Thread pitch, lead angle and pitch diameter
Pitch and threads-per-inch describe the same spacing in two unit systems. Enter either one, and this also works out the lead angle and pitch diameter a given major diameter and pitch imply.
Getting a number you can act on
- 01Enter pitch or TPI, in whichever you have
The other field updates automatically — there's no need to look up which direction the conversion runs.
- 02Enter the major diameter
The nominal outer diameter of the thread — 10mm for an M10, for instance — needed for the pitch diameter and lead angle calculations.
- 03Use pitch diameter for a precise fit check
This is the theoretical diameter where the thread's load-bearing flanks actually meet, and it's the dimension thread gauges and go/no-go checks are built around.
- 04Use lead angle to understand self-locking behaviour
A shallower lead angle generally resists loosening under vibration better than a steeper one, all else equal — part of why fine threads are sometimes chosen specifically for that property.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
TPI = 25.4 ÷ pitch (mm)An inch is exactly 25.4mm, so this conversion is exact rather than approximate.
pitch diameter = major diameter − 0.6495 × pitchThe standard approximation for ISO metric and Unified (UN) 60-degree thread profiles.
lead angle = arctan(pitch ÷ (π × major diameter))The helix angle the thread makes as it wraps around the shaft — steeper for a coarser pitch on the same diameter.
Why pitch diameter matters more than major diameter for a real fit
Major diameter — the number in a bolt's designation — is the outer edge of the thread crests. It is not where the male and female threads actually bear load against each other; that happens partway down the thread flank, at a diameter smaller than the major diameter.
That load-bearing diameter is the pitch diameter, and it is the dimension precision thread gauging is actually built around. Two threads can share an identical major diameter and pitch while differing slightly in pitch diameter due to manufacturing tolerance, which is exactly what thread gauges are designed to catch.
Thread Pitch Calculator FAQ
How do I convert thread pitch to TPI?+
Divide 25.4 by the pitch in millimetres. A 1.5mm pitch converts to approximately 16.9 threads per inch.
What is pitch diameter?+
The theoretical diameter at which the male and female thread flanks actually meet and bear load — smaller than the major (outer) diameter, and the dimension thread gauges are built around.
What is thread lead angle?+
The helix angle the thread makes as it wraps around the fastener's shaft. A coarser pitch on the same diameter produces a steeper lead angle.
Does lead angle affect how easily a bolt loosens?+
Generally yes — a shallower lead angle tends to resist loosening under vibration better than a steeper one, which is part of why fine threads are sometimes specifically chosen for vibration-prone applications.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01Dyno correction factors — SAE J1349 against STDExplains the J1349 reference conditions of 77°F, 0% humidity and 29.234 in-Hg, and why STD-corrected figures read higher.↗02Ford Performance — dynamometer testing and engine performance tech tipsManufacturer guidance on dyno correction and how quoted power figures are arrived at.↗A starting point — always check the manufacturer's own specification. Torque figures here come from generic formulas and standard grade tables, not from any specific vehicle's service manual. Critical fasteners — wheels, suspension, engine and safety-related hardware — have a torque specification set by the manufacturer, verified for that exact application, and it takes priority over a generic calculation every time.