# RPM to MPH on outer edge of hard drive **URL:** \ **Category:** Factual Questions\ **Created:** [October 31, 2002, 6:36am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669 "2002-10-31T06:36:12Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![ScottHaneda](https://avatars.discourse-cdn.com/v4/letter/s/9d8465/32.png) [@ScottHaneda](https://boards.straightdope.com/u/ScottHaneda)\ **Post date:** [October 31, 2002, 6:36am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/1 "2002-10-31T06:36:12Z")
3.5" hard drive, spinning at 10,000 RPM’s, how many MPH at the outer edge of the platter? Can someone give me the calculation as well, thanks. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [October 31, 2002, 6:43am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/2 "2002-10-31T06:43:46Z")
Well, a 3.5" diameter means an 11" circumference, so a point on the out edge covers 110,000 inches/minute. = 0.029 mph ---
**Author:** ![ScottHaneda](https://avatars.discourse-cdn.com/v4/letter/s/9d8465/32.png) [@ScottHaneda](https://boards.straightdope.com/u/ScottHaneda)\ **Post date:** [October 31, 2002, 6:52am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/3 "2002-10-31T06:52:23Z")
No way, 10,000 revolutions per minute, if I were to hook a peanut to the drive, with some sticky stuff that released as soon as the drive spun up to 10,000 RMP’s, I am certain it would fling off at somewhere greater than 0.029 MPH, more like close to 100MPH is my estimation, but I would like to figure it out. ---
**Author:** ![Mikahw](https://avatars.discourse-cdn.com/v4/letter/m/e9bcb4/32.png) [@Mikahw](https://boards.straightdope.com/u/Mikahw)\ **Post date:** [October 31, 2002, 7:06am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/4 "2002-10-31T07:06:27Z")
110,000 inches/minute converts to 104Mph, if I’m doing my calculations right. let’s see here, you multiply by 60 to turn minutes to hours, divide by 12 to turn inches to feet, then divide by 5280 to turn feet to miles. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [October 31, 2002, 7:06am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/5 "2002-10-31T07:06:37Z")
110,000 in/min is about 104 mph. **Bryan** , you have to _multiply_ by 60 min/hr, not divide. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [October 31, 2002, 7:13am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/6 "2002-10-31T07:13:20Z")
11 inches circumference x7200 rpm = 79k inches/minute= 4.74 million inches/hour 1 mph = 63k inches/hour total about 75 miles an hour. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [October 31, 2002, 7:38am UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/7 "2002-10-31T07:38:32Z")
Oops. Y’know, I thought that result was kinda low. ---
**Author:** ![scotth](https://avatars.discourse-cdn.com/v4/letter/s/b9e5f3/32.png) [@scotth](https://boards.straightdope.com/u/scotth)\ **Post date:** [October 31, 2002, 2:45pm UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/8 "2002-10-31T14:45:24Z")
[hijack] This thread is pretty useful to show just how damn amazing the technology is getting in equipment you can buy for $100 or there abouts. Consider that the magnetic read head sits practically on top of the platter whizzing by at 60-100mph (depending on the drive). Next consider how tightly packed those bits are on the platter. These bits are laid down at 100,000 bits per linear inch or higher(some of the drivers are approaching 500,000 bits). Imagine a strip laid out on the side of a highway that placed 100,000 1’s or 0’s per inch of strip. Now imagine driving past that highway speeds and never missing a digit for miles on end. It makes the mind boggle. [/hijack] ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [October 31, 2002, 5:49pm UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/9 "2002-10-31T17:49:39Z")
Note that 3.5" is the nominal size. The actual size may be more like [3.74"](http://www.pcguide.com/ref/hdd/op/mediaSize-c.html). Makes a diff. of about +7% in the speed. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [October 31, 2002, 5:52pm UTC](https://boards.straightdope.com/t/rpm-to-mph-on-outer-edge-of-hard-drive/134669/10 "2002-10-31T17:52:48Z")
After this thread, I now know why so many countries went metric. 🙂