Workshop & Hardware
What Size Is This Bolt?
Align the bolt shaft with actual-size references to find the closest diameter, then estimate length on a calibrated ruler.
Confirm calibration
Calibrate your display for best accuracy before comparing objects.
Identify this bolt
Avoid pressing sharp metal against the display. Hold the object just above the reference or use a thin protective layer when needed. Do not drag sharp threads across the screen.
1. Unit system
2. Shaft diameter
Compare the major diameter across the thread crests — not the tip.
Align shaft with the gap
3. Approximate length
For common protruding heads, measure from under the head to the tip. For flat / countersunk heads, overall length usually includes the head.
Thread pitch check (on-screen pitch gauge) is planned — not available yet. Do not assume pitch from diameter alone.
Can't find what you're measuring?
Bolt diameter and length
Bolts and machine screws often share the same diameter standards. The difference is usually head style, length, and how they are used — not a different major-diameter chart.
Common mistake: treating a measured 4.9 mm shaft as exact when the nearest standard is M5 (5.0 mm). This tool always shows measured vs nominal.
Metric vs imperial can look similar on screen (#10 ≈ 4.83 mm vs M5 = 5 mm). Use the "Not sure†mode and nearby list when you are unsure which system you have.
Metric bolt diameter chart
| Nominal | Major Ø (mm) | Coarse pitch |
|---|---|---|
| M3 | 3 | 0.5 mm |
| M3.5 | 3.5 | 0.6 mm |
| M4 | 4 | 0.7 mm |
| M5 | 5 | 0.8 mm |
| M6 | 6 | 1 mm |
| M8 | 8 | 1.25 mm |
| M10 | 10 | 1.5 mm |
| M12 | 12 | 1.75 mm |
| M14 | 14 | 2 mm |
| M16 | 16 | 2 mm |
| M20 | 20 | 2.5 mm |
Imperial bolt diameter chart
| Nominal | Major Ø (mm) | UNC TPI |
|---|---|---|
| #10 | 4.826 | 24 |
| #12 | 5.486 | 24 |
| 1/4" | 6.350 | 20 |
| 5/16" | 7.938 | 18 |
| 3/8" | 9.525 | 16 |
| 7/16" | 11.113 | 14 |
| 1/2" | 12.700 | 13 |
Source: ASME B1.1 Unified Inch Screw Threads (UNC / UNF). Closest standard size. Calibrate for best accuracy.
Common questions
How do I measure bolt diameter on screen?
Calibrate your display, align the bolt shaft (major diameter across the thread crests) with actual-size references, then estimate length on the calibrated ruler. Results show the closest metric or imperial major diameter.
What does M8 mean on a bolt?
M8 means a metric major diameter of 8 mm. Pitch, length, and head style are separate properties. This checker identifies closest major diameter and approximate length—not a complete buy-spec.
Is wrench size the same as bolt diameter?
No. Bolt diameter is the threaded shaft’s major diameter (for example M8 is 8 mm). Wrench size is the across-flats dimension of the hex head or nut—a different measurement. Do not use wrench size to identify thread diameter.
Is a measured 4.9 mm shaft exactly M5?
Not automatically. The nearest standard may be M5 (5.0 mm). This tool always shows measured versus nominal so you can judge closeness, and lists nearby alternatives when the match is ambiguous.
Can metric and imperial bolts look the same?
Yes. For example #10 is about 4.83 mm while M5 is 5 mm. Use Not sure mode and the nearby list when the system is unclear, and verify pitch separately.
Does this checker find thread pitch?
No. Pitch still needs a pitch gauge or packaging. Diameter and length alone do not fully specify a bolt.