Thread Pitch Calculator

Thread Pitch Calculator finds pitch from TPI, metric pitch, or measured thread length using P=1/TPI or P=L/N, then shows inch/mm pitch, thread density, 60° profile height, and flats.

This Thread Pitch Calculator converts between Threads Per Inch (TPI) and metric pitch, resolves measurements taken directly from a physical sample, and derives the basic thread profile geometry — all from a single input value. Whether you are working with Unified inch threads or ISO metric fasteners, the tool handles both unit systems and shows every result with full conversions.

What Is Thread Pitch?

Thread pitch is the axial distance between two adjacent thread crests, measured along the axis of the fastener or workpiece. In metric standards, pitch is expressed directly in millimetres — a common M8 bolt, for example, carries a standard pitch of 1.25 mm. In the inch system, pitch is expressed indirectly as Threads Per Inch (TPI), which counts how many complete thread peaks occur within one inch of length.

Because TPI counts threads per unit length, TPI and pitch are mathematically inverse. A higher TPI means the threads are packed more densely, so the individual pitch distance is smaller. A lower TPI means the threads are coarser with a larger pitch distance.

Inch pitch from TPI: P (in) = 1 / TPI
For a 20 TPI thread: P = 1 / 20 = 0.0500 in

Metric pitch from TPI: P (mm) = 25.4 / TPI
For a 20 TPI thread: P = 25.4 / 20 = 1.270 mm

TPI from metric pitch: TPI = 25.4 / P (mm)
For a 1.25 mm pitch thread: TPI = 25.4 / 1.25 = 20.32 TPI

Calculator Modes

Select the mode that matches your starting information. The calculator adjusts its input fields accordingly.

Mode 1 — Known TPI

Enter a TPI value directly. The calculator computes pitch in inches and millimetres using P = 1 / TPI and P = 25.4 / TPI. Use this for fasteners whose TPI is printed on a thread gauge, drawing callout, or specification sheet.

Mode 2 — Known Metric Pitch (mm)

Enter a pitch value in millimetres. The calculator derives TPI using TPI = 25.4 / P. This is the standard entry point for ISO metric thread specifications where pitch is listed explicitly.

Mode 3 — Measure from Sample (Inches)

Enter a measured length in inches and the number of thread crests counted across that span. The calculator resolves pitch as P = L / N, where L is the measured length and N is the thread count. Useful for identifying unmarked or legacy fasteners.

Mode 4 — Measure from Sample (Metric mm)

Same measurement approach as Mode 3 but with the span entered in millimetres. The resulting pitch is output in millimetres first, then converted to inches and TPI.

Understanding the Results

Every calculation returns five result panels. Each is explained below in the same order it appears in the calculator, including the exact formula used and what the output value means physically.

01 — Calculated Pitch

The primary output of the calculator. Pitch is the axial distance between two corresponding points on consecutive thread flanks — in practice, from one crest to the next crest measured parallel to the thread axis. The display unit follows the input mode: inch-based modes show pitch in inches to four decimal places; metric modes show pitch in millimetres to three decimal places.

For a single-start thread, pitch equals lead — the axial distance the threaded component advances per full rotation. For multi-start threads, lead = pitch × number of starts; this calculator does not compute lead for multi-start forms.

02 — Thread Density

Thread density expresses how many thread crests exist within a given linear span. The primary density value is TPI. The calculator also derives two metric-referenced density figures useful when working alongside gauges or standards that use 10 mm or 100 mm reference lengths.

The 10 mm and 100 mm figures are a unit-converted restatement of TPI over metric spans — useful for cross-referencing thread pitch gauges calibrated in metric units, or for sanity-checking measurements taken with a metric rule across a threaded surface.

03 — Pitch Conversion

This panel displays the calculated pitch simultaneously in both inches and millimetres. The conversion is exact, based on the internationally defined relationship of 1 inch = 25.4 mm exactly.

Most useful in mixed-standard environments — for example, a metric ISO drawing specifying M10 × 1.5 needs to be cross-referenced against an inch-dimensioned fixture or tooling callout. The pitch pair displayed here gives both values at a glance without a separate conversion step.

04 — Sharp V Height & Basic Profile Depths

The sharp-V height H is the full theoretical depth of a 60° V-form thread with no truncation at the crest or root. It is the geometric baseline from which both Unified (UN) inch and ISO metric basic profile depths are derived by applying standardised truncation ratios.

These are basic form values only. They do not represent actual machined thread dimensions. Tolerance classes, allowances, and deviations applied to real threads mean the actual pitch diameter, major diameter, and minor diameter will differ from these theoretical baseline figures. Do not use these values to set a lathe depth of cut or to specify thread gauging without consulting the full tolerance tables in ASME B1.1 or ISO 965.

05 — Crest Flat, Root Flat & Total Flat Width

Neither the Unified nor the ISO metric basic thread form has a sharp point at the crest or root. Both standards define a flat truncation at both the tip of the external thread (crest flat) and the bottom of the groove (root flat). These flats prevent stress concentrations at the sharpest geometry points and are part of the defined basic form.

The root flat (P/4) is twice the crest flat (P/8). This asymmetry is intentional in the basic UN and ISO profile: the root requires a wider flat to accommodate the full depth of the mating thread crest without interference at the base of the groove. In practice, crests and roots are often rounded (especially on rolled threads) and the actual flat geometry is governed by tolerance class and manufacturing process. Consult ASME B1.1 Appendix B or ISO 68-1 for actual profile boundary conditions.

Worked Example — 20 TPI

Using the default calculator value of 20 TPI in Known TPI mode, the following results are produced step by step.

Assumptions and Limits

All profile depth and flat width values produced by this calculator are theoretical basic geometry values only, derived from the idealised sharp-V thread form and standard truncation ratios. They are provided for reference and education. These values are not a substitute for:

• Full thread tolerance calculations (tolerance class, allowance, deviation)
• Thread fit class determination (Classes 1A/1B through 3A/3B for inch; 4H/6H/6g etc. for metric)
• Pitch diameter, major diameter, or minor diameter limits
• Tap drill size selection
• Thread strength or engagement length calculations
• Multi-start thread lead calculations (for multi-start threads, lead = pitch × number of starts)

The inch–millimetre conversion used throughout is the exact international definition: 1 inch = 25.4 mm. All results are rounded for display; the underlying arithmetic is performed in full floating-point precision.