The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947. The "M" designation for metric screws indicates the nominal outer diameter of the screw thread, in millimetres. This is also referred to as the "major" diameter in the information below. It indicates the diameter of smooth-walled hole that an externally threaded component (e.g. on a bolt) will pass through easily to connect to an internally threaded component (e.g. a nut) on the other side. For example, an M6 screw has a nominal outer diameter of 6 millimetres and will therefore be a well-located, co-axial fit in a hole drilled to 6 mm diameter.
Basic profile
The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter, D (Dmaj in the diagram), and its pitch, P. ISO metric threads consist of a symmetric V-shaped thread. In a cross-section along the thread axis, the sides of the V have an angle of 60° to each other. The thread depth is 0.54125 × pitch. The outermost 1⁄8 and the innermost 1⁄4 of the height H of the V-shape are cut off from the profile. The relationship between the height H and the pitch P is found using the following equation where θ is half the included angle of the thread, in this case 30°:
H = 1 tan θ ⋅ P 2 = 3 2 ⋅ P ≈ 0.8660 ⋅ P {\displaystyle H={\frac {1}{\tan \theta }}\cdot {\frac {P}{2}}={\frac {\sqrt {3}}{2}}\cdot P\approx 0.8660\cdot P}
or
P = 2 tan θ ⋅ H = 2 3 ⋅ H ≈ 1.1547 ⋅ H {\displaystyle P=2\tan \theta \cdot H={\frac {2}{\sqrt {3}}}\cdot H\approx 1.1547\cdot H}
Because only 5⁄8 of this height is cut, the difference between major and minor diameters is 5⁄4 × 0.8660 × P = 1.0825 × P, so the tap drill size can be approximated by subtracting the thread pitch from the major diameter. In an external (male) thread (e.g. on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g. in a nut), the major and minor diameters are minimum dimensions; therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj. In practice this means that one can measure the diameter over the threads of a bolt to find the nominal diameter Dmaj, and the inner diameter of a nut is Dmin. The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as
D min = D maj − 2 ⋅ 5 8 ⋅ H = D maj − 5 3 8 ⋅ P ≈ D maj − 1.082532 ⋅ P D p = D maj − 2 ⋅ 3 8 ⋅ H = D maj − 3 3 8 ⋅ P ≈ D maj − 0.649519 ⋅ P {\displaystyle {\begin{aligned}D_{\text{min}}&=D_{\text{maj}}-2\cdot {\frac {5}{8}}\cdot H=D_{\text{maj}}-{\frac {5{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-1.082532\cdot P\\[3pt]D_{\text{p}}&=D_{\text{maj}}-2\cdot {\frac {3}{8}}\cdot H=D_{\text{maj}}-{\frac {3{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-0.649519\cdot P\end{aligned}}}
Tables of the derived dimensions for screw diameters and pitches defined in ISO 261 are given in ISO 724.
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