Overview
ISO 725:2009 - "ISO inch screw threads - Basic dimensions" specifies the basic dimensions in inches for ISO inch screw threads. It defines the nominal (major), pitch and minor diameters for internal and external threads based on the ISO basic thread profile (ISO 68-2) and the general plan for inch threads (ISO 263). The standard provides the mathematical relationships used to calculate these basic dimensions and a comprehensive Table 1 giving rounded values for common nominal sizes and thread series.
Key topics
- Scope and units: All dimensions are given in inches and relate to the ISO inch thread system as referenced in ISO 263.
- Basic profile reference: Dimensions refer to the basic profile defined in ISO 68-2.
- Symbols and definitions: Includes standard symbols such as D / d (basic major diameter), D2 / d2 (basic pitch diameter), D1 / d1 (basic minor diameter), H (height of fundamental triangle) and n (threads per inch).
- Dimensional equations: Provides equations to derive pitch, major, pitch and minor diameters from thread pitch (P = 1/n) and H; values in Table 1 are rounded to the fourth decimal place.
- Comprehensive dimension table: Table 1 lists basic dimensions for a wide set of nominal sizes (from small numbered sizes through fractional-inch sizes) and common thread series (UNC, UNF, UNEF, UN, etc.).
Applications
- Design and engineering: Engineers and CAD designers use ISO 725:2009 to select correct basic dimensions when specifying inch screw threads for fasteners, mating parts and assemblies.
- Manufacturing and quality control: Machinists, tooling designers and inspection teams rely on these basic dimensions to set up thread cutting, tapping and gauging operations and to verify thread geometry.
- Specification and procurement: Procurement specialists and technical writers referencing standardized thread dimensions ensure interchangeability and compliance across suppliers and international projects.
- Product documentation: Technical drawings, BOMs and assembly instructions reference the standard values to avoid mismatches in thread fit.
Related standards
- ISO 68-2 - Basic profile for inch screw threads (referenced for the thread form).
- ISO 263 - General plan and selection for ISO inch screw threads (diameter and series planning).
- ISO 5408 - Screw threads - Vocabulary (terminology used by ISO 725).
Using ISO 725:2009 ensures consistent, internationally recognized inch-thread basic dimensions for design, manufacturing and inspection workflows, improving interchangeability and reducing fit-related errors.
