The Challenge
Screw sizing is one of the most misunderstood measurement systems in DIY and construction. An imperial #10 screw is not 10 inches — it's 0.190 inches (4.83mm) in shaft diameter. A 1/4-20 UNC bolt means 1/4-inch diameter with 20 threads per inch. None of this is obvious, and substituting a metric M5 for a 3/16-inch screw will strip threads or leave dangerous play in a joint. The conversion itself is simple: multiply inches by 25.4 to get millimeters. A 1/4-inch screw shaft is 1/4 × 25.4 = 6.35mm. But the harder problem is finding the metric fastener that actually fits — M6 (6.0mm) is the nearest standard metric size, and whether that's an acceptable substitute depends on thread pitch and application load.
Imperial Screw Gauge to Millimeters Conversion Chart
| Screw Gauge | Diameter (inches) | Diameter (mm) | Nearest Metric |
|---|---|---|---|
| #0 | 0.060 in | 1.52 mm | M1.6 |
| #1 | 0.073 in | 1.85 mm | M2 |
| #2 | 0.086 in | 2.18 mm | M2 |
| #3 | 0.099 in | 2.51 mm | M2.5 |
| #4 | 0.112 in | 2.84 mm | M3 |
| #5 | 0.125 in | 3.18 mm | M3 |
| #6 | 0.138 in | 3.51 mm | M3.5 |
| #7 | 0.151 in | 3.84 mm | M4 |
| #8 | 0.164 in | 4.17 mm | M4 |
| #9 | 0.177 in | 4.50 mm | M4.5 |
| #10 | 0.190 in | 4.83 mm | M5 |
| #12 | 0.216 in | 5.49 mm | M5.5 |
| #14 | 0.242 in | 6.15 mm | M6 |
Fractional Inch Bolt Sizes to mm with Thread Pitch
| Imperial Size | Shaft (mm) | Thread (UNC) | Pitch (mm) | Metric Equivalent |
|---|---|---|---|---|
| 1/4 in | 6.35 mm | 1/4-20 UNC | 1.27 mm | M6×1.0 |
| 1/4 in fine | 6.35 mm | 1/4-28 UNF | 0.91 mm | M6×0.75 |
| 5/16 in | 7.94 mm | 5/16-18 UNC | 1.41 mm | M8×1.25 |
| 3/8 in | 9.53 mm | 3/8-16 UNC | 1.59 mm | M10×1.5 |
| 7/16 in | 11.11 mm | 7/16-14 UNC | 1.81 mm | M11×1.5 |
| 1/2 in | 12.70 mm | 1/2-13 UNC | 1.95 mm | M12×1.75 |
| 9/16 in | 14.29 mm | 9/16-12 UNC | 2.12 mm | M14×2.0 |
| 5/8 in | 15.88 mm | 5/8-11 UNC | 2.31 mm | M16×2.0 |
| 3/4 in | 19.05 mm | 3/4-10 UNC | 2.54 mm | M20×2.5 |
| 7/8 in | 22.23 mm | 7/8-9 UNC | 2.82 mm | M22×2.5 |
| 1 in | 25.40 mm | 1-8 UNC | 3.18 mm | M24×3.0 |
Why Imperial and Metric Screws Are Not Interchangeable in Tapped Holes
The shaft diameter difference between a 1/4-inch bolt (6.35mm) and an M6 bolt (6.0mm) is only 0.35mm — close enough to pass through a clearance hole. The thread geometry is the problem. Both systems use a 60-degree thread angle, but metric ISO threads are defined by pitch in millimeters while Unified threads are defined by threads per inch. A 1/4-20 UNC bolt has 1.27mm pitch; M6 standard has 1.0mm pitch. These threads will engage for the first turn then bind or cross-thread, stripping the softer material. In aluminum or plastic housings this destroys the joint permanently. The only safe substitution is re-tapping the hole for the new thread specification.
How to Identify an Unknown Screw Size
- Measure shaft diameter with digital calipers — measure the unthreaded shank or across thread roots, not crests
- If under 6.35mm, compare to gauge chart — check whether it matches a #4 through #14 imperial gauge or M3–M6 metric
- Count threads per inch using a thread gauge or ruler — 20 TPI = UNC 1/4-20, 25.4 TPI = 1.0mm metric pitch
- Check head drive type — Pozidriv is common on metric screws, Phillips on imperial, Torx on both
- When in doubt, bring the screw to a fastener supplier — attempting to match by eye costs more in stripped holes than the price of a proper identification
Imperial vs Metric Fasteners: When Each Applies
Structural Fastener Warning
- Never replace structural bolts (deck ledgers, joist hangers, steel connections) with a 'close enough' metric or imperial substitute — load ratings are specific to fastener grade, diameter, and thread engagement length
- Grade 5 imperial and 8.8 metric bolts have similar tensile strength but are not rated identically — always match the specification called out in engineering drawings
- In the US, IRC and IBC building codes specify fastener types by exact designation — substitutions require engineer approval
Step-by-Step Workflow
Enter shaft diameter in inches as decimal — e.g. 0.190 for a #10 screw
Millimeter result appears — compare against metric screw chart below
Check thread pitch separately — diameter match alone does not guarantee compatibility
Specifications
- Formula
- mm = inches × 25.4
- #6 screw shaft
- 0.138 in = 3.51 mm
- #8 screw shaft
- 0.164 in = 4.17 mm
- #10 screw shaft
- 0.190 in = 4.83 mm
- 1/4 in shaft
- 6.35 mm → nearest metric M6
- 5/16 in shaft
- 7.94 mm → nearest metric M8
- 3/8 in shaft
- 9.53 mm → nearest metric M10
Best Practices
- Imperial screw gauge numbers (#0–#14) are not inches — use the chart below to find actual shaft diameter
- Thread pitch does not convert directly: 1/4-20 UNC has 1.27mm pitch; nearest metric M6 has 1.0mm pitch — not interchangeable in precision applications
- Wood screws: gauge × 0.013 + 0.060 = shaft diameter in inches (approximate)
- For structural applications, never substitute imperial and metric fasteners — thread form (60° unified vs 60° metric ISO) differs in root geometry
- Torque specs from imperial fastener datasheets cannot be applied directly to metric substitutes — recalculate from material and diameter
Frequently Asked Questions
What does a #10 screw size mean in millimeters?
A #10 screw has a shaft diameter of 0.190 inches = 4.83mm. The gauge number system for wood and machine screws uses the formula: diameter (inches) = (gauge × 0.013) + 0.060. A #10 gives (10 × 0.013) + 0.060 = 0.190 inches. The nearest metric equivalent is M5 (5.0mm diameter), though thread pitch differs.
Is a 1/4-inch screw the same as M6?
Not exactly. A 1/4-inch shaft is 6.35mm; M6 is 6.0mm — a 0.35mm difference. For clearance holes and most wood applications this is close enough. For threaded metal joints it is not: 1/4-20 UNC has 1.27mm thread pitch while M6 standard pitch is 1.0mm, making them incompatible in tapped holes.
How do I convert fractional inch screw sizes to mm?
Multiply the fraction by 25.4. Examples: 3/16 in = 0.1875 × 25.4 = 4.76mm. 5/16 in = 0.3125 × 25.4 = 7.94mm. 3/8 in = 0.375 × 25.4 = 9.53mm. 1/2 in = 0.500 × 25.4 = 12.70mm. These are shaft diameters — thread pitch and head size convert separately.
What is the metric equivalent of a #8 wood screw?
A #8 screw shaft diameter is 0.164 inches = 4.17mm. The closest metric wood screw is 4.0mm or 4.2mm diameter, depending on manufacturer. European and Asian flat-pack furniture almost universally uses 4.0mm Pozi or Torx wood screws for this size range.
What does 1/4-20 mean and what is it in mm?
1/4-20 is a UNC (Unified National Coarse) thread specification. The 1/4 is the shaft diameter: 0.25 inches = 6.35mm. The 20 is threads per inch, which gives a thread pitch of 25.4 ÷ 20 = 1.27mm. The nearest metric thread is M6×1.0 (6.0mm diameter, 1.0mm pitch) — same head tooling but not thread-compatible.
Can I use metric screws instead of imperial in an existing tapped hole?
Only if the hole is untapped (clearance hole) and the diameter difference is within tolerance. For pre-tapped holes, no — imperial UNC/UNF threads and metric ISO threads are not interchangeable. The thread angle is the same (60°) but pitch and root profile differ. Forcing a metric bolt into an imperial tapped hole will gall the threads on first installation and fail under load.
How do machine screw sizes compare between imperial and metric?
Common pairings by shaft diameter: #4 (2.84mm) ≈ M3, #6 (3.51mm) ≈ M3.5, #8 (4.17mm) ≈ M4, #10 (4.83mm) ≈ M5, #12 (5.49mm) ≈ M5, 1/4 in (6.35mm) ≈ M6, 5/16 in (7.94mm) ≈ M8, 3/8 in (9.53mm) ≈ M10. These are diameter approximations only — thread pitches differ and direct substitution in tapped holes is not safe.