Workshop & Hardware
What Size Is This Screw?
Put the shaft against the gauge and find the closest diameter when the packaging is gone, the drawer is mixed, or you do not own calipers.
Confirm calibration
Calibrate your display for best accuracy before comparing objects.
Identify this screw
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?
How screw size is measured
Diameter is the major diameter across the thread crests. Metric sizes use the M designation (M5 = 5 mm). Imperial numbered and fractional sizes use Unified major diameters (ASME B1.1).
Length depends on head style. Protruding heads are usually measured from under the head to the tip. Flat / countersunk heads often include the head in overall length.
Thread pitch is a separate property. Diameter and length alone do not fully identify a screw: verify pitch before buying a replacement.
Metric screw diameter chart
Nominal major diameter equals the M size. Coarse pitch is the common default when pitch is omitted.
| Nominal | Major Ø (mm) | Coarse pitch |
|---|---|---|
| M2 | 2 | 0.4 mm |
| M2.5 | 2.5 | 0.45 mm |
| 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 |
Source: ISO 261 / ISO 262 metric screw threads. Verified 2026-08-08.
Imperial screw diameter chart
Major diameter is shared by UNC and UNF for the same nominal size. TPI still needs a pitch gauge or packaging.
| Nominal | Major Ø (in) | Major Ø (mm) | UNC TPI | UNF TPI |
|---|---|---|---|---|
| #2 | 0.086 | 2.184 | 56 | 64 |
| #4 | 0.112 | 2.845 | 40 | 48 |
| #5 | 0.125 | 3.175 | 40 | 44 |
| #6 | 0.138 | 3.505 | 32 | 40 |
| #8 | 0.164 | 4.166 | 32 | 36 |
| #10 | 0.19 | 4.826 | 24 | 32 |
| #12 | 0.216 | 5.486 | 24 | 28 |
| 1/4" | 0.25 | 6.350 | 20 | 28 |
| 5/16" | 0.3125 | 7.938 | 18 | 24 |
| 3/8" | 0.375 | 9.525 | 16 | 24 |
| 7/16" | 0.4375 | 11.113 | 14 | 20 |
| 1/2" | 0.5 | 12.700 | 13 | 20 |
Source: ASME B1.1 Unified Inch Screw Threads (UNC / UNF). Closest standard size. Calibrate for best accuracy.
Common questions
How do I find screw size without calipers?
Calibrate your screen, put the shaft against the diameter gauge (major diameter across the thread crests, not the tip), then estimate length on the calibrated ruler. The tool reports the closest metric or imperial major diameter—not a full buy-spec.
What does M5 mean on a screw?
M5 means a metric major diameter of 5 mm. When pitch is omitted from the marking, coarse pitch is the common default—but pitch is still a separate property you should verify before buying a replacement.
Can I buy a replacement screw from diameter alone?
Not safely for critical fits. Confirm pitch, length, head style, and drive type. This checker identifies closest major diameter and approximate length only.
How is screw length measured?
Protruding heads are usually measured from under the head to the tip. Flat or countersunk heads often include the head in overall length. Head style is separate from diameter.
How do I tell metric screws from imperial?
Metric sizes use an M prefix and millimeters (for example M5). Imperial machine screws often use gauge numbers (for example #8, #10) or fractional inches. Nearby diameters can look similar—use Not sure mode and the nearby list when the system is unclear.
Does diameter alone identify the thread?
No. Thread pitch is a separate property. Verify pitch with a pitch gauge or packaging before buying a replacement.