Thread Pitch and TPI: Converting Metric and Imperial Screw Threads

Pitch in millimetres is 25.4 divided by TPI. How to convert screw thread specifications, and why M8 and 5/16 inch are close enough to destroy each other.

Introduction

Screw threads are specified two incompatible ways. Metric fasteners state the pitch — the distance between adjacent thread crests, in millimetres. Imperial fasteners state threads per inch, which is the same information inverted. Converting between them takes one division, but the conversion is not the dangerous part. The dangerous part is that several metric and imperial sizes are close enough in diameter to start threading into each other and then destroy both parts.

The Core Mathematics

Pitch and TPI are reciprocals scaled by the inch:

pitch (mm) = 25.4 ÷ TPI

TPI = 25.4 ÷ pitch (mm)

So a 20 TPI thread has a pitch of 25.4 ÷ 20 = 1.27 mm. A 1.5 mm pitch is 25.4 ÷ 1.5 = 16.93 TPI, which is not a whole number — and that is the first useful observation. Metric pitches almost never correspond to a sensible TPI, and imperial TPI values almost never give a round pitch. The two systems do not interleave.

How Each System Writes a Fastener

Metric: M8 × 1.25 means 8 mm nominal diameter, 1.25 mm pitch. If the pitch is omitted, as in plain "M8", the coarse pitch is implied.

Imperial: 1/4"-20 means quarter-inch diameter, 20 threads per inch. The suffix UNC or UNF distinguishes the coarse and fine series.

Note the inversion: the metric second number gets smaller as the thread gets finer, while the imperial second number gets larger. An M8 × 1.0 is finer than an M8 × 1.25, and a 1/4"-28 is finer than a 1/4"-20.

The Near-Misses That Cause Damage

This is the part worth knowing before any conversion table. Several pairs are close enough in diameter to engage:

M8 and 5/16": 8.000 mm against 7.9375 mm — just 0.0625 mm apart, under 1 percent
M6 and 1/4": 6.000 mm against 6.350 mm
M10 and 3/8": 10.000 mm against 9.525 mm
M5 and #10: 5.000 mm against 4.826 mm

The M8 and 5/16" pair is the genuinely hazardous one. The diameters differ by less than the thickness of a sheet of paper, so a 5/16"-18 bolt will enter an M8 nut and turn for a thread or two. But the pitches are 1.411 mm and 1.25 mm, nearly 13 percent apart, so it binds and then strips — usually ruining the softer of the two.

The rule that follows: diameter is not enough to identify a thread. A fastener needs its diameter and its pitch confirmed before it goes anywhere near a tapped hole, and the cheapest way to do that is a thread pitch gauge rather than a feel for whether it "seems about right".

Common Pitches and Their TPI Equivalents

Metric coarse series, with the TPI each pitch works out to:

M3 × 0.5 = 50.8 TPI
M4 × 0.7 = 36.3 TPI
M5 × 0.8 = 31.8 TPI
M6 × 1.0 = 25.4 TPI
M8 × 1.25 = 20.3 TPI
M10 × 1.5 = 16.9 TPI
M12 × 1.75 = 14.5 TPI

And the imperial series converted the other way:

13 TPI = 1.954 mm pitch (1/2"-13 UNC)
16 TPI = 1.587 mm (3/8"-16 UNC)
18 TPI = 1.411 mm (5/16"-18 UNC)
20 TPI = 1.270 mm (1/4"-20 UNC)
24 TPI = 1.058 mm
28 TPI = 0.907 mm (1/4"-28 UNF)
32 TPI = 0.794 mm

The M8 × 1.25 entry at 20.3 TPI sitting beside 1/4"-20 at exactly 20 TPI is a good illustration of how close the two systems come without ever meeting.

Pitch Is Not the Only Difference

Two threads can match on diameter and pitch and still be incompatible, because the thread profile differs between standards.

ISO metric and the American Unified series both use a 60 degree thread angle. British Standard Whitworth and BSF use 55 degrees. A Whitworth thread of the same nominal size will not seat correctly in a Unified hole even where the numbers agree, which is why older British machinery needs its own taps and dies rather than a conversion chart.

Pipe Threads Are Measured Differently Again

A trap that catches people who have mastered everything above. Pipe threads are named by nominal bore, not outside diameter, so the number bears little relation to anything you can measure.

1/8" BSP has an outside diameter of 9.73 mm, not 3.18 mm
1/4" BSP is 13.16 mm, not 6.35 mm
1/2" BSP is 20.96 mm, not 12.70 mm
1" BSP is 33.25 mm, not 25.40 mm

There is also a taper question. BSPT and NPT are tapered and seal on the threads themselves; BSPP is parallel and seals on a washer or an O-ring. Fitting a parallel thread where a tapered one belongs gives a joint that tightens and still leaks.

Quick Reference Conversion Table

0.5 mm pitch = 50.8 TPI
0.8 mm = 31.8 TPI
1.0 mm = 25.4 TPI
1.25 mm = 20.3 TPI
1.5 mm = 16.9 TPI
1.75 mm = 14.5 TPI
2.0 mm = 12.7 TPI
20 TPI = 1.27 mm
18 TPI = 1.411 mm
13 TPI = 1.954 mm

Common Mistakes Worth Avoiding

Identifying a thread by diameter alone

M8 and 5/16" differ by 0.0625 mm. Measure or gauge the pitch as well.

Reading the two second numbers the same way

A larger metric pitch is coarser; a larger TPI is finer. The notations run in opposite directions.

Assuming matching numbers mean a matching thread

Whitworth's 55 degree profile will not mate properly with a 60 degree Unified or ISO thread even at the same size.

Treating a pipe thread size as a measurement

Pipe threads are named by bore. A 1/2" BSP fitting measures nearly 21 mm across the threads.

Conclusion

Divide 25.4 by TPI for pitch in millimetres, and divide 25.4 by the pitch to go back. Remember that the metric and imperial series never actually align, that a larger metric pitch means coarser while a larger TPI means finer, and that diameter alone identifies nothing — the M8 and 5/16" pair are 0.06 mm apart and will happily ruin each other.

Try our millimeters to inches converter for the measurements behind these specifications.