A fast card cannot overcome a reader that supports a slower interface. If a card is consistently slow, try a reader known to support its card type and speed interface. A fast microSD card can also crawl in a basic reader, as explained in why a fast microSD card crawls in a cheap reader.
3. Close other file transfers. Pause large copies, backups, and downloads while testing. Those tasks compete for the same computer, port, or storage connection.
4. Check the card's free space and condition. A nearly full card can behave differently from an empty one. If results remain inconsistent, back up the files and test the card in another compatible reader or device.
5. Choose for the actual device. If the card is for a camera, check the camera's supported card type and recording requirements. A fast result in a computer does not prove that a camera supports the same interface or recording mode.

What this test cannot prove

A browser speed test measures transfers through the card, reader, connection, and computer together. It does not certify the printed speed class, prove that a card is genuine, or guarantee reliable video recording in a particular camera. It also does not test every kind of storage work, such as repeated small-file access. Use the numbers to compare cards in the same setup, then check the device's requirements before relying on one for important recording.