Thread pitch is the axial distance between corresponding points on adjacent threads. Metric thread designations give pitch in millimeters. Unified inch designations give threads per inch (TPI). M10 × 1.5 has a 1.5 mm pitch; 1/4-20 UNC has 20 TPI, corresponding to a 1.27 mm pitch.
A bolt can look right, measure close to the size you expect, and still bind after a turn. Diameter alone doesn’t identify a thread, so check the pitch before you reach for a wrench.
This guide covers ISO metric (M) threads and Unified inch UNC/UNF threads used on many machine fasteners. Use the charts and measurement steps to identify a thread. For acceptance limits, tap drill sizes or pipe threads, consult the applicable drawing and standard.
Thread Pitch, TPI and Lead: Three Different Measurements
How thread measurements differ
| Term | What it describes | Example |
|---|---|---|
| Pitch | Axial spacing between adjacent thread forms | M8 × 1.25: pitch = 1.25 mm |
| TPI | Number of thread pitches per inch of axial length | 1/4-20: 20 TPI |
| Lead | Axial travel in one complete revolution | Single-start, 1.25 mm pitch: lead = 1.25 mm |
On a single-start thread, lead equals pitch. On a multi-start thread, lead is pitch times the number of starts, so a two-start screw with a 2 mm pitch has a 4 mm lead. If you don’t know the number of starts, don’t read pitch from travel per turn.
ISO metric and Unified threads both use a 60° thread angle. Their diameter and pitch combinations still come from different systems.
How to Read Metric and Inch Thread Sizes
Metric example: M10 × 1.5
M is the ISO metric thread form.
10 is the nominal major diameter in millimeters.
1.5 is the pitch in millimeters.
M10 × 1.25 has the same diameter but a finer pitch, and it won’t mate with M10 × 1.5. Coarse pitch is often left off the label, but write it out when you order a replacement.
A socket head screw’s thread size is separate from its length, head dimensions and hex-key size. Check these fields independently when selecting socket head screws.
Inch example: 1/4-20 UNC-2A
1/4 is the nominal major diameter in inches.
20 is the TPI.
UNC is the Unified coarse series.
2A is the fit class for an external thread. Internal threads use B.
On numbered machine screws, the first number is a size code, not a diameter. #10-24 UNC has a nominal major diameter of 0.190 in, not 10 mm.
- M · Thread system
- ISO metric thread
- 10 · Nominal diameter
- 10 mm major diameter
- 1.5 · Pitch
- 1.5 mm axial spacing
- 1/4 · Nominal diameter
- 1/4 inch major diameter
- 20 · Threads per inch
- 20 TPI, pitch 1.27 mm
- UNC · Thread series
- Unified coarse series
- 2A · Thread class
- Class 2, external thread
Right-hand and left-hand threads
Most standard machine fasteners have right-hand threads. Viewed from the end being turned, they tighten clockwise. Left-hand threads tighten counterclockwise and are identified by LH in the designation, such as M10 × 1.5 LH. Confirm the thread direction before ordering a replacement.
Metric Thread Pitch Chart: Common Coarse and Fine Sizes
All pitches below are in millimeters. These diameter-and-pitch combinations follow the ISO 261 general plan. The fine column lists selected options, not every permitted pitch or a guarantee of stock availability. ISO 262 selects combinations for commercial bolts, screws, studs and nuts.
Common metric thread pitches
| Nominal size | Coarse pitch, mm | Selected fine pitch, mm |
|---|---|---|
| M2 | 0.4 | 0.25 |
| M2.5 | 0.45 | 0.35 |
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25; 1.0 |
| M12 | 1.75 | 1.5; 1.25 |
| M14 | 2.0 | 1.5 |
| M16 | 2.0 | 1.5 |
| M18 | 2.5 | 2.0; 1.5 |
| M20 | 2.5 | 2.0; 1.5 |
| M22 | 2.5 | 2.0; 1.5 |
| M24 | 3.0 | 2.0 |
| M27 | 3.0 | 2.0 |
| M30 | 3.5 | 2.0 |
| M36 | 4.0 | 3.0 |
| M42 | 4.5 | 3.0 |
| M48 | 5.0 | 3.0 |
An M8 coarse thread is M8 × 1.25; M8 × 1.0 uses a finer pitch. For hex head screws, confirm the diameter and pitch in the product specification.
Inch Thread Pitch Chart: UNC and UNF
UNC is the coarse series and UNF is the fine series at the same nominal diameter. Higher TPI means a smaller pitch. ASME B1.1 covers Unified thread forms, series, classes, tolerances and designations. The millimeter pitch columns below are calculated as 25.4 ÷ TPI and rounded to three decimal places.
Selected Unified coarse and fine thread sizes
| Nominal size | Nominal major diameter, in | UNC TPI | UNC pitch, mm | UNF TPI | UNF pitch, mm |
|---|---|---|---|---|---|
| #2 | 0.086 | 56 | 0.454 | 64 | 0.397 |
| #3 | 0.099 | 48 | 0.529 | 56 | 0.454 |
| #4 | 0.112 | 40 | 0.635 | 48 | 0.529 |
| #5 | 0.125 | 40 | 0.635 | 44 | 0.577 |
| #6 | 0.138 | 32 | 0.794 | 40 | 0.635 |
| #8 | 0.164 | 32 | 0.794 | 36 | 0.706 |
| #10 | 0.190 | 24 | 1.058 | 32 | 0.794 |
| #12 | 0.216 | 24 | 1.058 | 28 | 0.907 |
| 1/4 in | 0.250 | 20 | 1.270 | 28 | 0.907 |
| 5/16 in | 0.3125 | 18 | 1.411 | 24 | 1.058 |
| 3/8 in | 0.375 | 16 | 1.588 | 24 | 1.058 |
| 7/16 in | 0.4375 | 14 | 1.814 | 20 | 1.270 |
| 1/2 in | 0.500 | 13 | 1.954 | 20 | 1.270 |
| 9/16 in | 0.5625 | 12 | 2.117 | 18 | 1.411 |
| 5/8 in | 0.625 | 11 | 2.309 | 18 | 1.411 |
| 3/4 in | 0.750 | 10 | 2.540 | 16 | 1.588 |
| 7/8 in | 0.875 | 9 | 2.822 | 14 | 1.814 |
| 1 in | 1.000 | 8 | 3.175 | 12 | 2.117 |
On smaller screens, swipe horizontally to view all columns.
1/4-20 UNC and 1/4-28 UNF share a nominal major diameter of 0.250 in, but their nuts are not interchangeable.
Metric vs Inch: Close Sizes That Don’t Interchange
Some metric and inch threads have very similar nominal diameters. Check both diameter and pitch to distinguish them; visual similarity is not evidence of compatibility.
Nearby metric and inch threads compared
| Metric thread | Nearby inch thread | Nominal diameters, mm | Pitches, mm |
|---|---|---|---|
| M6 × 1.0 | 1/4-20 UNC | 6.00 vs 6.35 | 1.00 vs 1.27 |
| M8 × 1.25 | 5/16-18 UNC | 8.00 vs 7.94 | 1.25 vs 1.41 |
| M10 × 1.5 | 3/8-16 UNC | 10.00 vs 9.53 | 1.50 vs 1.59 |
| M12 × 1.75 | 1/2-13 UNC | 12.00 vs 12.70 | 1.75 vs 1.95 |
| M16 × 2.0 | 5/8-11 UNC | 16.00 vs 15.88 | 2.00 vs 2.31 |
M16 and 5/8-11 have nominal diameters that differ by about 0.12 mm, but their pitches differ by about 0.31 mm. Mismatched parts may begin to engage and then bind. If resistance develops, stop and identify the thread instead of applying more force.
Convert Thread Pitch to TPI
Use the exact conversion 1 in = 25.4 mm:
Pitch (mm) = 25.4 ÷ TPI
TPI = 25.4 ÷ pitch (mm)
Calculated pitch conversions
| Thread or pitch | Calculation | Result |
|---|---|---|
| 20 TPI | 25.4 ÷ 20 | 1.2700 mm |
| 28 TPI | 25.4 ÷ 28 | 0.9071 mm, rounded |
| 1.25 mm pitch | 25.4 ÷ 1.25 | 20.32 TPI |
Converting TPI to millimeters compares spacing only; it does not turn an inch thread into a metric thread. Mating parts must also have compatible nominal diameters, thread forms, directions and tolerance classes.
How to Measure Thread Pitch
1. Clean the thread and measure its diameter
Brush off debris and look at several undamaged turns. On an external thread, measure across the crests with a caliper and compare the result with the nominal size. Wear and tolerances can make it read a little small.
The inside diameter measured across a nut’s thread crests is not its nominal thread diameter. Identify an internal thread using the drawing, suitable thread-identification tools or a verified matching reference part. Do not force the parts together.
2. Match a thread pitch gauge
Select a metric or inch pitch-gauge leaf. Align its row of teeth with the screw axis and seat the teeth against several undamaged threads. The profiles should match without visible gaps or rocking. Read the pitch or TPI marked on the leaf and compare neighboring leaves if the fit is uncertain.
3. Without a gauge, measure several intervals
Measure along the axis from one crest to a later crest, then divide by the number of intervals, not the number of crests. Eleven crests contain ten intervals.
Pitch = axial span ÷ number of intervals. Example: 15.0 mm ÷ 10 = 1.50 mm.
Dividing a measured span across ten intervals by ten reduces the effect of a fixed span-reading error. For example, a 0.2 mm error in that span changes the pitch estimate by 0.02 mm. This does not account for every source of measurement error. Use a pitch gauge when nearby pitches are difficult to distinguish.
4. Record the full identification
Write down the thread system, diameter, pitch or TPI, direction and fit class if one is specified. When you replace hex nuts, match the mating thread as well as the strength grade. Never force a nut onto an unknown thread to test it.
Coarse or Fine Thread: How to Choose
Coarse and fine describe pitch relative to nominal diameter, not material or surface finish. For replacement work, retain the thread specified for the existing joint. Changing the series requires compatible mating parts and a review of the joint requirements.
Coarse threads are common on general-purpose fasteners. They advance farther per turn than fine threads of the same diameter and are generally easier to assemble without cross-threading. Their deeper thread profile can be useful in softer tapped materials, but stripping resistance also depends on material strength and effective engagement length.
Fine threads advance less per turn on a single-start screw, allowing finer adjustment. At the same nominal diameter, standard fine threads have a larger tensile stress area than the corresponding coarse threads. Their shallower profile can help where the radial wall thickness around a tapped hole is limited. Adequate axial engagement is still necessary. Fine threads are more sensitive to dirt, burrs and cross-threading. A smaller lead angle does not replace an appropriate locking method under vibration; preload and joint design remain essential.
When specifying threaded rods and studs, include diameter, pitch or TPI, thread direction, length, material, strength requirement and finish. A cut length alone does not identify a replacement.
Standards and Ordering Reference
For metric size selection, ISO 262 identifies selected diameter-and-pitch combinations from ISO 261. Purdue University’s fastener identification guide explains practical inch and metric identification. Use the applicable drawing and thread standard for inspection limits.
If you can’t identify a thread, contact us with the drawing, measured diameter, pitch-gauge result, thread direction and required material. Include clear photographs if the identification remains uncertain.
Frequently Asked Questions
Why can a thread bind even when the diameter and pitch appear to match?
Pitch and nominal diameter are only part of the specification. Check thread form, direction and tolerance class, then inspect for burrs, damage or contamination. Coatings can also affect fit. Stop if the parts bind; do not use a wrench to force engagement.
Can I identify a thread if only two or three turns are exposed?
A short exposed length makes nearby pitches harder to distinguish. Try a pitch gauge on undamaged threads and compare the result with a drawing or known specification. If identification remains uncertain, use appropriate inspection equipment rather than ordering from a short ruler measurement alone.
Should I remove a coating before measuring a thread?
Do not strip a functional coating just to identify a thread. Clean off loose dirt without damaging the surface, then use the part specification and suitable gauges. If coating thickness is suspected of affecting fit, have the finished thread inspected against its specified limits.
Does a pitch gauge prove a thread is within tolerance?
No. A pitch gauge helps identify thread spacing; it does not establish acceptance of the thread. Use the specified GO/NO-GO gauges and any additional dimensional or surface checks required by the drawing and inspection standard.
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