What this calculator does
Thread pitch describes how closely spaced the threads are on a screw, bolt or tapped hole. Metric fasteners state this as pitch in millimetres, the distance from one thread crest to the next. Imperial fasteners instead state threads per inch (TPI), the number of thread crests counted over one inch. This calculator converts between the two, since a fastener specified one way often needs to be matched against a system that uses the other.
Measuring which pitch or TPI a fastener actually has is a separate, physical step: the standard technique is a thread pitch gauge, a set of interlocking blades each cut to a known pitch that is held against the fastener until one blade's teeth sit flush against the threads. Failing that, counting the number of thread crests over a measured length, such as 25 mm or one inch, and dividing gives the same answer by hand. Once that raw count or gauge reading is in hand, this calculator turns it into the other unit.
The formula
Pitch in millimetres and threads per inch are reciprocals scaled by the number of millimetres in an inch: TPI equals 25.4 divided by the pitch in millimetres, and pitch in millimetres equals 25.4 divided by the TPI. A finer thread, meaning threads packed more closely together, has a smaller pitch value and a larger TPI value, so the two move in opposite directions.
| Term | Meaning |
|---|---|
| Pitch | The distance from one thread crest to the next, in millimetres, as used on metric fasteners. |
| TPI | Threads per inch: the number of thread crests counted over one inch of length, as used on imperial (UNC/UNF) fasteners. |
| 25.4 | The fixed number of millimetres in one inch, the constant that links the two systems. |
The inputs explained
| Field | What to enter |
|---|---|
| Convert | Choose which direction to convert. |
| Value to convert | The pitch in millimetres, or the threads per inch, depending on the direction chosen above. |
When to use it
Matching a metric bolt to an imperial reference
A metric datasheet quoting pitch in millimetres can be compared directly against an imperial parts list quoted in TPI once both are expressed the same way.
Reading a thread pitch gauge result
A pitch gauge typically reads directly in millimetres or in TPI depending on which side of the gauge is used; converting the reading confirms it against a spec sheet quoted in the other unit.
Ordering a replacement tap or die
Taps and dies are sold by exact thread specification, so confirming pitch or TPI before ordering avoids buying the wrong one for an existing hole or fastener.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What TPI do common metric thread pitches correspond to?
Standard metric coarse-thread pitches converted to threads per inch.
| Pitch | Threads per inch (TPI) |
|---|---|
| 0.5 mm | 50.80 |
| 0.7 mm | 36.29 |
| 0.8 mm | 31.75 |
| 1 mm | 25.40 |
| 1.25 mm | 20.32 |
| 1.5 mm | 16.93 |
| 1.75 mm | 14.51 |
| 2 mm | 12.70 |
What metric pitch do common imperial TPI values correspond to?
Standard UNC and UNF thread counts converted to metric pitch.
| Threads per inch | Pitch (mm) |
|---|---|
| 8 | 3.175 mm |
| 11 | 2.309 mm |
| 13 | 1.954 mm |
| 16 | 1.588 mm |
| 18 | 1.411 mm |
| 20 | 1.270 mm |
| 24 | 1.058 mm |
| 28 | 0.907 mm |
Questions
How do I measure thread pitch without a gauge?
Count the number of complete threads over a measured length, such as 25 mm or one inch, then divide the length by that count for pitch, or the count by the length in inches for TPI. A pitch gauge is quicker and more reliable but this works when one is not on hand.
Is a smaller pitch a finer or coarser thread?
Finer. Pitch is the distance between threads, so a smaller pitch value means the threads are packed more closely together, which is the definition of a fine thread. A larger pitch is a coarse thread.
Can I use this to check if a metric and imperial fastener will mate?
It confirms whether the pitch matches closely enough to be worth testing, but metric and imperial threads also differ in thread angle and profile, so a close pitch match does not guarantee the fasteners are interchangeable.
Why is 25.4 used in the formula?
25.4 is the exact number of millimetres in one inch, by definition. It is the fixed conversion factor between a pitch measured in millimetres and a thread count measured per inch.
For other length and unit conversions, see the square metre calculator.