Thread counts and pitches in both systems, with the three things a pitch alone does not tell you — the angle, the taper, and whether the name refers to anything you can measure.
Calculator
Try:
20 threads per inch
1.2700 mm pitch
and 1.25 mm is 20.32 TPI · the two are reciprocals, so their product is always exactly 25.4
1/4"-20 UNC in millimetres
Outside diameter6.35 mm0.25 inch, exactly ×25.4
Pitch1.270 mm25.4 ÷ 20
Nearest metric threadM6-0.35 mm on diameter, -0.270 on pitch
Will they mate?Nonearest is not compatible, ever
1/4"-20 UNC is 6.35 mm across with a 1.270 mm pitch; M6 is 6.00 mm with 1.00 mm. Close is not compatible — a thread mates only on an exact match of diameter, pitch and flank angle. A mismatched pair will usually start turning and then jam, which is worse than not fitting at all — by the time it binds, the softer of the two threads has already been damaged.
Eight series, and what separates them
A thread form is an angle and a taper as well as a pitch
Series
Angle
Form
The size in the name is
ISO metric coarse (M)
60°
parallel
Outside diameter, in millimetres
ISO metric fine (M×p)
60°
parallel
Outside diameter and pitch, both in millimetres
Unified coarse (UNC)
60°
parallel
Outside diameter in inches or a gauge number, plus threads per inch
Unified fine (UNF)
60°
parallel
Outside diameter plus threads per inch
British Standard Whitworth (BSW)
55°
parallel
Outside diameter in inches
BSP parallel (G)
55°
parallel
The BORE of the pipe it was designed for
BSP taper (R / Rc)
55°
tapered
The bore of the pipe, with a 1:16 taper
National Pipe Taper (NPT)
60°
tapered
The bore of the pipe, with a 1:16 taper
Two rows matter more than the rest. The 55° entries — Whitworth and both BSP forms — have a different flank angle from everything else, so they do not mate with a 60° thread even at identical diameter and pitch. And the pipe rows are named by the bore, which is why measuring the thread and looking up the number gives the wrong answer every time.
A half-inch pipe thread is not half an inch
BSP sizes: the name, and what a caliper reads
Called
Name in mm
Thread actually measures
TPI
G1/8
3.2 mm
9.73 mm
28
G1/4
6.4 mm
13.16 mm
19
G3/8
9.5 mm
16.66 mm
19
G1/2
12.7 mm
20.96 mm
14
G3/4
19.0 mm
26.44 mm
14
G1
25.4 mm
33.25 mm
11
Read the middle two columns together. A G1/2 fitting is called half an inch and measures 20.96 mm across the thread — closer to three quarters of an inch. The name refers to the bore of the pipe the fitting was designed around, not to anything you can measure on the outside. Someone measuring 21 mm and asking for a 21 mm fitting will be sold the wrong thing.
Will these two mate?
VerdictThey do notthe first failure is enough
The diameters differ by 0.35 mm.
The checks run in the order the metal applies them: flank angle first, then taper, then diameter, then pitch. An angle mismatch is fatal however well the numbers line up, which is why a 55° BSP taper and a 60° NPT of the same nominal size will screw part way together and never seal.
Coarse and fine at the same diameter
Two threads, one name, no compatibility
Diameter
Coarse
Fine
Difference
6 mm
M6 — 1 mm
M6×0.75 — 0.75 mm
0.25 mm of pitch
8 mm
M8 — 1.25 mm
M8×1 — 1 mm
0.25 mm of pitch
10 mm
M10 — 1.5 mm
M10×1.25 — 1.25 mm
0.25 mm of pitch
12 mm
M12 — 1.75 mm
M12×1.25 — 1.25 mm
0.50 mm of pitch
16 mm
M16 — 2 mm
M16×1.5 — 1.5 mm
0.50 mm of pitch
20 mm
M20 — 2.5 mm
M20×1.5 — 1.5 mm
1.00 mm of pitch
An M8 bolt and an M8 bolt can be two different threads. Coarse is the default and is what “M8” means with no pitch written after it; fine is written out, as M8×1. They are the same diameter, they will start, and they will bind within a couple of turns. If a fastener goes in easily and then stops, this is the first thing to check.
What this converter covers
TPI to millimetre pitch, thirteen unified sizes in metric, eight thread series with their flank angles, BSP names against real diameters, and a mating check.
Threads per inch to millimetre pitch, in both directions
Unified inch sizes converted to millimetres, with the nearest metric thread
Eight series with their flank angles, tapers and naming conventions
BSP pipe sizes, where the name is the bore and not the thread
A mating check that vetoes on angle and taper before it looks at the numbers
TPI ↔ mm pitch Nearest ≠ compatible 55° against 60° Pipe names are bores
Free, no signup — exact by definition, not an estimate.
Updated 8 September 2026
At a glance
Formula shown
pitch in mm = 25.4 ÷ threads per inch, and back the same way
Scenario support
20 TPI is a 1.27 mm pitch · a G1/2 thread measures 20.96 mm, not 12.7
Educational estimate
Planning support from the values you enter — not professional advice.
More threads means finer
The inch world counts threads per inch and the metric world measures the pitch — the distance between adjacent crests. The two are reciprocals, and that runs against the intuition that a bigger number is a bigger thing.
Threads per inch and the pitch it implies
TPI
Pitch
Where you meet it
11
2.309 mm
G1 pipe thread
13
1.954 mm
1/2"-13 UNC
14
1.814 mm
G1/2 and G3/4 pipe
20
1.270 mm
1/4"-20 UNC
28
0.907 mm
1/4"-28 UNF, G1/8 pipe
32
0.794 mm
#6-32 and #8-32
Because they multiply to exactly 25.4, halving the thread count doubles the pitch. A 40 TPI thread is finer than a 20 TPI one, not coarser, and the number going up while the thread gets smaller is the first thing to hold on to.
The conversion itself is exact in both directions, because the inch is exactly 25.4 mm by international agreement. Everything difficult on this page is about what the pitch does not tell you.
Nearest is never compatible
The most useful thing a converter can do here is refuse to be encouraging. Inch and metric threads come close and never touch.
Close pairs, none of them interchangeable
Inch thread
In millimetres
Nearest metric
1/4"-20 UNC
6.35 × 1.27
M6 — 6.00 × 1.00
5/16"-18 UNC
7.94 × 1.41
M8 — 8.00 × 1.25
3/8"-16 UNC
9.52 × 1.59
M10 — 10.00 × 1.50
1/2"-13 UNC
12.70 × 1.95
M12 — 12.00 × 1.75
Every row is within a millimetre on diameter and a fifth of a millimetre on pitch, and every row is incompatible. The 5/16" against M8 pair is the worst of them, because the diameters are within a tenth of a millimetre and it will screw in several turns before it stops.
That is the actual hazard. A thread that does not fit at all is an inconvenience; a thread that starts, turns, and then binds has already damaged the softer of the two — usually the aluminium casting rather than the steel bolt — and repairing it is a thread insert rather than a different fastener.
So the calculator above names the nearest metric thread and says, every time, that nearest is not compatible. There is no branch in the code where it says otherwise.
The angle is part of the thread
Diameter and pitch are the two numbers everyone quotes, and a thread has a third property that vetoes both.
The flank angle is the included angle of the V. ISO metric, UNC and UNF are all 60°. Whitworth and both BSP forms are 55°, with rounded crests and roots rather than flat ones.
Two threads at different angles do not mate properly even at identical diameter and identical pitch. They touch at the wrong part of the flank, so the load concentrates on a fraction of the intended contact area, and the joint either leaks or strips.
The practical instance is pipe fittings. BSPT and NPT are both tapered at the same 1:16, both are called by the same nominal sizes, and they are 55° and 60° respectively. A BSPT male will screw into an NPT female and it will not seal — and because both are tapered, it will feel like it is tightening properly right up until it does not hold pressure.
Which is why the mating check above tests the angle first, then the taper, and only then looks at the numbers. That is the order the metal applies them in.
Pipe threads are named by the bore
This one produces more wrong orders than anything else on the page, and it is not an error so much as a convention nobody explains.
A G1/2 BSP thread measures 20.96 mm across its major diameter. Half an inch is 12.7 mm. The name has nothing to do with any dimension of the thread.
What a caliper reads against what it is called
Called
Name would suggest
Actually measures
G1/8
3.2 mm
9.73 mm
G1/4
6.4 mm
13.16 mm
G1/2
12.7 mm
20.96 mm
G1
25.4 mm
33.25 mm
The size refers to the approximate bore of the wrought-iron pipe the fitting was originally designed to screw onto. The thread had to go around the outside of that pipe, so it is always larger, and proportionally much larger at the small end — a G1/8 thread is three times its nominal name.
The consequence is direct: measuring a fitting and asking for that size gets you the wrong part. Measure 21 mm on a BSP thread and you need a half inch, not a 21 mm anything. NPT works the same way, with the same trap.
Coarse and fine share a name
The last incompatibility is within a single system, which makes it easy to miss.
Every metric diameter has a coarse pitch and one or more fine pitches. M8 coarse is 1.25 mm; M8 fine is 1.0. Both are 8 mm across, both are 60°, both are correctly called M8, and they do not mate.
The convention is that a bare designation means coarse. “M8” is M8×1.25; a fine thread is always written out as M8×1. The inch series does the same thing differently — 1/4"-20 and 1/4"-28 are both “quarter inch”, with the thread count doing the distinguishing.
Fine threads exist for real reasons: more thread engagement per unit length, better resistance to loosening under vibration, and finer adjustment. They are also more easily cross-threaded and less tolerant of dirt and damage, which is why coarse is the default for general assembly.
The diagnostic is simple and worth knowing. A fastener that starts freely and then stops within two turns is nearly always a pitch mismatch at the right diameter — and continuing to turn it is how a repairable mistake becomes a thread insert.
Related calculators
Other workshop and gauge tools:
Tap Drill and Drill SizeTap drills at any thread engagement plus the nearest bit in all four drill series — and why “subtract the pitch” leaves 92% of the thread.
Wire GaugeAWG and SWG to mm, mm² and circular mils, with resistance and voltage drop — and the 14–21% gap the equivalence charts hide.
Sheet Metal GaugeGauge to thickness for steel, galvanised, stainless and aluminium — four standards sharing one set of numbers, up to 38% apart.
Fractional InchInch fractions, decimals, thou and millimetres — showing the answer at every denominator and what each rounding costs.
LengthMillimetres to miles on the exact 1959 factors, with the mil kept clearly apart from the millimetre — they differ 25-fold.
TorqueNewton-metres, pound-feet, pound-inches and kgf·m — with inch-pounds and foot-pounds named apart, since they differ by twelve.
Three standards bodies, because three separate traditions are in play: the Unified inch series, the British pipe threads that kept Whitworth’s 55° form, and the ISO metric series that replaced both nearly everywhere else.
ASME Codes & StandardsASME · verified 2026-09-08 · ASME B1.1, the Unified inch screw thread standard that defines the UNC and UNF series, their 60° flank angle and the diameter-and-threads-per-inch designation used throughout this page
British Standards InstitutionBSI · verified 2026-09-08 · BS EN ISO 228 for BSP parallel threads and BS 21 for BSP taper, including the 55° Whitworth form and the convention of designating a pipe thread by the nominal bore rather than by any measurable diameter
International Electrotechnical Commission standards storeIEC · verified 2026-09-08 · The ISO metric screw thread series as adopted in electrical equipment standards — ISO 68-1 for the 60° basic profile and ISO 261 for the coarse and fine pitch series at each nominal diameter
Conversion note
This identifies and compares thread designations; it does not specify a joint. Whether a particular fastener is right for a particular job depends on the material, the load, the required preload and torque, the engagement length, corrosion and vibration — none of which follows from a pitch. Thread classes and tolerance grades are not covered here and matter: two threads of the same designation can be a loose fit or an interference fit depending on class. Pipe threads seal in different ways — a tapered thread on its flanks, a parallel one on a washer or O-ring face — and a joint that holds pressure is a matter for the fitting manufacturer's instructions and the applicable piping code, not for a conversion. Where safety, pressure or structural load is involved, identify threads with proper gauges and follow the relevant standard rather than a measured diameter and a guessed series.
Published the TPI to mm Pitch Converter: threads per inch and millimetre pitch in both directions, thirteen unified inch sizes converted to millimetres, eight thread series with their flank angles and tapers, and a mating check.
Records that threads per inch is a RECIPROCAL, so a higher count is a finer thread -- the two multiply to exactly 25.4, which makes the conversion exact and the intuition backwards.
Reports the nearest metric thread to any inch size and states, in every case and as a typed constant rather than a computed one, that nearest is NOT compatible: a 1/4 inch 20 UNC is 6.35 by 1.27 mm and the nearest M6 is 6.00 by 1.00, and the pair will start turning and then jam.
Names the 5/16 inch against M8 pair as the worst of them, since the diameters are within a tenth of a millimetre and several turns go in before it binds -- by which point the softer thread is already damaged.
Treats the flank ANGLE as part of the thread. Whitworth and both BSP forms are 55 degrees while ISO metric, UNC and UNF are 60, so an identical diameter and pitch still will not mate -- which is why a BSP taper male screws into an NPT female and never seals.
Explains that a pipe thread is named by the BORE of the pipe it was designed around: a G1/2 thread measures 20.96 mm across, closer to three quarters of an inch than to a half, and the gap is proportionally largest at the small end.
Shows coarse and fine at the same diameter -- M8 is 1.25 mm and M8 by 1 is 1.0, both correctly called M8 -- and gives the diagnostic: a fastener that starts freely and stops within two turns is nearly always a pitch mismatch at the right diameter.
Runs the mating check in the order the metal applies it: flank angle, then taper, then diameter, then pitch.
Verified by 161 automated cases, asserting that pitch times threads per inch is exactly 25.4 at every size in every table, that nearest-metric returns a literal false for compatibility for all thirteen unified sizes, that a 55-degree and a 60-degree thread are refused even at identical diameter and pitch, and that every BSP thread measures more than its nominal name.
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