The Challenge
Metric and imperial screws are not interchangeable — thread pitch, diameter, and head size all differ. A metric M5 screw is 5mm in diameter (0.197in), but the closest US equivalent is a #10 screw at 0.190in. Close enough to fit loosely into the wrong hole, tight enough to strip it. IKEA hardware ships M6 bolts; American lumber stores sell 1/4-20 UNC. A 3mm pilot hole needs a different bit than a #6 imperial screw. The diameter conversion is straightforward — divide millimeters by 25.4. The complication is that metric screws are specified by diameter and thread pitch (M6×1.0 means 6mm diameter, 1.0mm between threads), while US screws use gauge numbers with threads per inch (TPI). These systems don't map cleanly onto each other, so matching by converted diameter alone can give you the wrong screw.
Metric Screw Diameter: mm to Inches Conversion Chart
| Metric Size | Diameter (mm) | Diameter (inches) | Closest US Size | Coarse Thread Pitch |
|---|---|---|---|---|
| M1.6 | 1.6 mm | 0.063 in | #0 (0.060 in) | 0.35 mm |
| M2 | 2.0 mm | 0.079 in | #2 (0.086 in) | 0.40 mm |
| M2.5 | 2.5 mm | 0.098 in | #3 (0.099 in) | 0.45 mm |
| M3 | 3.0 mm | 0.118 in | #4 (0.112 in) | 0.50 mm |
| M3.5 | 3.5 mm | 0.138 in | #6 (0.138 in) | 0.60 mm |
| M4 | 4.0 mm | 0.157 in | #8 (0.164 in) | 0.70 mm |
| M5 | 5.0 mm | 0.197 in | #10 (0.190 in) | 0.80 mm |
| M6 | 6.0 mm | 0.236 in | 1/4 in (0.250 in) | 1.00 mm |
| M7 | 7.0 mm | 0.276 in | — | 1.00 mm |
| M8 | 8.0 mm | 0.315 in | 5/16 in (0.313 in) | 1.25 mm |
| M10 | 10.0 mm | 0.394 in | 3/8 in (0.375 in) | 1.50 mm |
| M12 | 12.0 mm | 0.472 in | 1/2 in (0.500 in) | 1.75 mm |
| M14 | 14.0 mm | 0.551 in | 9/16 in (0.563 in) | 2.00 mm |
| M16 | 16.0 mm | 0.630 in | 5/8 in (0.625 in) | 2.00 mm |
| M20 | 20.0 mm | 0.787 in | 3/4 in (0.750 in) | 2.50 mm |
| M24 | 24.0 mm | 0.945 in | 1 in (1.000 in) | 3.00 mm |
Common Screw Lengths: mm to Inches
| Length (mm) | Length (inches) | Common Use |
|---|---|---|
| 6 mm | 0.236 in | Electronics, PCB standoffs |
| 8 mm | 0.315 in | Thin panel fastening |
| 10 mm | 0.394 in | General electronics, PC builds |
| 12 mm | 0.472 in | PC case, appliances |
| 16 mm | 0.630 in | Light fixtures, thin timber |
| 20 mm | 0.787 in | Furniture flatpack, brackets |
| 25 mm | 0.984 in | General woodwork, framing hardware |
| 30 mm | 1.181 in | Cabinet hardware, hinges |
| 35 mm | 1.378 in | Door hinges (standard EU) |
| 40 mm | 1.575 in | Structural timber connections |
| 50 mm | 1.969 in | Timber framing, decking |
| 60 mm | 2.362 in | Heavy timber, structural |
| 75 mm | 2.953 in | Decking, joinery |
| 100 mm | 3.937 in | Heavy structural, landscaping |
Why Metric and Imperial Screws Are Not Interchangeable
The thread pitch incompatibility between metric and imperial fasteners is the core problem. Metric screws follow ISO 68-1 thread profiles with pitch measured in millimeters per thread. Imperial screws follow Unified National Thread Standard (UNS) with pitch measured in threads per inch. Even when diameters nearly match — M6 at 6mm vs 1/4 inch at 6.35mm — the pitch difference means threads won't engage cleanly. Forcing a metric bolt into an imperial threaded hole damages the thread form and produces a joint that fails under vibration or load. The only safe approach is to match both diameter and thread standard. When replacing a fastener, always identify both the diameter and the thread pitch before ordering a substitute.
How to Identify an Unknown Screw Size
- Measure the outer diameter with calipers — a round number in mm (3.0, 4.0, 5.0, 6.0) indicates metric; a fractional inch or gauge number indicates imperial
- Count threads per 10mm using a thread pitch gauge or ruler — divide 10 by the count to get pitch in mm (e.g. 8 threads per 10mm = 1.25mm pitch = M8 coarse)
- Check head markings — metric bolts show a property class number (8.8, 10.9); imperial bolts show radial grade lines or no marking for Grade 2
- Cross-reference diameter and pitch against the chart above to confirm the full callout (e.g. M6×1.0 or 1/4-20 UNC)
Metric vs Imperial Screws: When Each System Applies
Camera Gear Exception: 1/4-20 and M6 Confusion
- Camera tripod sockets use 1/4-20 UNC (0.250in) as the standard thread in North America
- Many European and Asian camera accessories use M6 (0.236in, 1.0mm pitch) instead
- These look identical but have different thread pitches — forcing one into the other will cross-thread the socket
- Some tripod heads ship with both a 1/4-20 and 3/8-16 adapter — check the thread before mounting
Step-by-Step Workflow
Enter the screw diameter in millimeters — this is the M-number on metric screws
Inch equivalent appears instantly — use this to find the nearest US gauge or fractional size
Cross-reference the chart below to confirm thread pitch compatibility before ordering
Specifications
- Formula
- inches = mm ÷ 25.4
- M3 diameter
- 3mm = 0.118 in (close to #4 screw)
- M4 diameter
- 4mm = 0.157 in (close to #8 screw)
- M5 diameter
- 5mm = 0.197 in (close to #10 screw)
- M6 diameter
- 6mm = 0.236 in (close to 1/4 in)
- M8 diameter
- 8mm = 0.315 in (close to 5/16 in)
- M10 diameter
- 10mm = 0.394 in (close to 3/8 in)
Best Practices
- Diameter conversion alone is not enough — always verify thread pitch separately before substituting metric for imperial
- M6 (0.236in) and 1/4-20 UNC (0.250in) look similar but are not interchangeable — thread pitch differs (1.0mm vs 1.27mm)
- For electronics and PC builds: M3 is the most common screw type for case hardware, drives, and standoffs
- Pilot hole sizes: M3 = 2.5mm, M4 = 3.3mm, M5 = 4.2mm, M6 = 5.0mm, M8 = 6.8mm
- Screw length is always in mm for metric, inches for imperial — a 30mm screw is approximately 1.18 inches
Frequently Asked Questions
What is the imperial equivalent of an M6 screw?
M6 has a 6mm diameter, which converts to 0.236 inches. The nearest US fractional size is 1/4 inch (0.250in). They are close but not interchangeable — M6 has a 1.0mm thread pitch while the common 1/4-20 UNC has 1.27mm pitch (20 threads per inch). In a pinch, an M6 bolt will thread into a 1/4-20 nut loosely and will strip under load. Use the correct fastener.
What is an M3 screw in inches?
M3 has a 3mm diameter = 0.118 inches. The nearest US size is a #4 screw (0.112in) or #5 (0.125in). M3 is the standard screw for PC case components, 2.5-inch hard drives, SSD brackets, and most consumer electronics. Thread pitch is 0.5mm (coarse) or 0.35mm (fine).
Is M5 the same as 10-32?
Close but not the same. M5 diameter is 5mm (0.197in). The 10-32 UNF screw has a 0.190in diameter. The diameters differ by 0.007 inches — enough that M5 will not properly thread into a 10-32 hole. Thread pitch also differs: M5 coarse is 0.8mm, while 10-32 has 32 TPI (0.794mm). These are functionally incompatible despite being nearly identical in size.
How do I convert screw length from mm to inches?
Divide millimeters by 25.4. Common lengths: 10mm = 0.394in, 16mm = 0.630in, 20mm = 0.787in, 25mm = 0.984in (close to 1 inch), 30mm = 1.181in, 40mm = 1.575in, 50mm = 1.969in. Screw length is always measured from under the head to the tip, except for countersunk screws which are measured overall.
What does M mean in screw sizes?
M stands for metric, specifically the ISO metric thread standard. The number after M is the nominal outer diameter in millimeters. M6 = 6mm outer diameter. A full metric screw callout includes diameter and pitch: M6×1.0 means 6mm diameter with 1.0mm thread pitch. Coarse pitch is the default and most common — fine pitch threads (e.g. M6×0.75) are used in precision applications.
How do I know if a screw is metric or imperial by looking at it?
The most reliable visual check is head markings. Metric bolts often have a raised number (property class: 8.8, 10.9, 12.9) on the head. Imperial bolts use radial lines — 3 lines for Grade 5, 6 lines for Grade 8. No markings usually means Grade 2 (imperial) or Class 4.6 (metric). When unmarked, measure with calipers and compare against the chart — metric sizes are round numbers in mm, imperial are fractions of an inch.
Can I use a metric screw in an imperial hole?
Only if the diameters and thread pitches happen to align — which is rare. M6 (1.0mm pitch) forced into a 1/4-20 UNC hole (1.27mm pitch) will cross-thread and damage both the screw and the workpiece. The one common exception is certain camera tripod sockets: 1/4-20 UNC and M6 are both used, and their near-identical diameter causes frequent confusion on camera gear.