Thread
Calculate metric thread dimensions for ISO screw threads: pitch diameter, minor diameter, fundamental triangle height, and tolerance limits for external threads by grade and position.
About This Calculator
This metric thread dimensions calculator helps engineers, machinists, and DIY enthusiasts determine the key dimensions of ISO metric screw threads (M profile) under ISO 68-1 and ISO 261 standards. Whether you are designing bolted connections, selecting fasteners for a project, or verifying thread callouts, this tool computes the fundamental triangle height (H), pitch diameter (d₂), and minor diameter (d₁) from the basic major diameter (d) and thread pitch (P).
The calculations follow the standard ISO metric thread formulas: the height of the fundamental triangle H = P × √3 / 2, the pitch diameter d₂ = d − 2 × (3/8) × H, and the minor diameter d₁ = d − 2 × (5/8) × H. These formulas apply to both external threads (bolts, screws, studs) and internal threads (nuts, tapped holes), as basic diameters are identical for matching thread pairs. For external threads with tolerance class requirements, you can enable the tolerance section to select the tolerance position (e, f, g, h) and grades for pitch and major diameters. The calculator then outputs the maximum and minimum allowable diameters per ISO 965-1, using fundamental deviation (es) and tolerance (Td, Td₂) values derived from the selected grade.
Threads are helical grooves cut into cylindrical fasteners. Metric threads use a 60° symmetrical V-profile. Understanding thread dimensions is critical for ensuring proper fit, strength, and interchangeability of threaded fasteners in mechanical assemblies, automotive parts, construction hardware, and industrial machinery. This calculator supports the worldwide ISO metric thread standard, which is the predominant standard used in Europe, Asia, and most of the world. For imperial (UNC/UNF) threads, refer to a separate unified thread calculator.
Regional Notes
Worldwide (ISO metric): The ISO metric thread system is the global standard for threaded fasteners, used in virtually every country. Threads are specified by the letter M followed by the nominal diameter in mm and the pitch in mm (e.g., M10 × 1.5). Coarse threads (standard pitch) use a single number (M10, pitch = 1.5 mm) while fine threads specify the pitch explicitly.
Frequently Asked Questions
How do I calculate the pitch diameter of a metric thread?
First calculate the height of the fundamental triangle H = P × (√3 / 2). For a bolt with major diameter d = 10 mm and pitch P = 1.5 mm, H = 1.5 × (√3 / 2) = 1.299 mm. Then pitch diameter d₂ = d − 2 × (3 / 8) × H = 10 − 0.75 × 1.299 = 9.026 mm.
How do I calculate the minor diameter of a thread?
For a bolt with major diameter d = 20 mm and pitch P = 2 mm, first calculate H = 2 × (√3 / 2) = 1.732 mm. Then minor diameter d₁ = d − 2 × (5 / 8) × H = 20 − 1.25 × 1.732 = 17.835 mm.
What is the diameter of an M6 thread?
An M6 thread has a basic major diameter of 6 mm. Metric thread callouts also include the thread pitch, so an M6 × 1 bolt has a 6 mm major diameter and a 1 mm thread pitch. Always check both diameter and pitch when purchasing threaded fasteners.
What does the thread callout M10 × 1.5 × 25 mean?
M stands for metric (ISO Metric Standard). 10 is the basic major diameter in mm. 1.5 is the thread pitch in mm. 25 is the length of the shaft in mm. So M10 × 1.5 × 25 means a metric bolt 10 mm in diameter with 1.5 mm thread pitch and 25 mm length.
What are metric thread tolerance classes?
Metric thread tolerance classes indicate the tolerance grade and position for the thread. For example, 5g6g means pitch diameter tolerance grade 5, major diameter tolerance grade 6, both with g tolerance position (small allowance). Tolerance positions are e (large allowance), f (medium), g (small), and h (no allowance).
What is the height of the fundamental triangle in a thread?
The fundamental triangle is the equilateral triangle formed by the thread's cross-section. Its height H = P × √3 / 2, where P is the thread pitch. For a 1.5 mm pitch thread, H = 1.5 × 0.866 = 1.299 mm.
What is the nominal diameter of a metric thread?
The nominal diameter of a metric thread is the diameter used for general identification, equal to the basic major diameter. For a thread callout M10 × 1.5, the nominal diameter is 10 mm.
What is the difference between major, pitch, and minor diameter?
The major diameter is the largest diameter of the thread (crest for external, root for internal). The minor diameter is the smallest diameter (root for external, crest for internal). The pitch diameter is the diameter where thread thickness equals the space between threads, approximately the average of major and minor diameters.