What is the difference between Nominal Diameter and Actual Diameter of a screw?

In simple terms:

Nominal Diameter = the "label size" of the screw

Diameter (actual) = the real measured thickness

People often use them interchangeably in daily speech, but they are technically different.

• This is what we call M-number: M3, M4, M5, M6, etc.
• It is a standard designation that indicates the size class of the screw.
• It is a theoretical, nominal value, not an exact measured dimension.

For example:

For an M6 screw, the nominal diameter is 6 mm.

The real size measured with calipers is usually slightly smaller than the nominal diameter.

Take M6 as an example:

• Nominal diameter: 6 mm
• Actual major diameter of the thread: about 5.8–5.9 mm

Reason:

Screws have crests and roots. To fit smoothly into a nut, the actual thread is made slightly smaller than its nominal size.

• Nominal Diameter: The labeled size; M-number directly equals millimeters.
• Actual Diameter: The real measured thickness, slightly smaller.

When buying screws, just refer to the nominal diameter — the small difference in actual size does not matter.