Choosing an SD or microSD card is less about finding the highest number on the package and more about matching sustained write speed, bus compatibility, capacity, and workload. This guide explains UHS, U1/U3, V30/V60/V90, A1/A2, and capacity ratings, then shows how to estimate the card you need for a camera, drone, phone, or handheld console.
Overview
Memory-card labels describe different characteristics, so they should not be read as one combined speed score. A card can have a fast bus rating but still be unsuitable for a demanding video mode if its sustained write-class rating is too low. Conversely, a high-speed video card may not be the best value for storing games or everyday phone files.
For most buying decisions, use this order:
- Check the device manual for supported card type, capacity, and speed class.
- Identify the workload: continuous video, burst photography, app storage, games, or general files.
- Choose the required sustained write rating, such as V30, V60, or V90.
- Choose enough capacity for a session, trip, or replacement cycle.
- Compare price per usable gigabyte only among cards that meet the first four requirements.
The practical meaning of common markings is summarized below:
| Marking | What it describes | Practical use |
|---|---|---|
| Class 10 | A minimum sequential write-speed category of 10 MB/s | Basic recording and general storage when the device requires no higher class |
| U1 | UHS Speed Class with a 10 MB/s minimum sustained write category | Lower-bitrate video and everyday storage |
| U3 | UHS Speed Class with a 30 MB/s minimum sustained write category | Many demanding photo and video modes, if supported by the device |
| V30 | Video Speed Class with a 30 MB/s minimum sustained write category | A common starting point for compatible 4K video modes |
| V60 | Video Speed Class with a 60 MB/s minimum sustained write category | Higher-bitrate video or devices that specifically require V60 |
| V90 | Video Speed Class with a 90 MB/s minimum sustained write category | The most demanding supported video workflows |
| A1 or A2 | Application-performance categories for random access | Apps and game data on devices that support application performance features |
These are minimum performance categories under specified testing conditions, not guaranteed transfer speeds in every camera, reader, phone, or computer. A device may also limit performance even when the card has a higher rating.
How to estimate
For video, begin with the recording bitrate shown in the camera, drone, or phone specifications. Convert megabits per second to megabytes per second by dividing by eight:
Approximate data rate in MB/s = bitrate in Mb/s ÷ 8
Then select a card whose sustained write class comfortably exceeds that rate. The video class is often easier to interpret than a headline read-speed number because it is specifically intended to communicate sustained recording performance. Leave headroom for variation, metadata, multiple streams, and the fact that real-world performance depends on the device.
To estimate capacity, multiply the recording rate by the planned recording time:
Approximate storage in GB = bitrate in Mb/s × seconds ÷ 8,000
This uses decimal gigabytes and provides a planning estimate. Formatting overhead, file-system space, existing files, and the difference between advertised and usable capacity mean that a card should not be filled to its label capacity.
For still photography, estimate the size of one image and multiply it by the number of images you expect to capture before transferring files. RAW files, high-resolution JPEGs, and simultaneous RAW-plus-JPEG recording can produce very different storage needs. Burst shooting also makes write performance important: a card that handles occasional photos may slow down when the camera’s buffer is full.
For phones and handheld consoles, capacity is usually the first constraint. A1 and A2 markings can be relevant when the device runs apps from the card, but they do not automatically make a card faster in every phone or console. A2 performance depends on device and host support, so verify compatibility before paying extra for that label.
Inputs and assumptions
Device compatibility
Confirm whether the device accepts full-size SD, microSD, microSDHC, microSDXC, or another format. An adapter changes the physical size but does not change the card’s underlying capabilities. Also check the maximum supported capacity and file-system requirements. A card that works in one device may not be recognized by an older camera, drone, or console.
Bus speed versus speed class
UHS-I and UHS-II describe the card interface or bus. They are not the same as U1 and U3, which describe a UHS Speed Class. A UHS-II card may require a compatible host and reader to reach its potential, while a UHS-II reader cannot make a UHS-I card behave like a UHS-II card. Treat the device’s stated bus support as a ceiling, not a promise of a particular real-world transfer rate.
Capacity and endurance
Choose capacity based on how often you can transfer or replace files. A larger card reduces card swapping, but it also concentrates more photos or footage on one removable device. For important work, copy files to another destination rather than treating the card as a backup. A practical workflow might include a laptop, external SSD, or NAS; the restore-verification checklist for computer backups explains why a copy is not fully useful until it has been checked.
Authenticity and condition
Buy from a reputable seller and be cautious of unusually cheap cards, inconsistent packaging, or listings that combine unrelated speed and capacity claims. Counterfeit cards can report a false capacity and begin overwriting earlier files once the real storage limit is reached. Test a new card with a suitable capacity-verification tool before trusting it with irreplaceable footage. Format the card in the device when the manufacturer recommends it, and safely eject it before removal.
Worked examples
4K camera or drone video
Suppose a camera records at 200 Mb/s. The estimated write rate is 25 MB/s. A V30 card provides a stated 30 MB/s minimum sustained write category, but selecting a higher class may provide additional margin if the camera supports it. For 30 minutes of recording, the capacity estimate is 200 × 1,800 ÷ 8,000, or about 45 GB. In practice, choose more than that to allow for multiple clips, setup footage, still photos, and usable-space differences.
High-resolution burst photography
Here, the relevant questions are the size of each image, the number of images in a burst, and how quickly the camera clears its buffer. A card with sufficient sustained write performance can reduce the time the camera remains busy after a burst. Check the camera manual for recommended card classes, especially if it records RAW files or high-frame-rate video as well as stills.
Handheld console or phone
Start with the device’s supported capacity and file-system limits, then estimate the space needed for installed games, offline media, photos, and future additions. If the card will hold applications, look for the required application-performance category and confirm that the host device supports it. Do not choose V90 solely because it has a larger number; that rating may add cost without helping a workload dominated by game storage.
For a broader capacity decision involving games and other files, compare the card’s role with the storage guidance in this console and PC expansion guide. If your workflow involves frequent large transfers, an external SSD may be a better companion than repeatedly moving files through a memory card reader; see the portable SSD comparison for photo and video work.
When to recalculate
Revisit the decision whenever the device, recording mode, or workflow changes. Recalculate if you move from standard video to a higher-bitrate or higher-frame-rate mode, enable RAW capture, start recording longer trips, install games with larger files, or switch to a device with a different capacity limit.
Pricing also changes the value calculation. Compare the total price divided by usable capacity, but only after filtering for the required speed class and compatibility. Recheck prices when buying several cards, preparing for a major shoot, or replacing cards as part of routine media management. The cheapest card is not a bargain if it fails verification or cannot sustain the device’s recording mode.
Before buying, use this short checklist:
- Record the device’s supported format, maximum capacity, bus, and required speed class.
- Write down the actual video bitrate or estimated photo and game-storage needs.
- Calculate the minimum capacity, then add room for existing files and transfer schedules.
- Choose V30, V60, V90, or another required class based on sustained write needs—not peak read speed.
- Confirm whether A1 or A2 matters for the host device.
- Buy from a reliable source, verify a new card’s capacity, and maintain a separate backup of important files.
Use the same process whenever a new card or device enters your setup. Standards and product pricing evolve, but matching the workload to the device’s documented requirements remains the most reliable way to choose the right SD or microSD card.
