How are 1.8" and 2.5" hard drives measured?

[kumbaya]

Wikipedia article on hard drives:

"2½ inch" drive: (2.75 in x 0.374 in x 3.945 in = 69.85 mm x 9.5 mm x 100 mm)

"1.8 inch" drive: (54 mm × 8 mm × 71 mm)

I can't see how the 2.5" and 1.8" descriptions are derived..

What do the 2.5" and 1.8" refer to??

...driving me mad...



Ooops, saw this just now in the same article:

The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are for more recent form factors specified in millimeters), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.

Hate that companies can just make up stuff when we think it's smaller than it is, but when it's TVs and monitors they tell us that's 60" when it's the diagonal measurement!!!!

Corporate pathology

 

[flopticalcube]

Platter diameter.

http://www.pcguide.com/ref/hdd/op/mediaSize-c.html

Lots of popups on that site so I will quote the salient info:

Platter Size

The size of the platters in the hard disk is the primary determinant of its overall physical dimensions, also generally called the drive's form factor; most drives are produced in one of the various standard hard disk form factors. Disks are sometimes referred to by a size specification; for example, someone will talk about having a "3.5-inch hard disk". When this terminology is used it usually refers to the disk's form factor, and normally, the form factor is named based on the platter size. The platter size of the disk is usually the same for all drives of a given form factor, though not always, especially with the newest drives, as we will see below. Every platter in any specific hard disk has the same diameter.

The first PCs used hard disks that had a nominal size of 5.25". Today, by far the most common hard disk platter size in the PC world is 3.5". Actually, the platters of a 5.25" drive are 5.12" in diameter, and those of a 3.5" drive are 3.74"; but habits are habits and the "approximate" names are what are commonly used. You will also notice that these numbers correspond to the common sizes for floppy disks because they were designed to be mounted into the same drive bays in the case. Laptop drives are usually smaller, due to laptop manufacturers' never-ending quest for "lighter and smaller". The platters on these drives are usually 2.5" in diameter or less; 2.5" is the standard form factor, but drives with 1.8" and even 1.0" platters are becoming more common in mobile equipment.

 

[kumbaya]

thanks!