Hard disk drives have been around since the 1950s, with IBM launching the first commercial hard drive in 1956 [1]. Early hard drives were enormous in physical size and very limited in capacity. Over the decades, hard drives have progressively gotten smaller while exponentially increasing in storage capacity.
Hard drives come in different physical form factors, specifying their overall dimensions and connectors. The 3.5-inch form factor, first introduced by IBM in 1983, became the standard desktop hard drive size in the late 1980s and is still widely used today. 3.5-inch drives provide a good balance of storage capacity and physical size for desktop computers. Other common form factors include 2.5-inch for laptops and 1-inch for smaller devices.
What is a 3.5-inch Hard Drive?
The 3.5-inch form factor has been the standard size for desktop computer hard drives since the early 1990s. Despite the name, 3.5-inch hard drives are not actually 3.5 inches wide. The “3.5-inch” measurement refers to the size of the disk platters inside the hard drive enclosure. 3.5-inch hard drives have the following exterior dimensions:
- Width: 101.6 mm
- Length: 146 mm
- Height: Varies, typically between 9.5 mm to 26.1 mm
So in metric units, a typical 3.5-inch hard drive is about 10 cm wide, 15 cm long, and 1-2.5 cm tall. The 3.5-inch platters inside the drive are actually 4 inches (about 10 cm) in diameter.
Drive Thickness
3.5 inch hard drives come in a few common thicknesses, with 15mm and 9.5mm being the most popular heights today. According to Seagate, 15mm drives can have a height up to 26.11mm, while 9.5mm drives max out at 19.99mm.
The 26.11mm dimension for thicker 15mm drives was common in older models, but most modern 15mm drives are around 20.2mm thick. The 9.5mm drives are designed to fit in slimmer bays and cases where vertical space is limited.
According to Wikipedia, while the 3.5 inch form factor technically allows for drives up to 42mm thick, such large drives have not been manufactured for many years now. Most drives today come in the thinner 9.5mm to 15mm size range.
So in summary, the two most common 3.5 inch hard drive thicknesses today are:
- 15mm – Typically 20.2mm thick
- 9.5mm – Up to 19.99mm thick
Thicker and thinner models exist, but are less common than these two drive heights.
15mm Models
15mm hard drives are commonly used in desktop PCs to provide more internal space for components. The thicker design allows for larger platters and motors inside the drive, enabling higher capacities. While most 3.5″ hard drives are 9.5mm thick, larger 15mm models have become popular in recent years as data storage needs have grown.
The extra 5.5mm of thickness may not seem like much, but it provides around 50% more internal volume for drive mechanisms. This allows manufacturers to engineer hard drives with up to 8TB or more of capacity in the 3.5″ form factor. For example, Seagate’s BarraCuda Pro desktop HDDs go up to 10TB for 15mm models, while the 9.5mm versions max out at 5TB.
The downside is that 15mm drives require more vertical clearance to fit properly in desktop PC cases. Many modern mid and full tower cases provide support for 15mm models, but smaller form factors may only have room for standard 9.5mm drives. So when selecting a high capacity 3.5″ hard drive, it’s important to verify your PC case can accommodate the taller 15mm height.
9.5mm Models
9.5mm is a common thickness for 2.5″ hard drives. These thinner models are often used in external enclosures since they can fit into more compact spaces. The 9.5mm height allows the drive to be used in some laptops and portable devices where space is limited.
For example, a 2TB 2.5″ 9.5mm hard drive costs around $127, while a similar 1TB drive is around $65. The thinner size comes at a premium but allows for larger capacity drives to be used where ultra-slim models are required (AnandTech).
Overall, 9.5mm 2.5″ drives provide a good balance of capacity and physical size, making them a popular choice for external and mobile storage needs.
Other Heights
While 9.5mm and 15mm are the most common thicknesses for 3.5-inch hard drives, some other heights exist as well. For example, some older 3.5-inch drives used a height of 12.5mm. According to the Wikipedia page on disk drive form factors, some HDD manufacturers have released drives as slim as 7mm in height [1]. These ultra-slim drives provide a minimized form factor while still using the standard 3.5-inch mounting points. However, the reduced thickness also typically corresponds with lowered storage capacity. While 15mm drives can offer multi-terabyte capacities, 7mm models often max out at 500GB to 1TB. Nonetheless, the small size makes them convenient for use in compact PC builds where space is at a premium.
Choosing Drive Thickness
The thickness of the 3.5-inch hard drive you choose depends on your intended use case. The most common thicknesses for 3.5-inch hard drives are 15mm and 9.5mm.
15mm hard drives are best suited for desktop computers, where there is enough clearance and space to accommodate the larger drive height. 15mm drives tend to have higher storage capacities compared to thinner 9.5mm models. For example, common 3.5-inch hard drives up to 10TB are 15mm thick [1].
9.5mm thick 3.5-inch hard drives are recommended for external enclosures and some small form factor cases that have tighter clearance. 9.5mm models also work well for laptop hard drive upgrades using a HDD caddy to replace the optical drive. Though 9.5mm drives generally max out at 2TB for a 3.5-inch form factor [2].
So in summary, if you need maximum storage capacity in a desktop PC, go with a thicker 15mm drive. But for external use or tight clearance cases, a 9.5mm high model is preferable.
Height Compatibility
When choosing a 3.5″ hard drive, it’s important to consider the height or thickness and ensure it will be compatible with your computer case or external enclosure. There are a few key points to keep in mind:
Check chassis or enclosure specs – Most computer cases and external drive enclosures that accept 3.5″ drives will indicate the maximum drive height supported. This is often listed in the specs as something like “supports up to 15mm HDDs.”
15mm may not fit smaller cases – While 15mm is the max thickness for standard 3.5″ drives, it does not fit all enclosures. Many smaller form factor cases only accept the slimmer 9.5mm models. Make sure to verify your specific case or enclosure can handle the full 15mm height if choosing a drive this thick.[1]
Some larger cases allow taller drives – On the flip side, some full tower or enterprise cases allow for taller specialty drives up to 26.1mm thick. However, 15mm remains the most common maximum.
Check for drive bay obstacles – Even if a case lists a drive thickness capacity, also look out for potential clearance issues around the drive bays, as cables or other components could limit the space. Always measure the actual open vertical space before purchasing any thicker HDD.
Thickness and Capacity
Thicker hard drives are able to accommodate more platters, which allows for increased storage capacity. As explained on Quora, higher capacity hard drives generally have more platters inside. For example, a 750GB laptop hard drive will be noticeably heavier than a 160GB model due to the increased number of platters. More platters means each one can be made thinner while still providing greater overall capacity.
Hard drive thickness is therefore closely tied to maximum potential storage space. Thinner 9.5mm drives are limited to 1-2 platters, restricting their capacity to under 1TB for laptop form factors. Meanwhile, 15mm drives have room for 3 or more platters and can offer multi-terabyte capacity. This explains why high capacity external portable drives meant for mass storage are thicker than basic internal laptop drives.
Conclusion
In summary, the most common thicknesses for 3.5-inch hard drives are 15mm and 9.5mm. 15mm drives were initially more popular, offering higher storage capacities. But over time, 9.5mm models have become more prevalent as capacities have increased and thinner drives are needed for ultra-slim laptops. While less common, 3.5-inch drives are also made in 12.5mm and 5mm heights for niche applications. When selecting a hard drive, you need to ensure it matches the height supported by your computer’s drive bay to avoid compatibility issues. Thinner 9.5mm drives sacrifice some capacity but can fit into the widest range of devices.