Technical guide
Thread Pitch Chart: UNC, UNF & Metric Thread Sizes
Identify common Unified inch and ISO metric screw threads with UNC, UNF, TPI, and metric-pitch charts, plus practical measurement guidance.
Thread diameter alone is not enough to identify a screw. A #4-40 screw and a #4-48 screw have nearly the same outside diameter, but their threads will not mate. The same is true for M3 × 0.5 and M3 × 0.35 fasteners.
Use the charts below to identify common Unified inch and ISO metric machine-screw threads. Then confirm the fit with a thread-pitch gauge or a known matching nut before ordering.
Quick answer: Inch threads are normally written as diameter followed by threads per inch, such as #4-40 UNC. Metric threads are written as diameter followed by the distance between adjacent thread crests, such as M3 × 0.5.
Inch machine-screw thread chart: numbered sizes
In the Unified Thread Standard, numbered screw sizes are used below 1/4 inch. The first number identifies nominal diameter; the second is the number of thread crests in one inch, or threads per inch (TPI).
| Screw size | Nominal major diameter | UNC coarse TPI | UNF fine TPI | Metric diameter equivalent* |
|---|---|---|---|---|
| #0 | 0.0600 in. | — | 80 | 1.52 mm |
| #1 | 0.0730 in. | 64 | 72 | 1.85 mm |
| #2 | 0.0860 in. | 56 | 64 | 2.18 mm |
| #3 | 0.0990 in. | 48 | 56 | 2.51 mm |
| #4 | 0.1120 in. | 40 | 48 | 2.84 mm |
| #5 | 0.1250 in. | 40 | 44 | 3.18 mm |
| #6 | 0.1380 in. | 32 | 40 | 3.51 mm |
| #8 | 0.1640 in. | 32 | 36 | 4.17 mm |
| #10 | 0.1900 in. | 24 | 32 | 4.83 mm |
| #12 | 0.2160 in. | 24 | 28 | 5.49 mm |
\*The millimeter value is only a conversion of nominal diameter. It does not make an inch screw interchangeable with a metric screw.
What does #4-40 mean?
#4-40 UNC identifies a Unified coarse-thread machine screw with:
- #4 nominal diameter, approximately 0.112 inch;
- 40 threads per inch; and
- UNC thread series.
Planet Fasteners carries #4-40 female-female PCB standoffs and #4-40 male-female PCB standoffs. The body length may be metric while the mating thread remains #4-40; always read both specifications.
Inch thread pitch chart: common fractional sizes
| Nominal diameter | UNC coarse TPI | UNF fine TPI | Coarse pitch | Fine pitch |
|---|---|---|---|---|
| 1/4 in. | 20 | 28 | 0.0500 in. | 0.0357 in. |
| 5/16 in. | 18 | 24 | 0.0556 in. | 0.0417 in. |
| 3/8 in. | 16 | 24 | 0.0625 in. | 0.0417 in. |
| 7/16 in. | 14 | 20 | 0.0714 in. | 0.0500 in. |
| 1/2 in. | 13 | 20 | 0.0769 in. | 0.0500 in. |
| 9/16 in. | 12 | 18 | 0.0833 in. | 0.0556 in. |
| 5/8 in. | 11 | 18 | 0.0909 in. | 0.0556 in. |
| 3/4 in. | 10 | 16 | 0.1000 in. | 0.0625 in. |
| 7/8 in. | 9 | 14 | 0.1111 in. | 0.0714 in. |
| 1 in. | 8 | 12 | 0.1250 in. | 0.0833 in. |
Pitch in inches equals 1 ÷ TPI. For example, a 1/4-20 screw has a pitch of 0.050 inch because 1 ÷ 20 = 0.050.
Metric thread pitch chart
Metric fastener designations use nominal outside diameter and pitch, both in millimeters. M4 × 0.7 means a 4 mm nominal major diameter and 0.7 mm between adjacent thread crests.
When pitch is omitted, the designation commonly refers to the standard coarse pitch—but the product specification should still state it explicitly whenever confusion is possible.
| Metric thread | Standard coarse pitch | Common fine pitch options | Approx. coarse TPI |
|---|---|---|---|
| M1.6 | 0.35 mm | 0.20 mm | 72.6 |
| M2 | 0.40 mm | 0.25 mm | 63.5 |
| M2.5 | 0.45 mm | 0.35 mm | 56.4 |
| M3 | 0.50 mm | 0.35 mm | 50.8 |
| M4 | 0.70 mm | 0.50 mm | 36.3 |
| M5 | 0.80 mm | 0.50 mm | 31.8 |
| M6 | 1.00 mm | 0.75 mm | 25.4 |
| M8 | 1.25 mm | 1.00 / 0.75 mm | 20.3 |
| M10 | 1.50 mm | 1.25 / 1.00 mm | 16.9 |
| M12 | 1.75 mm | 1.50 / 1.25 mm | 14.5 |
Fine pitches listed here are common standardized choices, but not every diameter-pitch combination is stocked or appropriate for every product standard.
UNC vs. UNF: what is the difference?
UNC means Unified National Coarse. UNF means Unified National Fine. For a given diameter, the UNF version has more threads per inch and therefore a shorter distance between thread crests.
Coarse threads are commonly chosen because they assemble quickly, tolerate minor damage and contamination better, and provide deeper thread engagement in softer materials. Fine threads provide finer adjustment and more engaged threads over a given length, but require the matching female thread and appropriate installation controls.
Neither series is universally stronger or more vibration-resistant in every joint. Joint material, engagement length, preload, tightening method, fastener strength, and service conditions all matter. Do not replace UNC with UNF—or vice versa—without checking the assembly design.
Metric versus inch: close is not compatible
Some thread diameters appear nearly identical:
| Inch thread | Nominal diameter | Nearby metric size | Why it does not fit correctly |
|---|---|---|---|
| #2-56 | 2.18 mm | M2 × 0.4 | Different diameter and pitch |
| #4-40 | 2.84 mm | M3 × 0.5 | Different diameter and pitch |
| #6-32 | 3.51 mm | M3.5 × 0.6 | Different pitch system |
| #8-32 | 4.17 mm | M4 × 0.7 | Different diameter and pitch |
| #10-24 | 4.83 mm | M5 × 0.8 | Different diameter and pitch |
| 1/4-20 | 6.35 mm | M6 × 1.0 | Different diameter and pitch |
A screw that turns one or two revolutions is not proof of compatibility. Forcing near-matching threads can damage the screw, nut, threaded insert, or standoff.
How to measure screw diameter and thread pitch
1. Measure the major diameter
Use calipers across the thread crests. Compare the reading with the nominal values in the chart. Measurements can be slightly below nominal because of manufacturing tolerances, plating, wear, and caliper placement.
2. Determine whether it is inch or metric
If the diameter falls near a numbered or fractional inch size, check TPI. If it falls near a whole or standard metric diameter, check pitch in millimeters. Do not decide from diameter alone.
3. Measure the pitch
The best tool is a thread-pitch gauge. Match a gauge leaf to the thread until there is no visible gap.
Without a gauge:
- For inch threads, count the number of thread intervals over a known length and convert to TPI.
- For metric threads, measure across ten thread intervals and divide by ten. Measuring several intervals reduces error on small fasteners.
4. Verify with a known mating part
A correctly matched nut or internally threaded standoff should turn smoothly by hand. Stop if it binds. Never use a wrench to “test” an uncertain thread.
How to read common fastener callouts
#4-40 × 3/8 in.
- #4 nominal screw diameter
- 40 TPI
- 3/8-inch nominal length
- UNC when the series is stated or established by the governing specification
M3 × 0.5 × 10 mm
- 3 mm nominal screw diameter
- 0.5 mm pitch
- 10 mm nominal length
M3 × 10 mm
The omitted pitch typically implies the standard coarse M3 pitch of 0.5 mm, but confirm the product data before mating it with an existing part.
Thread standards and fastener product standards
Unified inch threads are defined by ASME B1.1. General M-profile metric threads used in US practice are covered by ASME B1.13M, based on the ISO 68-1 profile.
DIN and ISO designations shown in the Planet Fasteners catalog—such as ISO 4762 / DIN 912 for socket head cap screws—describe the fastener product's geometry and requirements. They should not be mistaken for a complete thread callout. Diameter, pitch, material, property class, drive size, head dimensions, and length remain separate selection details.
Use the Planet Fasteners reference center alongside each product listing to confirm dimensions, tap-drill information, clearance holes, standards, and the nearest inch or metric size.
Buying checklist
Before ordering a screw, nut, washer, or standoff, confirm:
- Inch or metric thread system
- Nominal diameter
- TPI or metric pitch
- Thread series or tolerance class when specified
- Required engagement and fastener length
- Head style and drive size
- Material and finish
- Strength or property class when the joint requires it
- Applicable product standard
Washers do not have mating threads, but their inside diameter must suit the selected screw. Browse flat washers and split lock washers by compatible nominal size.
Frequently asked questions
Is thread pitch the same as TPI?
They describe the same spacing in different ways. TPI counts threads per inch. Metric pitch measures the distance between adjacent thread crests in millimeters. Convert with metric pitch = 25.4 ÷ TPI.
Are #4-40 and M3 the same?
No. A #4 thread has a nominal diameter of 0.112 inch (about 2.84 mm); M3 has a nominal diameter of 3 mm. Their pitches also differ: #4-40 is 40 TPI, equivalent to 0.635 mm pitch, while standard coarse M3 is 0.5 mm.
How can I tell whether a screw is coarse or fine?
Identify its diameter, then compare its TPI or metric pitch with the standard chart. “Coarse” and “fine” are relative to the thread system and diameter—not a visual judgment alone.
Can I use a metric nut on an inch screw?
No, unless both parts were specifically manufactured with the same thread standard—which ordinary metric and Unified fasteners are not. Similar diameters do not make the threads compatible.
What does 2A or 2B mean?
Unified thread classes describe fit and tolerance. The letter A identifies an external thread and B an internal thread. Class 2A/2B is widely used for general-purpose fasteners; use the class required by the drawing or product specification.
What do 6g and 6H mean on metric threads?
They are metric tolerance classes. A common general-purpose pairing is an external 6g thread with an internal 6H thread. They are not strength grades.
Find the fasteners and tools you need
Products selected to support the measurements and applications covered in this guide.
