Thread Pitch Calculator
Identify and convert thread pitch. Enter a known TPI or pitch, or measure a span over a counted number of threads — get the pitch in inch and mm, threads per inch and per mm, the lead and helix angle for your number of starts, and the nearest standard pitch, plus a common-pitch reference table and a downloadable PDF.
| TPI | Pitch (in) | Pitch (mm) | vs yours |
|---|---|---|---|
| Enter a thread to compare | |||
| Starts | Lead | Helix angle | Type |
|---|---|---|---|
| Enter a thread to compare | |||
How to use the thread pitch calculator
Four quick steps identify and convert a thread pitch — pitch in inch and mm, threads per inch and per mm, and the lead and helix angle for any number of starts, with a common-pitch reference and a downloadable report.
Pick a system
Choose inch (TPI) or metric (mm) — the tool converts between them.
Enter or measure
Type a known TPI or pitch, or measure a span over a counted number of threads.
Set starts & diameter
Choose single or multi-start and the pitch diameter for the helix angle.
Read results & save a PDF
Get the pitch, lead, helix angle and nearest standard — then download a report.
The formulas behind the pitch
Pitch converts between systems with the 25.4 mm inch; lead and helix angle follow from the pitch, the starts and the diameter.
▶ Worked example
A 20 TPI single-start thread on a 0.2175 in pitch diameter.
Pitch = 1 ÷ 20 = 0.05 in = 0.05 × 25.4 = 1.27 mm; lead = pitch (single start).
Helix = atan(1.27 ÷ (π × 5.52)) = 4.19° (diameter 0.2175 in = 5.52 mm).
Pitch 0.05 in / 1.27 mm · lead 0.05 in · helix 4.19° · nearest 20 TPI
As a double start the lead doubles to 0.10 in and the helix angle rises to about 8.33°.
Thread pitch quick reference
The conversion formulas, common inch pitches in both systems, and common metric pitches with their TPI equivalents.
| Quantity | Formula |
|---|---|
| Pitch (inch) | 1 ÷ TPI |
| Pitch (mm) | 25.4 ÷ TPI |
| Threads per mm | 1 ÷ pitch(mm) |
| Lead (per turn) | pitch × number of starts |
| Helix (lead) angle | atan(lead ÷ (π × pitch diameter)) |
| From measurement | TPI = count ÷ span |
| TPI | Pitch (inch) | Pitch (mm) | Typical use |
|---|---|---|---|
| 8 | 0.1250″ | 3.175 mm | 1″ coarse bolt |
| 10 | 0.1000″ | 2.540 mm | 3/4″ coarse |
| 13 | 0.0769″ | 1.954 mm | 1/2″ coarse |
| 16 | 0.0625″ | 1.587 mm | 3/8″ coarse |
| 20 | 0.0500″ | 1.270 mm | 1/4″ coarse |
| 24 | 0.0417″ | 1.058 mm | #10 coarse |
| 32 | 0.0312″ | 0.794 mm | #6 / #8 coarse |
| Pitch (mm) | TPI equivalent | Pitch (inch) | Typical size |
|---|---|---|---|
| 0.80 mm | 31.75 TPI | 0.0315″ | M5 coarse |
| 1.00 mm | 25.40 TPI | 0.0394″ | M6 coarse |
| 1.25 mm | 20.32 TPI | 0.0492″ | M8 coarse |
| 1.50 mm | 16.93 TPI | 0.0591″ | M10 coarse |
| 1.75 mm | 14.51 TPI | 0.0689″ | M12 coarse |
| 2.00 mm | 12.70 TPI | 0.0787″ | M14 / M16 coarse |
| 2.50 mm | 10.16 TPI | 0.0984″ | M18 / M20 coarse |
Note how close some pairs are: 20 TPI (1.270 mm) is near M-coarse 1.25 mm, and 1.0 mm metric is near 25 TPI — a reminder to confirm the system before assuming a match.
Everything the calculator returns
One lookup gives the full pitch picture — both systems, the lead and helix, and the nearest standard.
Thread pitch at a glance
The conversion constant, the key relationships, and the default result.
Built for the way you work
Whether you are identifying a mystery screw, designing a leadscrew, or buying the right fastener, the pitch data starts from the same inputs.
Have an unknown screw or part and need to pin down its pitch and system before cutting a mating thread or buying a tap.
- Measure over many threads
- Check the nearest standard
- Confirm inch vs metric
Designing a screw drive and need the lead and helix angle for travel per turn and whether the thread is self-locking.
- Use lead for travel per turn
- Add starts for faster motion
- Watch the helix for back-drive
Replacing a bolt or screw and want to be sure of the pitch so the new one threads in cleanly without stripping.
- Match pitch, not just diameter
- Tell M6 from a 1/4-20
- Note the full designation
Identify a thread with confidence
Seven things that make pitch identification quicker and more reliable.
Measure over many threads
Counting threads across an inch or 25 mm, then dividing, averages out error far better than measuring a single thread space.
Keep a pitch gauge handy
A thread pitch gauge with both inch and metric leaves settles an identification in seconds — a measured span is only a close estimate.
Beware the close pairs
20 TPI (1.27 mm) sits right next to metric 1.25 mm, and 1.0 mm is near 25 TPI. Always confirm the pitch, not just the diameter.
Lead, not pitch, sets travel
On a multi-start thread the nut moves one lead per turn, not one pitch. Use the lead when working out travel per revolution.
Mind the helix for self-locking
A shallow helix angle tends to hold under load; a steep one can back-drive. It matters for leadscrews, clamps and jacks.
Use the pitch diameter
The helix angle is figured on the pitch diameter, not the major. Using the nominal diameter gives a close but slightly low estimate.
Record the full designation
A pitch alone isn’t a part number. Note diameter, pitch, series and class before buying taps, dies or fasteners.
Frequently asked questions
Common questions about thread pitch, converting between TPI and millimeters, lead and multi-start threads.
Thread pitch is the distance from one thread crest to the next. If you know the thread’s designation you can read it off directly: an inch thread is quoted in threads per inch (TPI), so the pitch is 1 divided by the TPI; a metric thread quotes the pitch in millimeters, such as the 1.0 in M6 by 1.0.
If you don’t know the designation, you can measure it — lay a rule along the threads and count how many thread spaces fall within a measured length. For inch threads, the TPI is the count divided by the length in inches; for metric, the pitch is the length in millimeters divided by the count.
For example, a thread with 20 TPI has a pitch of 1 divided by 20, or 0.05 inch, which is 1.27 mm. This calculator converts between TPI and pitch and works out the lead and helix angle.
Because one inch is 25.4 mm, you convert between inch threads per inch (TPI) and metric pitch with that factor. The pitch in millimeters is 25.4 divided by the TPI; going the other way, the TPI is 25.4 divided by the pitch in millimeters.
So a coarse inch thread quoted as TPI can be compared directly against a metric pitch in millimeters, which is handy when you have a thread in one system and a gauge or chart in the other.
For example, a 20 TPI thread has a pitch of 25.4 divided by 20, which is 1.27 mm; a 1.0 mm metric pitch works out to 25.4 TPI. This calculator shows the pitch in both inch and millimeter form at once.
Lay a steel rule along the threads, line the zero up with one thread crest, and count how many thread spaces fall within a measured length. Counting over as many threads as you can — say an inch or 25 mm — makes the result far more accurate than measuring a single thread.
For an inch thread, divide the number of thread spaces by the measured length in inches to get the threads per inch. For a metric thread, divide the measured length in millimeters by the number of spaces to get the pitch.
For example, 20 thread spaces measured over one inch is 20 TPI; 20 spaces over 25 mm is a pitch of 1.25 mm. A thread pitch gauge is still the most reliable way to confirm a thread, so use one for a positive identification.
Pitch is the distance between adjacent threads, crest to crest. Lead is the distance the thread advances along its axis in one full turn. On an ordinary single-start thread the two are the same, so one turn advances the nut by exactly one pitch.
On a multi-start thread they differ: the lead is the pitch times the number of starts. A double-start thread advances two pitches per turn, a triple-start three, and so on, so it moves much faster for the same thread fineness.
For example, a 0.05 inch pitch single-start thread has a 0.05 inch lead, but the same pitch as a double-start has a 0.10 inch lead. This calculator reports both the pitch and the lead for any number of starts.
The helix angle, also called the lead angle, is how steeply the thread winds around the cylinder. It is the angle whose tangent is the lead divided by pi times the pitch diameter — so it rises as the lead increases and falls as the diameter increases.
It matters for how a thread behaves: a shallow helix angle tends to be self-locking, holding position under load, while a steep one, from a coarse pitch or multiple starts, moves freely, which is what you want in a leadscrew or a fast-action cap.
For example, a 20 TPI single-start thread on a 0.2175 inch pitch diameter has a helix angle of about 4.19 degrees. This calculator computes the helix angle from the lead and the pitch diameter you enter.
A multi-start thread has two or more separate thread helices running side by side, instead of the single helix of an ordinary thread. The number of these is the number of starts — two for a double-start, three for a triple, and so on.
The point is speed: because the lead is the pitch times the number of starts, a multi-start thread advances several pitches per turn while keeping each thread shallow and strong. They are used for fast-action screws, bottle caps, lens focus mechanisms and leadscrews.
For example, a 0.05 inch pitch triple-start thread advances 0.15 inch per turn — three times as fast as a single start of the same pitch. This calculator works out the lead and helix angle for 1, 2, 3 or 4 starts.
No. They are close in diameter — a 1/4 inch is about 6.35 mm against M6’s 6 mm — which is exactly why they get confused, but the pitches differ: a 1/4-20 has 20 TPI, a 1.27 mm pitch, while M6 coarse has a 1.0 mm pitch. Forcing one into the other will cross-thread and strip.
This near-miss between inch and metric sizes is common, so it is worth confirming the pitch, not just the diameter, before assuming two parts match.
This calculator converts the pitch into both systems and names the nearest standard, so you can tell quickly whether a thread is really the inch or the metric size you expected.
Start by measuring the pitch and the diameter and comparing them against both systems. If the pitch lands neatly on a round millimeter value like 1.0 or 1.5 mm, it is likely metric; if it matches a whole number of threads per inch like 20 or 13 TPI, it is likely inch.
The diameter helps confirm it, but because some inch and metric sizes are close, the pitch is the more reliable tell. A thread pitch gauge with both inch and metric leaves settles it quickly.
This calculator shows the pitch in inch and millimeter form and names the nearest standard pitch in the chosen system, which helps you decide whether a thread is inch or metric.
Disclaimer: Inch (TPI) and metric (mm) threads. Pitch = 1 ÷ TPI (inch); pitch(mm) = 25.4 ÷ TPI. Lead = pitch × number of starts. Helix (lead) angle = atan(lead ÷ (π × pitch diameter)). A pitch worked out from a span over a counted number of threads is only as accurate as the measurement — use a thread pitch gauge for a positive identification. A reference aid only — confirm the full designation (diameter + pitch + series + class) before buying taps, dies or fasteners.