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Screw Thread Size Converter

Science

Compare metric screw thread sizes (M3-M24) to their nearest UNC/imperial equivalent, with diameter in mm and inches and thread pitch or TPI for each.

Reviewed by the thecalcu.com team · Last updated July 19, 2026

Nearest Imperial Equivalent (UNC)
1/4-20 UNC
Metric Diameter / Pitch
6 mm / 1 mm
Imperial Diameter / TPI
0.25" / 20 TPI

Nearest-equivalent pairing by diameter — metric and imperial thread standards don't align exactly, so treat this as a reference match, not an exact substitute.

All Thread Sizes

What is a Thread Size?

A Screw Thread Size Converter cross-references metric screw thread sizes (like M6 × 1.0) against their nearest equivalent in the UNC (Unified National Coarse) imperial standard used predominantly in the US. Because metric and imperial threads are entirely separate standards, one based on millimetres, the other on fractions of an inch, there's no exact mathematical conversion between them, only commonly published "nearest equivalent" pairings based on similar nominal diameter.

This tool shows both the metric thread's diameter and pitch, and the paired UNC thread's diameter and threads-per-inch (TPI), letting you compare hardware specified in either standard. It complements the Wire Gauge Converter for another common metric-versus-imperial sizing challenge in electronics and hardware work.

Why Use a Screw Thread Size Converter?

Hardware catalogues, imported equipment, and DIY project instructions frequently specify fasteners in whichever standard the manufacturer's home country uses, US components in UNC/UNF, and most of the rest of the world (including India) in metric ISO threads. Sourcing a replacement bolt or matching a nut to an unfamiliar thread designation requires knowing the nearest equivalent in your locally available standard.

Two concrete situations this solves: cross-referencing a US-sourced machine's 1/4-20 UNC bolt against locally available metric M6 hardware when a replacement part isn't available in the original standard, and converting a thread's TPI (threads per inch) rating into millimetre pitch to compare against a metric thread gauge.

Who Should Use This Converter?

  • DIY enthusiasts and makers sourcing replacement bolts, screws, or nuts for imported equipment specified in a different thread standard.
  • Mechanics and technicians working on vehicles or machinery that mix metric and imperial fasteners, such as US-made automotive parts.
  • Hardware store staff and procurement teams helping customers identify the closest available stock item to a specified foreign thread size.
  • Engineering students learning to cross-reference metric and imperial fastener standards for coursework or lab equipment repair.
  • Import/export businesses dealing with machinery or hardware specifications that need translating between regional standards.

What Insights Does the Thread Size Converter Give You?

Selecting a metric thread size shows its nearest imperial equivalent in UNC notation, the closest commonly stocked US thread size by nominal diameter. The metric diameter/pitch and imperial diameter/TPI fields give the exact specifications of both sizes side by side, so you can see precisely how close (or not) the match really is, rather than assuming they're interchangeable.

This distinction matters: a "nearest equivalent" pairing is useful for sourcing decisions (finding an available bolt that's close enough for a low-tolerance application) but is explicitly not a substitute for using the correct matching standard in a threaded hole or nut, since the thread pitch and profile differ even when the diameters are close.

How to use this Thread Size calculator

  1. Open the Metric Thread Size dropdown and select the metric size you want to cross-reference, such as M10 × 1.5.
  2. Read the Nearest Imperial Equivalent (UNC) result, showing the closest matching US thread designation.
  3. Check the Metric Diameter / Pitch box for the exact metric specification.
  4. Check the Imperial Diameter / TPI box for the exact UNC specification.
  5. Read the note below the results, which explains that this is a reference match, not an exact substitute.
  6. Browse the All Thread Sizes reference table and click any row to jump directly to that pairing.

Formula & Methodology

Metric-to-imperial thread pairings in this converter follow commonly published nearest-equivalent reference tables, matched primarily by nominal diameter (metric mm converted to inches, compared against standard UNC fractional/numbered sizes). Threads-per-inch (TPI) converts to metric pitch using the exact inch-to-millimetre relationship:

Pitch (mm) = 25.4 ÷ TPI

Worked example: A 1/4-20 UNC bolt has 20 threads per inch, so its pitch equivalent is 25.4 ÷ 20 = 1.27 mm per thread, close to, but not identical to, M6's standard 1.0 mm pitch, illustrating why the pairing is a nominal-diameter match rather than an exact thread-for-thread equivalent.

Frequently Asked Questions

M6 × 1.0 is the commonly published nearest equivalent to a 1/4-20 UNC bolt, since both have a nominal diameter close to 6.35 mm (1/4 inch = 6.35 mm vs M6's 6 mm). They are not identical, the threads won't interchange, but M6 is the standard metric size referenced against 1/4-20 in cross-reference charts.
No, even when a metric and imperial size are listed as 'nearest equivalent,' their thread pitch and profile differ enough that they will not thread together safely, and forcing it can damage both the bolt and the hole. Always match metric fasteners with metric-tapped holes and imperial (UNC/UNF) fasteners with imperial-tapped holes, or use a thread-repair insert designed for the conversion.
UNC (Unified National Coarse) has fewer, larger threads per inch and is the more common general-purpose standard, while UNF (Unified National Fine) has more threads per inch for the same nominal diameter, offering finer adjustment and higher resistance to loosening under vibration. This converter references UNC pairings, the more commonly cross-referenced standard against metric coarse threads.
Metric pitch (mm) = 25.4 ÷ TPI, since 25.4 mm equals exactly 1 inch. For example, a 1/4-20 UNC bolt (20 TPI) has a pitch equivalent of 25.4 ÷ 20 = 1.27 mm per thread, which this converter calculates automatically alongside each metric-imperial pairing.
Select a metric thread size from the dropdown, such as M8 × 1.25, and the tool immediately shows its nearest UNC imperial equivalent along with both sizes' exact diameter and pitch/TPI values. You can also click any row in the reference table below to jump directly to that size.
In a UNC designation, the number after the dash is the thread count, threads per inch (TPI), so '1/4-20' means a 1/4-inch nominal diameter bolt with 20 threads per inch. This converter shows the TPI value for each imperial equivalent alongside its metric pitch counterpart.
Drill bit sizing for tapping depends on the desired thread engagement and is a separate calculation from nominal thread size, for M6 × 1.0, a common tap drill size is approximately 5.0 mm, but always verify against a tap drill chart for your specific material and thread depth requirement. This converter provides thread size cross-referencing, not tap drill sizing.
Metric and imperial (UNC/UNF) threads were developed as entirely separate standards with different base units (millimetres vs inches) and different historical rounding conventions, so their diameters only coincidentally land close to each other rather than matching exactly. This is why cross-reference tables always describe pairings as 'nearest equivalent' rather than exact matches.
Most of the world, including India, Europe, and most of Asia, uses metric (ISO) threads as the standard for fasteners, while the US predominantly uses UNC/UNF imperial threads alongside metric in some industries like automotive and aerospace. This makes cross-referencing essential when working with imported hardware or US-sourced equipment in a metric-standard country.
No, for new engineering designs, always specify a single thread standard (either fully metric or fully imperial) rather than relying on nearest-equivalent substitutions, which are intended for quick reference and sourcing convenience, not interchangeable engineering tolerances. Use this converter to identify approximately comparable hardware, then verify exact specifications against the relevant ISO or ASME standard before finalising a design.
Also known as
metric to imperial bolt size chartm6 to imperialunc to metric threadbolt thread size convertertpi to pitch converterscrew thread chart