Identifying inch-thread screws is often easier than it may first appear. With just a few basic measurements, you can quickly and reliably determine the diameter, thread type (UNC/UNF), and length. On this page, you will find practical examples and helpful tips to correctly classify screws and avoid confusion between inch and metric sizes.
Whether by converting inches to millimeters, counting the threads per inch, or using a thread gauge – with these tips you can accurately identify screws and select the right replacement parts. This way, you are well prepared for repairs, restorations, and workshop projects on US vehicles, motorcycles, or machinery.
Step 1: Determine Head Shape, Drive & Length
First, take a look at the head – it determines how the length is measured and which types are available in our range.
Hex Head Bolts
Available from us: Grade 5 plain, Grade 5 zinc-plated, Grade 8, Stainless Steel 18-8Length measurement: under the head
Socket Head Cap Screws
Available from us: Black oxide steel, Zinc-plated steel, Stainless Steel 18-8Length measurement: under the head
Button Head Cap Screws
Available: Black oxide steel, Zinc-plated steel, Stainless steel 18-8Length measurement: measured under the head
Flat Head Socket Cap Screws
Available: Black oxide steel, Zinc-plated steel, Stainless steel 18-8Length measurement: including the head
Pan Head Machine Screws
Available: Zinc-plated steel, Stainless steel 18-8Length measurement: under the head
Socket Set Screws
Available: Black oxide steelLength measurement: overall length (end to end)
Threaded Rods
Available: Steel, Zinc-plated steel, Stainless steel 18-8Length measurement: supplied only in 3-foot (914.4 mm) rods
Step 2: Determine Material and Finish
Next, identify the material of the screw and its surface finish. This will significantly narrow down the search for the right replacement part.
Hex Head Bolts
- Steel Grade 5 plain (comparable to Class 8.8) – identified by 3 radial lines on the bolt head
- Steel Grade 5 zinc-plated (comparable to Class 8.8) – identified by 3 radial lines on the bolt head
- Steel Grade 8 plain (comparable to Class 10.9) – identified by 6 radial lines on the bolt head
- Stainless steel 18-8
Socket Head Cap Screws
- Alloy steel, black oxide (up to 1/2" 39 HRC, from 5/8" 37 HRC)
- Alloy steel, zinc-plated (up to 1/2" 39 HRC, from 5/8" 37 HRC)
- Stainless steel 18-8
Button Head Socket Cap Screws
- Alloy steel, black oxide (up to 1/2" 39 HRC, from 5/8" 37 HRC)
- Alloy steel, zinc-plated (up to 1/2" 39 HRC, from 5/8" 37 HRC)
- Stainless steel 18-8
Flat Head Socket Cap Screws
- Alloy steel, black oxide (up to 1/2" 39 HRC, from 5/8" 37 HRC)
- Alloy steel, zinc-plated (up to 1/2" 39 HRC, from 5/8" 37 HRC)
- Stainless steel 18-8
Socket Set Screws
- Hardened steel 45 H
Pan Head Machine Screws
- Steel Grade 2 zinc-plated (comparable to Class 4.6)
- Stainless steel 18-8
Threaded Rods
- Steel Grade 2 plain (comparable to Class 4.6)
- Steel Grade 2 zinc-plated (comparable to Class 4.6)
- Stainless steel 18-8
Step 3: Determine Thread Count
Determine the thread count, i.e. how many threads per inch are present. This will significantly narrow down the search for the right replacement part.
- Count the threads per inch (TPI = Threads per Inch).
- Place an inch ruler (1 inch = 25.4 mm) against the screw.
- Count the thread peaks over 1 inch.
- If the screw is shorter, count over ½ inch and multiply by 2, or over 10 mm and multiply by 2.54.
Thread Gauges
As an alternative, you can also use our thread gauges to determine the thread count.
Simply place the gauge against the thread and read off the number of threads per inch. In the example, the screw would have 13 threads per inch.
With a measured diameter of 12.7 mm, this results in a UNC coarse thread.
You can find the gauges under Tools.
Step 4: Determine Diameter
- Use a caliper to measure the outside diameter across the thread peaks.
- Record the value in millimeters and divide by 25.4 to obtain the inch value.
- In the thread table you will find the values listed in millimeters and inch decimal.
Step 5: Identify the Thread
? By combining the previously determined thread count (TPI) and the thread diameter, you can clearly identify the screw thread:
- The diameter defines the basic size (e.g. 1/4", 5/16", 1/2").
- The threads per inch complement this information and narrow down the exact size.
Using the thread table (further down the page), you can match both values to determine the exact UNC or UNF thread.
For example, if you measure a diameter of approx. 6.35 mm and a thread count of 2
UNC - Unified Coarse Thread Series - ANSI B1.1
American standard coarse thread. The flank angle is 60°.
| ø | Threads per Inch (TPI) | Millimeters | Inch Decimal |
| #2 | 56 | 2.1844 | 0.0860 |
| #3 | 48 | 2.5146 | 0.0990 |
| #4 | 40 | 2.8448 | 0.1120 |
| #5 | 40 | 3.1750 | 0.1250 |
| #6 | 32 | 3.5052 | 0.1380 |
| #8 | 32 | 4.1656 | 0.1640 |
| #10 | 24 | 4.7625 | 0.1875 |
| #12 | 24 | 5.4864 | 0.2160 |
| 1/4 | 20 | 6.3500 | 0.2500 |
| 5/16 | 18 | 7.9375 | 0.3125 |
| 3/8 | 16 | 9.5250 | 0.3750 |
| 7/16 | 14 | 11.1125 | 0.4375 |
| 1/2 | 13 | 12.7000 | 0.5000 |
| 9/16 | 12 | 14.2875 | 0.5625 |
| 5/8 | 11 | 15.8750 | 0.6250 |
| 3/4 | 10 | 19.0500 | 0.7500 |
| 7/8 | 9 | 22.2250 | 0.8750 |
| 1 | 8 | 25.4000 | 1.0000 |
UNF - Unified Fine Thread Series - ANSI B1.1
American standard fine thread. The flank angle is 60°.
| ø | Threads per Inch (TPI) | Millimeters | Inch Decimal |
| #2 | 64 | 2.1844 | 0.0860 |
| #3 | 56 | 2.5146 | 0.0990 |
| #4 | 48 | 2.8448 | 0.1120 |
| #5 | 44 | 3.1750 | 0.1250 |
| #6 | 40 | 3.5052 | 0.1380 |
| #8 | 36 | 4.1656 | 0.1640 |
| #10 | 32 | 4.7625 | 0.1875 |
| #12 | 28 | 5.4864 | 0.2160 |
| 1/4 | 28 | 6.3500 | 0.2500 |
| 5/16 | 24 | 7.9375 | 0.3125 |
| 3/8 | 24 | 9.5250 | 0.3750 |
| 7/16 | 20 | 11.1125 | 0.4375 |
| 1/2 | 20 | 12.7000 | 0.5000 |
| 9/16 | 18 | 14.2875 | 0.5625 |
| 5/8 | 18 | 15.8750 | 0.6250 |
| 3/4 | 16 | 19.0500 | 0.7500 |
| 7/8 | 14 | 22.2250 | 0.8750 |
| 1 | 12 | 25.4000 | 1.0000 |