SD cards were first introduced in 1999 as a joint effort between SanDisk, Panasonic (Matsushita Electric) and Toshiba, as an improvement over MultiMediaCards (MMC). The basis for SD card technology is flash memory, which was invented by Fujio Masuoka at Toshiba in 1980 and commercialized by Toshiba in 1987. [1]
Since their introduction, SD cards have evolved into various sizes and speed classes. The main sizes of SD cards available today include SD, SDHC, and SDXC. SD cards range from 1 GB to 2 GB, SDHC from 4 GB to 32 GB, and SDXC from 64 GB to 2 TB. In addition to standard sizes, there are also microSD cards, which are smaller versions used in mobile devices and cameras.
SD cards are commonly used for storage in digital cameras, camcorders, smartphones, tablets, and other mobile devices. They are also used in some laptops, gaming consoles, media players, and other electronics that have a built-in SD card slot.
MB/s – What is it?
MB/s stands for Megabytes per second and is a measurement of data transfer speed to and from a storage device like an SD card. It refers to the number of megabytes (MB) that can be read or written per second.
There is an important distinction between megabytes (MB) and megabits (Mb). A byte is a collection of 8 bits of data. A megabyte is equal to 1024 kilobytes or 8388608 bits. A megabit is equal to 1/8 of a megabyte. So 8 megabits = 1 megabyte.
Therefore, a transfer speed of 1 MB/s is equivalent to 8 Mbps (megabits per second). MB/s measures the actual volume of data that is transferred, while Mbps refers to the underlying data connection speed.
For example, an SD card rated at 95 MB/s can transfer 95 megabytes of data every second. This means it has an underlying data connection speed of 95 x 8 = 760 Mbps.
In summary, MB/s is used to measure the real-world data transfer rate, while Mbps measures just the connection speed. When comparing SD card performance, MB/s gives a better sense of actual read/write speeds.
SD Card Speed Classes
The SD Association has defined various Speed Classes to indicate the minimum guaranteed speeds that SD cards support https://www.sdcard.org/developers/sd-standard-overview/speed-class/. The Speed Classes give an indication of the card’s capabilities and performance.
The main Speed Classes are:
- Class 2 – Minimum write speed of 2 MB/s
- Class 4 – Minimum write speed of 4 MB/s
- Class 6 – Minimum write speed of 6 MB/s
- Class 10 – Minimum write speed of 10 MB/s
- U1 – Minimum sequential write speed of 10 MB/s
- U3 – Minimum sequential write speed of 30 MB/s
- V30 – Minimum sequential write speed of 30 MB/s for video recording
Higher Speed Classes indicate faster performance. However, the Speed Class only provides the guaranteed minimum speed – cards often exceed this in real-world usage. The Speed Class rating marked on a card refers to its minimum write speed capability.
MB/s vs Speed Class
MB/s refers to the maximum sequential read and write speed of an SD card, measured in megabytes per second. The speed class rating refers to the minimum sustained write speed guaranteed by the SD Association for a card of that class.
MB/s is a measure of the absolute maximum speed, while speed class represents the minimum speed that can be maintained over time during writes. For example, a card rated at 100MB/s may have speed bursts up to that rate, but its sustained write speed could be lower if it has a slower speed class.
A Class 10 card has a minimum write speed of 10MB/s, while a UHS Speed Class 3 (U3) card has a minimum write speed of 30MB/s, according to the SD Association specifications. So a card rated at 80MB/s with a U3 speed class will always write at a minimum of 30MB/s even if its maximum speed is higher.
The speed class provides a more standardized measure of real-world sustained performance. However, MB/s ratings allow you to compare maximum speeds between cards. Checking both ratings gives you a better sense of the overall capability and consistent performance to expect from a card in different devices and conditions.
According to Kingston, cards with higher MB/s ratings typically have faster speed classes as well, so both indicators provide useful guidance. But sustained write speed is most relevant for smooth performance in demanding applications like 4K video recording.
Factors Affecting Speed
There are several key factors that affect the speed of an SD card:
Bus Interface
The bus interface between the SD card and device plays a big role in speed. Common bus interfaces for SD cards include: UHS-I and UHS-II. UHS-II has faster transfer rates, allowing for speeds over 300MB/s.
Card Reader
If you’re using an external card reader, its speed capabilities will impact performance. Look for a reader that supports UHS-II/UHS-III for maximum speeds.
Device Hardware and Software
The device’s components like processor, RAM, storage controller, and OS version determine how fast it can read/write data. Older devices with UHS-I will have slower speeds than new UHS-II/UHS-III compatible devices.
File Sizes
Larger file sizes like 4K video require higher sustained write speeds compared to smaller photo files. So shooting high resolution video requires a faster SD card.
Uses that Require Faster Speeds
Some uses and devices require faster SD card speeds to function properly or optimize performance. Key examples include:
4K Video Recording
Recording high-resolution 4K video requires write speeds of at least 10 MB/s according to the SD Association, though speeds closer to 30 MB/s are recommended for reliable performance (SD Association). 4K video has very large file sizes, so a slower card is more likely to bottleneck recording.
Burst Photo Shooting
When capturing photos in burst mode, a faster SD card is needed to quickly save multiple images. Top-end DSLR and mirrorless cameras can shoot over 10 frames per second, creating a backup if the card can’t keep up (Kingston). UHS-I or UHS-II cards are best for burst shooting.
Gaming
Modern gaming consoles like the Nintendo Switch require SD cards with 100 MB/s write speeds and 90 MB/s read for optimal performance. Slower cards can lead to longer game load times and other lags (Wired).
HD Video Editing
Editing HD or 4K video requires reading and writing large files constantly. Insufficient card speeds can slow down previewing edits and exporting final videos. Cards with at least 30-90 MB/s speeds are recommended for seamless HD editing (SD Association).
Choosing the Right Card
When selecting an SD card, it’s important to consider the speed class required for your intended use. The speed determines how quickly data can be written to and read from the card. Faster cards allow you to take high resolution photos and video more rapidly without interruption.
For basic photo shooting with a point-and-shoot or low resolution DSLR camera, a Class 10 card is sufficient. This provides a minimum sequential write speed of 10MB/s. For shooting bursts, HD video, or using a high resolution DSLR, a faster UHS Speed Class card is recommended such as U3 or V30, with sequential write speeds up to 30MB/s or 90MB/s respectively. A V90 card with 90MB/s write speed is best for 4K or 8K video (1).
Faster cards do come at a higher price point. Consider both your camera’s capabilities and your own needs. Occasional photographers can save money with a more basic Class 10 card, while professionals will require a high performance V30 or V90 card.
Speed Test Methods
There are a few different ways to test the speed of your SD card to see if it lives up to its rated MB/s.
One method is to use benchmark apps that are designed specifically for testing SD card speeds. These apps will perform read and write tests on your card and give you metrics like sequential read/write speeds and random read/write speeds. Popular benchmark apps include SD Card Speed Tester and EaseUS Partition Master.
Another real-world test is to transfer a large file or folder of files to and from the SD card. Time the transfer and divide the amount of data by the time to get the effective read/write speed. Using large files of at least 1GB in size will get speeds closer to the rated MB/s.
You can also test the card’s speed by recording high bitrate video to it or viewing RAW image files directly from the card. If you experience lag, stuttering, or slow buffer clears, the card may not be fast enough for those types of files.
Whichever method you use, make sure to test the card in the device you intend to use it in when possible. The card reader, USB port, or device itself can sometimes be the speed bottleneck rather than the card.
Improving Speed
There are a few ways to potentially improve the speed of your SD card:
Firmware Updates: Check for any available firmware updates for your SD card reader or device that uses the SD card. Manufacturers will periodically release updates that can improve performance and fix bugs. Installing the latest firmware ensures your SD card reader hardware and controllers are running optimally.
Clear Storage Space: If your SD card is nearly full, the lack of free space can negatively impact write speeds and overall performance. Try deleting unused apps, photos, videos or other data to free up storage space. Having at least 20% of free space is ideal for optimal speed. The more free space the better.
Faster Card Reader: Using a newer, higher-quality USB card reader can provide faster transfer speeds compared to an older, built-in reader. Look for USB 3.0/3.1 readers or ones clearly marked as high-speed. Avoid no-name cheap card readers. The reader is often the bottleneck, so upgrading can significantly increase SD card speeds.
Properly formatting the SD card can also help in some cases. We covered that in more detail earlier in this guide.
Summary
In summary, the MB/s or megabytes per second rating on an SD card refers to the maximum read and write speeds that the card is capable of. Higher MB/s ratings indicate faster data transfer speeds.
SD card speed classes like Class 10 also provide a general indication of the minimum speed capabilities. However, the exact MB/s speed can vary widely among cards with the same speed class.
The actual real-world speeds of an SD card depend on many factors like the device used, file sizes, and data access patterns. Cards may not always achieve the maximum advertised speeds.
Faster SD card speeds are most beneficial and often required for recording and transferring large files like high resolution photos, 4K video, or high bitrate audio. For basic photo storage or other light uses, a slower card may be sufficient.
To get the optimal performance for your needs, it’s best to compare MB/s ratings and read speed test results before choosing the right SD card. Proper device compatibility and following best practices for improving SD card speed are also important.