CNC / machinist calculator
Thread Pitch Calculator
Inch threads are called out by threads per inch and metric threads by a pitch in millimetres, which makes cross-referencing a part awkward. This calculator converts cleanly between TPI, inch pitch and metric pitch so you can match a tap, a die or a lathe gear to the thread in front of you. It also returns the sharp-V and basic thread heights for a 60 degree thread, the numbers you need when setting a single-point threading tool or checking thread depth.
- Threads per inch
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- Pitch (inch)
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- Pitch (metric)
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- Basic thread height
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- Sharp-V height
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Basic thread height (about 0.541 x pitch) is the height of the basic 60-degree profile; the practical single-point infeed is usually a little deeper (near 0.61 x pitch) and depends on the thread standard you are cutting.
How it works
Pitch and threads per inch are reciprocals of a length. The pitch in inches is one divided by the TPI, so a 20 TPI thread has a 0.05 inch pitch. To express that in millimetres you multiply by 25.4, giving 1.27 millimetres, and the reverse divides 25.4 by the metric pitch to recover an equivalent TPI. That last conversion is what tells you a foreign fastener is metric rather than a near-miss inch size.
The thread profile itself is a 60 degree triangle. Its full sharp-V height is the pitch times the square root of three over two, about 0.866 times the pitch. Real threads truncate the crest and root, so the basic thread height, the height of the engaged profile, is five eighths of that, about 0.541 times the pitch. These are the standard geometric heights of the basic ISO and Unified profile.
The basic height is not the same as the infeed you crank in when single-pointing a thread. Practical cutting depth for a 60 degree external thread is usually a little deeper, commonly near 0.61 times the pitch, because the tool has to form the root, and the exact figure depends on the thread standard and the tool. Treat the basic height as the reference profile height and set the infeed to the standard you are cutting.
Worked example
A 20 TPI inch thread has a 0.05 in pitch, which is 1.27 mm. That sits right beside the 1.25 mm metric pitch, which is why a 20 TPI screw can start into an M-size nut and then bind.
Frequently asked questions
What is thread pitch?
Thread pitch is the distance from one thread crest to the next. Inch threads state it indirectly as threads per inch, while metric threads state the pitch directly in millimetres, such as M8 by 1.25.
How do I convert TPI to a metric pitch?
Divide 25.4 by the threads per inch to get the pitch in millimetres. A 20 TPI thread is a 1.27 millimetre pitch, which is why it is close to but not the same as a 1.25 millimetre metric thread.
How is thread pitch different from lead?
Pitch is the distance between adjacent threads. Lead is the axial travel in one turn. For a single-start thread they are equal, but a two-start thread has a lead of twice the pitch and advances faster.
What thread height do I use to set a threading tool?
The basic thread height, about 0.541 times the pitch, is the height of the engaged 60 degree profile. Practical single-point infeed is usually a little deeper, often near 0.61 times the pitch, because the tool must cut the root, and it depends on the thread standard you follow.
Why does a metric bolt sometimes almost fit an inch nut?
Because some metric and inch pitches are numerically close. A 1.27 millimetre inch pitch sits right next to a 1.25 millimetre metric pitch, so they start to thread together and then bind, which damages both parts.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.