Pitch Diameter
Calculate ISO metric thread pitch diameter for external and internal threads using d₂ = d − 0.6495 × P. Free calculator with tolerance classes (6g, 6H) and sizing charts.
About This Calculator
About the Pitch Diameter Calculator
This calculator helps engineers, machinists, manufacturers, and DIY enthusiasts determine the correct pitch diameter for ISO metric screw threads. The pitch diameter is a critical dimension that determines how well external threads (bolts, screws) fit into internal threads (nuts, tapped holes). Using the industry-standard formula d₂ = d − 0.6495 × P, the calculator computes the basic pitch diameter from the major diameter and thread pitch, then applies tolerance class adjustments per ISO 965-1.
The calculator supports both external and internal threads with selectable tolerance grades (3-9 for external, 4-8 for internal) and tolerance positions (e, f, g, h for external; G, H for internal). The comparison chart visualizes the basic, maximum, and minimum pitch diameters, while the sizing chart shows how pitch diameter scales across common metric thread sizes (M6 to M64) at the selected pitch. Understanding pitch diameter is essential for proper fastener selection, quality control, and ensuring threaded connections meet design specifications for strength and reliability.
Regional Notes
US: While inch-system threads (UN, UNF, UNC) are common, ISO metric threads per ASTM F568 are widely used in automotive, aerospace, and industrial applications. Tolerance classes follow ASME B1.13M which aligns with ISO 965-1.
UK: ISO metric threads are the standard for all general engineering applications per BS 3643. The most common tolerance class is 6g for bolts and 6H for nuts, providing a medium-fit for most industrial and construction uses.
India: IS 4218 (Parts 1-6) specifies ISO metric screw threads as the national standard, aligned with ISO 68-1 and ISO 965-1. Metric threads from M6 to M48 are most common in Indian manufacturing, construction, and automotive sectors.
Frequently Asked Questions
What is pitch diameter in a screw thread?
Pitch diameter is the diameter of an imaginary cylinder that passes through the thread profile at points where the thread width equals the groove width. It is one of the three critical thread dimensions alongside major diameter and minor diameter, and determines the fit quality between mating threads per ISO 68-1 standards.
How do you calculate pitch diameter for metric threads?
For ISO metric threads with a 60-degree angle, the basic pitch diameter is calculated using the formula d₂ = d − 0.6495 × P, where d is the basic major diameter and P is the thread pitch in mm. For example, an M10 × 1.5 bolt gives d₂ = 10 − 0.6495 × 1.5 = 9.026 mm. The actual limits depend on the tolerance class (e.g., 6g, 6H).
What is the difference between major diameter and pitch diameter?
The major diameter is the largest diameter of the thread — the outside diameter of a bolt or the inside diameter of the threads on a nut. The pitch diameter is smaller, located at the point where thread width and groove width are equal. For an M10 thread, the major diameter is 10 mm, while the pitch diameter is approximately 9.026 mm for a 1.5 mm pitch.
What does 6g and 6H mean in thread tolerance?
6g and 6H are ISO metric thread tolerance classes. The number (6) indicates the tolerance grade — lower numbers mean tighter tolerances. The letter indicates the tolerance position relative to the basic profile. For external threads, g places the thread below the basic profile (negative deviation). For internal threads, H starts from the basic profile (zero lower deviation). A common fit is 6g/6H for general-purpose metric threads.
What is the pitch diameter of an M8 thread?
For an M8 × 1.25 standard thread, the basic pitch diameter is 8 − 0.6495 × 1.25 = 7.188 mm. With a 6g tolerance, the maximum pitch diameter is 7.169 mm and the minimum is 7.042 mm. For a 6H internal thread, the minimum pitch diameter is 7.188 mm and maximum is 7.342 mm.
What are the standard ISO metric thread pitches?
Standard ISO metric thread pitches range from 0.2 mm for very fine threads (M1.6) to 6.0 mm for coarse threads (M60+). Common pitches include 0.5 mm (M3), 0.8 mm (M4-M5), 1.0 mm (M6), 1.25 mm (M8), 1.5 mm (M10-M12), 1.75 mm (M14), 2.0 mm (M16), 2.5 mm (M20), 3.0 mm (M24-M30), and 3.5-6.0 mm (M36+). These are defined in ISO 261 and ISO 262 standards.
How do tolerance classes affect thread fit?
Tolerance classes determine how tightly an external thread fits into an internal thread. A finer grade (e.g., 4g) provides a tighter fit suitable for precision applications, while a coarser grade (e.g., 8g) allows more clearance for easy assembly. The standard general-purpose fit is 6g/6H, which balances manufacturing tolerance with reliable function for most bolted connections.
What is the fundamental deviation in thread measurements?
Fundamental deviation is the intentional offset of the thread profile from the basic size to provide clearance or interference between mating threads. For external threads, positions e, f, g provide increasing negative deviations (h has zero deviation). For internal threads, G provides a positive deviation while H has zero deviation. The deviation value depends on the thread pitch and is calculated per ISO 965-1.