Unit conversion, not thread substitution

Thread Geometry Unit Converter

Convert the nominal diameter and axial pitch of one documented thread between millimetres, inches and threads per inch. The result does not select a mating thread or declare different thread systems interchangeable.

Exact inch conversionSigned-off input recordNo closest equivalentNo strength claim

Controlled-input conversion

Do not use converted nominal diameter and pitch alone to select a replacement or mating fastener. Compatibility also depends on the complete thread standard and designation, profile, tolerance class or fit, allowance, pitch diameter and crest/root limits, taper or parallel form, handedness, number of starts, coating and verified gauging.

Declared thread systemUsed only to identify the record boundary; it does not change the unit arithmetic
Nominal diameterPositive decimal; inch input may also be a fraction such as 1/2 or 1-1/4
Diameter unitUnit of the entered nominal diameter
Axial pitch or TPIPositive plain decimal; pitch is the axial spacing for this recorded thread
Pitch input unitUse mm for axial pitch or TPI for threads per inch
Identification and source recordRecord drawing/specification revision, gauge or measurement basis, complete designation and condition
I confirm that the nominal diameter and axial pitch/TPI refer to the same documented thread and that their units are known.
I understand that this is unit conversion only and does not establish fit, interchangeability, strength, proof load, tightening torque or vibration resistance.

Unit-conversion result

Declared system

—

Nominal diameter

—

Axial pitch

—

Threads per inch

—

Diameter-to-pitch ratio d/P

—

Arithmetic and units

d(mm) = 25.4 × d(in)
P(mm) = 25.4 / TPI
TPI = 25.4 / P(mm)
d/P = d(mm) / P(mm)

NIST lists 1 inch = 2.54 centimetres exactly, so 1 inch = 25.4 millimetres exactly. The same exact factor converts an inch-based axial repeat into millimetres.

The dimensionless d/P ratio is only a geometry descriptor. No basic, pitch, minor or tensile-stress area is inferred; those quantities require the applicable profile, formula, class and controlled standard data.

Standards boundary

ISO 261:1998, Edition 2 remains current after confirmation in 2024 and defines the general plan for ISO general-purpose metric threads. ISO 262:2023, Edition 3 selects commercial metric fastener diameter/pitch combinations from that plan.
ASME B1.1-2024 specifies unified inch thread form, series, class, allowance, tolerance and designation. This page does not reproduce its proprietary tables or declare a metric/Whitworth replacement.
BS 84:2007 is the current BSI specification for parallel Whitworth-form threads and replaced withdrawn BS 84:1956. BSW is not BSP, and neither is inferred from diameter and TPI alone.
Pitch choice alone does not guarantee resistance to vibration loosening. Active NASA-STD-5020B, revalidated in 2026, addresses preload, friction, cyclic slip, dynamic loads and locking features as joint-level factors.

Official evidence

ISO 261:1998, Edition 2 (confirmed 2024)

Official current-status and scope page for the metric general plan.

ISO 262:2023, Edition 3

Official page for selected commercial metric fastener sizes from ISO 261; it replaces ISO 262:1998.

ASME B1.1-2024

Official scope and current-edition page for unified inch threads.

BS 84:2007

Official BSI current-status page for parallel Whitworth-form thread requirements.

NIST SI Units – Length (updated 2025)

Official NIST page stating that the standard inch is exactly equivalent to 25.4 mm.

NASA-STD-5020B (active; revalidated 2026)

Official fastening-system requirements and joint-locking guidance for dynamic environments.

Controlled Thread Geometry Unit Converter

Controlled-input conversion

Unit-conversion result

Arithmetic and units

Standards boundary

Official evidence

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