Units for Form Factor? A Quick Guide

This guide explains what units are used for form factor, a term that describes the physical size, shape, and mounting specifications of hardware components like motherboards, power supplies, and drives. You’ll learn the standard units for each type and how to interpret them when choosing parts. The answer depends on the component, but the most common units are millimeters (mm) for dimensions, grams (g) or kilograms (kg) for weight, and standard named sizes like ATX or M.2 that imply specific measurements.

Formula

The calculator above converts a rectangular component’s dimensions into area, which is often used to compare form factors. The formula is:

Area = Length × Width

• Length (d) – the longer side of the board, in millimeters (mm).
• Width (w) – the shorter side, also in millimeters (mm).
• Area – the result in square millimeters (mm²). To convert to square inches (in²), divide by 645.16 (since 1 inch = 25.4 mm, and 25.4² = 645.16).

Worked Example

Using the defaults (length = 305 mm, width = 244 mm, which are the ATX motherboard dimensions):

1. Multiply length by width: 305 × 244 = 74,420 mm².
2. Convert to square inches: 74,420 ÷ 645.16 ≈ 115.35 in².

The calculator shows 74,420 mm² and 115.35 in², matching the formula.

What Is Form Factor?

Form factor is a set of specifications that define a component’s physical dimensions, mounting hole locations, connector positions, and sometimes thermal or power requirements. It ensures interchangeability—so a motherboard that follows the ATX form factor will fit in any ATX case, and a power supply with the SFX form factor will fit in an SFX-compatible case.

Units vary by what is being measured:

• Dimensions – almost always millimeters (mm) in official specs, even in countries that use inches for general measurements. For example, a standard ATX motherboard is 305 × 244 mm.
• Weight – grams (g) or kilograms (kg) for most components, though ounces (oz) and pounds (lb) appear in some retail listings.
• Volume (for cases) – liters (L) or cubic feet (ft³).
• Electrical specs – watts (W) for power, volts (V) for voltage, amps (A) for current.

Common Form Factors and Their Units

Here are the typical units you’ll encounter for each type of component:

• Motherboards – dimensions in mm. Examples: ATX (305×244 mm), Micro-ATX (244×244 mm), Mini-ITX (170×170 mm).
• Power supplies – dimensions in mm (e.g., ATX PS/2: 150×86×140 mm), plus wattage output (W).
• Drives – drive bay sizes in inches (3.5″ for hard drives, 2.5″ for SSDs), but the physical dimensions are given in mm.
• Expansion cards – height and length in mm, plus bracket height (e.g., full-height, low-profile).
• M.2 drives – dimensions in mm, like 2280 (22 mm wide, 80 mm long).
• Cases – external dimensions in mm, internal volume in liters.

Why Millimeters Are the Standard

The electronics industry uses the metric system for almost all mechanical specifications, regardless of the country. This is because most manufacturing happens in metric-based regions, and international standards (like those from the IEC or JEDEC) are written in millimeters. Even in the US, where inches are common for consumer products, you’ll find that motherboard manuals and case specs list dimensions in mm first, with inches in parentheses.

How to Choose the Right Units

When comparing components, always check the unit used in the spec sheet. A common mistake is mixing up inches and millimeters, especially for drive bay sizes (3.5 inches vs. 3.5 mm would be wildly different). Here are some tips:

• If a spec says “length: 200”, ask what unit—if it’s a motherboard, it’s likely mm; if it’s a case, it could be mm or cm.
• Use a conversion factor: 1 inch = 25.4 mm exactly. To convert mm to inches, divide by 25.4.
• For area, remember that 1 in² = 645.16 mm² (since 25.4² = 645.16).

Common Mistakes and Misconceptions

One common error is assuming that a smaller number means a smaller component when the unit differs. For example, a case volume of 40 liters is much larger than 1.4 cubic feet (which is about 39.6 L), but if you only look at the number 1.4 vs 40, you might think the latter is larger. Another misconception is that “form factor” only refers to size—it also includes mounting holes, connector placement, and airflow patterns.

Another mistake: using the wrong unit for screw threads. Motherboard standoffs use #6-32 UNC screws in the US, but M3 screws in metric countries. The form factor spec includes screw type, which is a unit of its own.

Limits of the Formula

The area calculation above is purely geometric. It doesn’t account for the actual shape (many boards have cutouts or protrusions), nor does it tell you if a component fits in a case—that depends on the case’s internal clearance, which is often different from the external dimensions. Always check the case’s max motherboard size and GPU length in mm, not just the board area.

Frequently Asked Questions

Q: Is form factor measured in inches or millimeters?
Official specs use millimeters for dimensions, but drive bay sizes are traditionally in inches (3.5″, 2.5″). For example, a 3.5″ hard drive is actually 101.6 mm wide, 146.99 mm long, and 26.1 mm thick.

Q: What does ATX stand for?
ATX stands for Advanced Technology eXtended. It’s a motherboard form factor introduced by Intel in 1995. The current standard dimensions are 305×244 mm, but earlier versions were slightly different.

Q: How do I measure my case’s form factor?
Measure the mounting hole positions for the motherboard, not just the overall case size. The distance between standoff holes is what determines compatibility. For a standard ATX board, the holes are in a specific pattern that you can find in the case manual.

Q: Can I use a larger form factor in a smaller case?
No, not usually. A larger board physically won’t fit. Smaller boards (like Mini-ITX) can fit in larger cases if the mounting holes align, but the case must have the right standoff positions. Always check the case’s supported motherboard sizes.

Q: What is the difference between form factor and size?
Size is just dimensions; form factor includes dimensions plus mounting, connectors, and sometimes electrical characteristics. For example, two power supplies can be the same size but have different form factors if their screw holes or connector layouts differ.

Related

• Gray Code Calculator: Binary Conversion Tool
• Disc Capacitor Code Calculator & Decoder
• E/M Code Calculator: How It Works

in d | Board length | mm | 305 | 0
in w | Board width | mm | 244 | 0
out area | Board area | mm² | d * w | 0
out area_in | Board area | in² | (d * w) / 645.16 | 2