Thread Pitch Calculator - Pitch, TPI, Thread Count
thread pitch calculator for ISO metric and Unified bolts - read pitch, TPI, and total thread count from any thread length and count.
Thread Pitch Calculator
Results
What Is the Thread Pitch Calculator?
Thread pitch is the distance between two adjacent thread crests on a screw, bolt, or threaded rod, and the thread pitch calculator turns that distance into a number a designer, machinist, or DIY user can act on. Type the length of the threaded section and the number of threads you count across that length, and the tool returns the pitch in millimetres, the pitch in inches, and the threads per inch (TPI) for ISO metric and Unified callouts.
- • Identify an unmarked bolt: measure the threaded shank with a ruler, count the threads, and read pitch or TPI from the result panel
- • Confirm an ISO metric callout: check that an M10 x 1.5 bolt really is 1.5 mm by counting threads across a known length
- • Confirm a Unified callout: verify that a 1/4-20 screw is really 20 TPI before tapping a hole or ordering replacements
- • Plan a thread repair: estimate how many engaged threads a stud needs to support a given load, using the same count to plan a tap drill size
Thread pitch shows up on every bolt bin label and every engineering drawing. Once the pitch is known, the same fastener feeds straight into the ISO metric thread dimensions calculator to read pitch diameter, minor diameter, and tolerance limits, or into a tightening torque tool to size the torque from the nut factor and bolt grade.
How the Thread Pitch Calculator Works
The thread pitch calculator rests on three small equations that all use the same two inputs: thread length L and thread count n.
- L: thread length measured along the threaded shank, in mm or inch
- n: total number of threads counted across L; whole numbers for hand counting, decimals if a partial thread is included
- TPI: threads per inch, used when the user already knows the inch callout such as 1/4-20 UNC
- lengthUnit: mm or inch, drives the conversion between TPI and metric pitch
The same form returns pitch in both units. Pitch in inches equals pitch in mm divided by 25.4, and TPI equals 1 divided by pitch in inches.
Worked example: M10 x 1.5 - 6g
L = 25 mm, n = 16.67 threads, lengthUnit = mm
pitchMm = 25 / 16.67 = 1.5 mm; pitchIn = 1.5 / 25.4 = 0.05906 in; TPI = 1 / 0.05906 = 16.93.
P = 1.5 mm, P = 0.05906 in, TPI = 16.93, n = 16.67 threads.
An M10 x 1.5 bolt with 25 mm of thread holds about 16.67 threads on the shank, matching the metric callout.
According to ISO 965-1:2013, the ISO metric thread callout is written as M followed by the basic major diameter, an x, and the pitch in millimetres, so an M10 x 1.5 bolt is 10 mm across the major diameter with 1.5 mm between thread crests.
According to ASME B1.1, the Unified National Coarse series 1/4-20 UNC designates a 1/4 inch major diameter with 20 threads per inch, which gives a pitch of 1 / 20 = 0.05 inch or 25.4 / 20 = 1.27 mm.
Once pitch is known, the same major diameter and pitch feed straight into the thread calculator to read pitch diameter, minor diameter, and tolerance class limits.
Key Concepts Explained
Four short definitions keep the result panel honest for any screw or bolt you measure.
Thread pitch versus threads per inch
Thread pitch is the distance between two adjacent thread crests, usually expressed in millimetres for ISO metric threads. Threads per inch (TPI) is the count of thread crests inside one inch, used for Unified inch threads. They are reciprocals when pitch is in inches, so P_in = 1 / TPI.
ISO metric callouts (M10 x 1.5)
An M callout reads as basic major diameter in mm, an x, and the pitch in mm. The pitch is the value after the x; the number before the x is the major diameter, not the pitch.
Unified callouts (1/4-20 UNC)
A Unified callout reads as nominal diameter in inches, a dash, and TPI. The number after the dash is threads per inch; the pitch in inches is 1 / TPI, and the pitch in millimetres is 25.4 / TPI.
Thread pitch gauges
A thread pitch gauge is a set of toothed shims, each labelled with its own pitch in mm or TPI in inches. Match a shim to the thread and read the pitch off the shim face. For ISO metric, the shim matches the mm mark on a ruler; for Unified, the shim matches an integer count of TPI across one inch.
ISO 965-1 also defines an internal companion to the external callout, so the same pitch feeds an internal thread or a thread plug gauge without changing the geometry.
Readers who need the pitch diameter of the same thread can cross-check the value with the pitch diameter calculator, which solves d2 from the same pitch in a standalone form.
How to Use the Thread Pitch Calculator
Pick the unit that matches the ruler you have, enter the length of the threaded shank, count the threads across that length, and read the pitch in mm, pitch in inches, and TPI from the result panel.
- 1 Measure the threaded length: Run a ruler along the threaded shank from the start to the end of the threads and read L to the nearest 0.5 mm or 1/32 inch.
- 2 Pick the length unit: Switch the unit selector to mm if you used a metric ruler, or to inch if you used an imperial ruler.
- 3 Count the threads: Count the thread crests across L and type that integer into the thread count box. Add a decimal if you only counted a partial thread at the end.
- 4 Enter TPI when the callout is known: Type the threads per inch from a Unified callout such as 1/4-20 UNC, and the calculator returns the pitch in mm and inches.
- 5 Read the result panel: The headline pitch is shown in the selected length unit. The smaller lines give the pitch in the other unit, the TPI, and the total thread count.
- 6 Cross-check with a thread pitch gauge: Place each gauge shim against the thread until one matches the crest spacing, and confirm the shim label matches the calculator reading.
A machinist pulls a 1 inch long bolt from a parts bin and sees a Unified callout that is partly worn. They enter L = 1, pick inch as the length unit, type n = 20, and the result panel shows P = 0.05 in, P = 1.27 mm, and TPI = 20. The bolt is a 1/4-20 UNC, and the same major diameter feeds a clearance check through the clearance hole calculator before drilling the mating part.
Before tapping the hole, the same major diameter feeds a clearance check through the clearance hole calculator so the bolt head and the mating part sit on the right Close, Medium, or Free fit.
Benefits of Using the Thread Pitch Calculator
The thread pitch calculator turns a ruler reading and a thread count into three useful numbers in one read, instead of flipping between chart pages and callout tables.
- • One tool for ISO metric and Unified threads: the same form returns pitch in mm, pitch in inches, and TPI from a single set of inputs
- • Solves any one unknown: enter length and count to read pitch, enter TPI to read pitch in both units, or enter length and TPI to read total thread count
- • Cross-checks a thread pitch gauge: enter the shim label and confirm it matches the pitch measured along the shank, instead of trusting one tool alone
- • Ties to fastener standards: the result panel labels ISO metric callouts as M dxP and Unified callouts as d-TPI so the reading maps to a bolt-bin label
- • Mobile-friendly form: length unit, length, thread count, and TPI all fit on a phone screen, so a measuring job in the shop or on site uses the same form as a desk review
- • Saves a lookup: TPI to mm and mm to TPI conversions run at the same time, so a metric callout can be cross-checked without a second tool
The thread pitch calculator also doubles as a thread-count planner. Enter the length of the threaded shank and a target pitch from a callout such as M8 x 1.25, and the calculator returns the number of engaged threads. That number feeds a torque check or a joint diagram.
For multi-bolt assemblies, the same major diameter and pitch feed a pattern of holes through the bolt circle calculator to lay out a flange, a pump base, or a wheel hub on a bolt circle.
Factors That Affect Your Results
Five short factors describe which inputs change the result panel.
Thread length L
The result panel scales linearly with the length you measure. A ruler reading that is 1 mm short under-estimates pitch by the same 1 mm divided by the thread count.
Thread count n
The thread count is the most error-prone input. Count crests, not grooves, and check the count twice before locking in the value.
Length unit mm versus inch
Switching from mm to inch changes the conversion path. A 25.4 mm threaded shank and a 1 inch threaded shank both return 1.27 mm pitch when n = 20, because the metric and inch systems agree on the value once the conversion is applied.
TPI entry
Typing TPI alone gives pitch in mm and inches but no thread count. To get the thread count, type both L and TPI; the calculator solves for n = L / P internally.
Coarse versus fine series
ISO metric coarse (M x 1.5) and fine (M x 1.25) threads use the same geometry but different pitch. The same callout prefix d can hide two pitches, so the calculator reads pitch from the inputs rather than from the diameter alone.
- • The calculator follows P = L / n for parallel single-start threads. It does not cover multi-start screws, ACME threads, or pipe threads.
- • A ruler reading is only as accurate as the ruler. A thread pitch gauge is still the right way to confirm pitch on a real bolt.
Before the Unified Thread Standard and ISO 965-1, the same Unified and ISO metric pitches were published in BSW and DIN 13, so the result panel lines up with the same callouts a machinist would see on a fastener bought anywhere the metric and inch systems meet.
According to NIST SI Units, 1 inch equals exactly 25.4 millimetres, the conversion that ties Unified thread pitch in inches to ISO metric thread pitch in millimetres.
The same major diameter and pitch feed a tightening torque through the bolt torque calculator once the pitch has been measured.
Frequently Asked Questions
Q: What is thread pitch and how is it measured?
A: Thread pitch is the distance between two adjacent thread crests on a screw, bolt, or threaded rod, measured from crest to crest or root to root with a ruler, caliper, or thread pitch gauge. The metric pitch is reported in millimetres, and the imperial pitch is reported as threads per inch (TPI).
Q: How do you calculate thread pitch from length and thread count?
A: Divide the measured thread length by the total number of threads counted across that length. For a 25 mm shank with 16.67 threads, P = 25 / 16.67 = 1.5 mm.
Q: What is the difference between thread pitch and threads per inch?
A: Thread pitch is a length between crests, while threads per inch (TPI) is the count of crests inside one inch. They are reciprocals when pitch is in inches: P_in = 1 / TPI. The same callout reads as M10 x 1.5 in metric and 1/4-20 UNC in imperial.
Q: How do you convert TPI to metric thread pitch?
A: Divide 25.4 by the TPI value. A 20 TPI Unified thread gives P = 25.4 / 20 = 1.27 mm, which sits between the 1.0 mm coarse pitch of an M6 and the 1.25 mm fine pitch of an M8; the 1/4 inch major diameter (6.35 mm) is not a direct ISO metric match, so treat the conversion as a read-back, not a metric replacement.
Q: What thread pitch does an M10 x 1.5 bolt have?
A: The pitch is 1.5 mm, the number after the x in the metric callout. Read on a ruler, the same bolt shows P = 1.5 mm and TPI = 25.4 / 1.5 = 16.93.
Q: How many threads does a 1 inch long 1/4-20 bolt have?
A: A 1 inch length with 20 threads per inch gives n = 1 x 20 = 20 threads. Use the calculator to confirm: L = 1 inch, TPI = 20, and the result panel shows n = 20.