Editing SD card (section)

== Card speed class ratings ==
'''Speed Class ratings''' are defined by the SDA to indicate the minimum data transfer performance of SDHC and SDXC memory cards, particularly in terms of sustained sequential write speed. These ratings are especially important for applications such as video recording, which require consistent throughput.<ref name="sda_speed_class" />

Where speed classes overlap, manufacturers often display multiple symbols on the same card to indicate compatibility with different host devices and standards.
{| class="wikitable" style="text-align: center;"
|+ Comparison of card speed class ratings<ref>{{Cite web |date=2020-12-11 |title=Speed Class {{!}} SD Association |url=https://www.sdcard.org/developers/sd-standard-overview/speed-class/ |access-date=2025-01-07 |website=www.sdcard.org |language=en-US |archive-date=4 January 2025 |archive-url=https://web.archive.org/web/20250104004951/https://www.sdcard.org/developers/sd-standard-overview/speed-class/ |url-status=live }}</ref>
! scope="col" rowspan="2" | {{Abbr|Min speed|Minimum serial data writing speed}}
! scope="colgroup" colspan="4" | Speed Class
! scope="colgroup" colspan="4" | [[Video format]]{{Efn|The necessary recording and playback speed class requirements may vary by device.}}
|-
! scope="col" | Original
! scope="col" | UHS
! scope="col" | Video
! scope="col" | SD Express
! scope="col" | [[Standard-definition television|SD]]
! scope="col" | [[High-definition video|HD]]
! scope="col" | [[4K resolution|4K]]
! scope="col" | [[8K resolution|8K]]
|-
! scope="row" | {{Nowrap|2 MB/s}}
| style="background:white" | {{Nowrap|Class 2 (C2)}}<br/>[[File:SDHC Speed Class 2.svg|x20px]]
| rowspan="3" {{N/a}}
| rowspan="2" {{N/a}}
| rowspan="7" {{N/a}}
| rowspan="11" {{Yes}}
| {{No}}
| rowspan="2" {{No}}
| rowspan="4" {{No}}
|-
! scope="row" | {{Nowrap|4 MB/s}}
| style="background:white" | {{Nowrap|Class 4 (C4)}}<br/>[[File:SDHC Speed Class 4.svg|x20px]]
| rowspan="10" {{Yes}}
|-
! scope="row" | {{Nowrap|6 MB/s}}
| style="background:white" | {{Nowrap|Class 6 (C6)}}<br/>[[File:SDHC Speed Class 6.svg|x20px]]
| style="background:white" | {{Nowrap|Class 6 (V6)}}<br/>[[File:SD Video Speed Class 6.svg|x20px]]
| rowspan="9" {{Yes}}
|-
! scope="row" | {{Nowrap|10 MB/s}}
| style="background:white" | {{Nowrap|Class 10 (C10)}}<br/>[[File:SDHC Speed Class 10.svg|x20px]]
| style="background:white" | {{Nowrap|Class 1 (U1)}}<br/>[[File:SD UHS Speed Class 1.svg|x20px]]
| style="background:white" | {{Nowrap|Class 10 (V10)}}<br/>[[File:SD Video Speed Class 10.svg|x20px]]
|-
! scope="row" | {{Nowrap|30 MB/s}}
| rowspan="7" {{N/a}}
| style="background:white" | {{Nowrap|Class 3 (U3)}}<br/>[[File:SD UHS Speed Class 3.svg|x20px]]
| style="background:white" | {{Nowrap|Class 30 (V30)}}<br/>[[File:SD Video Speed Class 30.svg|x20px]]
| rowspan="7" {{Yes}}
|-
! scope="row" | {{Nowrap|60 MB/s}}
| rowspan="6" {{N/a}}
| style="background:white" | {{Nowrap|Class 60 (V60)}}<br/>[[File:SD Video Speed Class 60.svg|x20px]]
|-
! scope="row" | {{Nowrap|90 MB/s}}
| style="background:white" | {{Nowrap|Class 90 (V90)}}<br/>[[File:SD Video Speed Class 90.svg|x20px]]
|-
! scope="row" | {{Nowrap|150 MB/s}}
| rowspan="4" {{N/a}}
| style="background:white" | {{Nowrap|Class 150 (E150)}}<br/>[[File:SD Express Class 150.svg|41x41px]]
|-
! scope="row" | {{Nowrap|300 MB/s}}
| style="background:white" | {{Nowrap|Class 300 (E300)}}<br/>[[File:SD Express Class 300.svg|41x41px]]
|-
! scope="row" | {{Nowrap|450 MB/s}}
| style="background:white" | {{Nowrap|Class 450 (E450)}}<br/>[[File:SD Express Class 450.svg|41x41px]]
|-
! scope="row" | {{Nowrap|600 MB/s}}
| style="background:white" | {{Nowrap|Class 600 (E600)}}<br/>[[File:SD Express Class 600.svg|41x41px]]
|}

=== Original speed class (C) ===
The original speed class ratings—Class 2, 4, 6, and 10—specify minimum sustained write speeds of 2, 4, 6, and 10&nbsp;MB/s, respectively. Class 10 cards assume a non-fragmented file system and use the High Speed bus mode.<ref name="SDHCspec" /> These are represented by a number encircled with a "C" (e.g., C2, C4, C6 and C10).

=== UHS speed class (U) ===
Ultra High Speed (UHS) speed class ratings—U1 and U3—specify minimum sustained write speeds of 10 and 30&nbsp;MB/s, respectively. These classes are represented by a number inside a "U" and are designed for high-bandwidth tasks such as [[4K video]] recording.<ref>{{cite web |title=NEW SDXC AND SDHC MEMORY CARDS SUPPORT 4K2K VIDEO |url=https://www.sdcard.org/home/New_SDXC_and_SDHC_Memory_Cards_Now_Support_4K2K_Video_11-6-2013_FINAL_2.pdf |url-status=live |archive-url=https://web.archive.org/web/20131113000144/https://www.sdcard.org/home/New_SDXC_and_SDHC_Memory_Cards_Now_Support_4K2K_Video_11-6-2013_FINAL_2.pdf |archive-date=2013-11-13 |access-date=2013-11-13 |publisher=SD Association}}</ref>

=== Video speed class (V) ===
Video speed class ratings—V6, V10, V30, V60, and V90—specify minimum sustained write speeds of 6, 10, 30, 60, and 90&nbsp;MB/s, respectively.<ref name="sda_video_speed" /><ref name="sda_speed_class" /><ref name="sda_speed_class_consumer">{{cite web |date=11 December 2020 |title=Speed Class Standards for Video Recording – SD Association |url=https://www.sdcard.org/consumers/choices/speed_class/index.html |url-status=live |archive-url=https://web.archive.org/web/20160407123207/https://www.sdcard.org/consumers/choices/speed_class/index.html |archive-date=7 April 2016 |access-date=28 April 2016 |website=sdcard.org}}</ref> These classes are represented by a stylized "V" followed by the number, were introduced to support high-resolution formats like 4K and [[Ultra-high-definition television|8K]], and to align with the performance characteristics of [[Multi-level cell|MLC NAND flash]] memory.<ref name="sd50_video_speed" />

=== SD Express Speed Class (E) ===
SD Express speed class ratings—E150, E300, E450, and E600—specify minimum sustained write speeds of 150, 300, 450, and 600&nbsp;MB/s, respectively.<ref name="SDver9.1" /> These classes are represented by a stylized "E" followed by the number, enclosed in a rounded rectangle. They are designed for data-intensive applications such as large-scale video processing, real-time analytics, and software execution.<ref name="SDver9.1" />

=== "×" rating ===
{{main|CD and DVD writing speed}}

{| class="wikitable" style="background:white; float:right; margin-left:1em;"
|-
! valign="top" | Rating
! valign="top" | Approx.<br />(MB/s)
! valign="top" | Comparable<br />speed class
|- style="text-align:center;"
| 16×
| 2.34
| [[File:SDHC Speed Class 2.svg|28px]] (13×)
|- style="text-align:center;"
| 32×
| 4.69
| [[File:SDHC Speed Class 4.svg|28px]] (27×)
|- style="text-align:center;"
| 48×
| 7.03
| [[File:SDHC Speed Class 6.svg|28px]] (40×)
|- style="text-align:center;"
| 100×
| 14.6
| [[File:SD Class 10.svg|28px]] (67×)
|}

Initially, some manufacturers used a "×" rating system based on the speed of a standard [[Optical disc drive|CD-ROM drive]] (150&nbsp;kB/s or 1.23&nbsp;[[Mbit/s]]),{{efn|reference=1 KB = 1024 B}} but this approach was inconsistent and often unclear. It was later replaced by standardized Speed Class systems that specify guaranteed minimum write speeds.<ref name="SDHCspec" /><ref name="sda_speed_class_consumer" /><ref name="sda_logos_2017">{{Cite web |title=SD Standards Brochure 2017 |url=https://www.sdcard.org/consumers/pdf/2017SDA_brochure_eng.pdf |url-status=dead |archive-url=https://web.archive.org/web/20170330174639/https://www.sdcard.org/consumers/pdf/2017SDA_brochure_eng.pdf |archive-date=2017-03-30 |access-date=2017-03-29}}</ref><ref>{{cite web |title=Flash Memory Cards and X-Speed Ratings |url=http://www.kingston.com/us/flash/x-speed |url-status=dead |archive-url=https://web.archive.org/web/20170702204220/http://www.kingston.com/us/flash/x-speed |archive-date=2017-07-02 |access-date=2017-08-05 |publisher=Kingston}}</ref>

=== Real-world performance ===
Speed Class ratings guarantee minimum write performance but do not fully describe real-world speed, which can vary based on factors such as [[file fragmentation]], [[write amplification]] due to flash memory management, controller retry operations for soft error correction and sequential vs. random write patterns.

In some cases, cards of the same speed class may perform very differently. For instance, random small-file write speeds can be significantly lower than sequential performance. A 2012 study found some Class 2 cards outperformed Class 10 cards in random writes.<ref name="necstudy" /> Another test in 2014 reported a 300-fold difference in small-write performance across cards, with a Class 4 card outperforming higher-rated cards in certain use cases.<ref name="Gough2014" />