This SD card speed test measures how fast your SD or microSD card really writes and reads, then checks the result against the speed classes printed on it, from Class 10 and U1 to U3, V30, V60 and V90. You choose the card in a folder dialog, the tester writes test files, reads them back and checks every block, then deletes its files. It works in Chrome, Edge and Opera on Windows, Mac, Linux and ChromeOS, with laptop card slots and USB card readers. A card in a microSD to SD adapter is tested the same way.
How to test an SD card's speed
- Insert the card into your reader. A USB 3 reader that supports UHS-I or UHS-II gives the truest figures. If the card has a lock switch, slide it away from LOCK.
- Press Choose and select the card in the folder dialog. If the browser refuses the top of the card, make a folder on it and choose that. Allow editing when asked.
- Pick 256 MB for a quick check or 1 GB or more for a steadier figure.
- Press Start test. The yellow line is the write speed. A line that sags partway through shows the card slowing after its first burst.
- When writing ends, eject the card, take it out and put it back. The read starts by itself, straight from the card.
- Read the verdict, then press Copy results to keep the figures.
What the results mean
| Readout | What it measures | Typical UHS-I card in a good reader |
|---|---|---|
| Write | Sustained speed saving large files | 20 to 90 MB/s |
| Read | Speed opening large files | 80 to 100 MB/s |
| Small file writes | 4 KB files saved per second | 10 to 200 per second |
| Small file reads | 4 KB files opened per second | 200 to 1,500 per second |
| Copy a 4 GB file | Time to save one 4 GB video file | 1 to 3 minutes |
The write figure is the one that matters for cameras, drones and dash cams, because a card that cannot keep up drops frames or stops recording.
SD speed classes and what they guarantee
Every class is a minimum sustained write speed. The card must hold it while recording, not just in a short burst.
| Class symbol | Minimum write | Typical use |
|---|---|---|
| Class 2, 4, 6 | 2, 4, 6 MB/s | Standard video, older cameras |
| Class 10, U1, V10 | 10 MB/s | Full HD video |
| U3, V30 | 30 MB/s | 4K video at most bitrates |
| V60 | 60 MB/s | 4K at high bitrates, burst photos |
| V90 | 90 MB/s | 8K and high bitrate 4K video |
App Performance Class cards add random speed for running apps and games from the card: A1 needs 1,500 read and 500 write operations per second on 4 KB blocks, and A2 needs 4,000 and 2,000, both with at least 10 MB/s sustained write. A2 only reaches those figures in devices that support its command queue, and many card readers do not.
Why the card reader matters
A card can never beat the bus between it and the reader, and many readers stop well short of the card's limit.
| Bus mode | Bus limit | Real large file reads |
|---|---|---|
| Default Speed | 12.5 MB/s | About 10 MB/s |
| High Speed | 25 MB/s | About 20 to 23 MB/s |
| UHS-I | 104 MB/s | 80 to 100 MB/s |
| UHS-II | 312 MB/s | 200 to 300 MB/s |
| SD Express and microSD Express | Up to 985 MB/s or more | Up to about 900 MB/s on current cards |
Some UHS-I cards sold as "up to 170 MB/s" or "up to 200 MB/s" use a maker's own faster mode that usually needs that maker's reader. In any other UHS-I reader they read at about 90 to 100 MB/s, which is normal.
The card reads at 20 MB/s or less
A read stuck near 20 MB/s, or below the USB 2.0 limit of about 40 MB/s, almost always points at the reader:
- A laptop's built-in slot. Many are wired to an internal USB 2.0 link or only support High Speed mode. A USB 3 UHS-I reader often doubles or quadruples the result.
- An old or cheap USB reader. Check the box for UHS-I or UHS-II support, and plug it into a USB 3 port on the computer itself.
- A USB 2.0 port or hub. The usb speed test explains how to spot one and what the port colors mean.
- A loose adapter. microSD to SD adapters wear out and drop to slower modes. Try another adapter or a microSD reader.
The card writes slower than its class
- It is a fake. Counterfeit cards often write at 2 to 10 MB/s whatever the label says, and lose data once they are filled past their real size. Prove the real capacity with the fake flash drive tester.
- It is worn. Cards in dash cams and security cameras wear out after months of constant recording. Speed drops first, then errors appear. Replace cards used this way regularly.
- It is nearly full or fragmented. Copy your files off, format the card in the camera or with the SD Association's SD Card Formatter, and test again.
- The burst is over. Many cards write fast for the first few hundred MB, then settle lower. The sustained level after the drop is what decides whether the card keeps up with video.
Testing on Windows, Mac, Android and Raspberry Pi
On Windows 10 and 11, CrystalDiskMark gives full figures, including the 4 KB random results that A1 and A2 depend on. On a Mac, Blackmagic Disk Speed Test or AmorphousDiskMark do the same. This page tests cards only on a computer, so on Android use a storage benchmark app. On a Raspberry Pi, the official Raspberry Pi Diagnostics tool (under Accessories, or sudo apt install agnostics) runs an SD card speed test against the A1 figures.
For a USB stick or an internal drive, use the ssd speed test, which also measures this computer's own storage.
What this test cannot measure
The tester works through files, like your camera and computer do, so its results match copying photos and videos. It cannot send the raw random commands that the A1 and A2 classes are rated on, so it cannot certify those classes, and its small file figures include the browser's own work. Results also depend on the reader: a fast card in a slow reader is measured as a slow card, which is exactly what you get when you copy files with that reader.
SD card speed test questions
How do I check my SD card's speed class?
The classes are printed on the card: a number in a C (Speed Class), a number in a U (UHS Speed Class), a V with a number (Video Speed Class) and A1 or A2 (Application Performance Class). Run this test to see whether your card's sustained write speed actually meets the class it shows.
Why is my SD card so slow?
The most common cause is the card reader, not the card. Many laptop slots and cheap USB readers top out around 20 to 40 MB/s. Other causes are a worn or fake card, a nearly full card, and many small files, which every card writes far more slowly than large ones.
Does the card reader affect SD card speed?
Yes, a lot. A UHS-I card rated at 100 MB/s reads at about 20 MB/s in a reader that only supports SD High Speed mode, and no more than about 40 MB/s in any reader on USB 2.0. Use a UHS-I or UHS-II reader on a USB 3 port to see the card's full speed.
Is the speed on the card the read or the write speed?
The big "up to" figure on the label is almost always the read speed. The class symbols (C10, U3, V30) are the minimum sustained write speed. A card sold as "up to 170 MB/s" may write at 60 to 90 MB/s.
Can this sd card speed tester find a fake card?
It can show warning signs: data that does not match when read back, or very low and uneven write speed. To prove a card's real capacity, run the full check on the fake usb test page, which fills and verifies the whole card.
Can I test an SD card on my phone?
Not on this page, which tests cards in a computer's reader with Chrome, Edge or Opera. On the phone itself, use a storage benchmark app.