Buying Guides
Metric vs Imperial Screws: Size Conversion Chart
M6 vs 1/4 inch — are they the same screw? A metric-to-imperial conversion chart for M3–M12, how to read both size systems, and why close in diameter does not mean interchangeable.
A buyer in the US asks for a 1/4-inch screw. A supplier in China lists the same job as M6. The diameters look close — 6 mm against 6.35 mm — so it is tempting to treat them as the same screw. They are not.
Metric and imperial screws differ in two ways at once: diameter and thread. A size that matches in diameter almost never matches in thread pitch, so a metric screw will not fit an imperial nut or tapped hole, and forcing it will damage both. This guide explains how each system is written, gives the conversion chart buyers actually use, and shows how to order the right screw when you only have the other system's size.
1. How metric sizes are written
Metric screws follow the ISO system, used in most of the world. A size is written as M + diameter in millimetres, then the thread pitch, then the length — for example M6 × 1.0 × 20:
- M6 — 6 mm nominal thread diameter
- 1.0 — 1.0 mm thread pitch, the distance from one thread peak to the next
- 20 — 20 mm length
If the pitch is left out, as in M6 × 20, it is the standard coarse pitch for that diameter — 1.0 mm for M6. The full list of standard coarse pitches is in the measuring guide.
2. How imperial sizes are written
Imperial screws follow the Unified Thread Standard (UTS), used mainly in the United States and Canada. A size is written as diameter, then threads per inch (TPI) — for example 1/4-20:
- 1/4 — 1/4 inch diameter
- 20 — 20 threads per inch (coarse, UNC); a fine version would be 1/4-28 (UNF)
Sizes smaller than 1/4 inch use a gauge number instead of a fraction: #8-32 means gauge #8, 32 threads per inch. The larger the gauge number, the larger the screw — #10 is thicker than #8, which is thicker than #6. The gauge number itself is not a measurement: #8 is 0.164 inches (4.17 mm) in diameter.
So the two systems do not just use different units. Metric states the distance between threads in millimetres; imperial states how many threads fit in one inch. That is why a conversion needs a table, not just a calculator.
3. Metric to imperial conversion chart
"Closest" in this table means closest in diameter only. Thread pitch and TPI still differ — see section 4 before swapping one for the other.
Metric | Diameter | Closest imperial | Imperial diameter | Metric coarse pitch | Imperial coarse TPI |
|---|---|---|---|---|---|
M3 | 3.00 mm | #4 | 2.84 mm (0.112″) | 0.5 mm | 40 |
M4 | 4.00 mm | #8 | 4.17 mm (0.164″) | 0.7 mm | 32 |
M5 | 5.00 mm | #10 | 4.83 mm (0.190″) | 0.8 mm | 24 |
M6 | 6.00 mm | 1/4″ | 6.35 mm (0.250″) | 1.0 mm | 20 |
M8 | 8.00 mm | 5/16″ | 7.94 mm (0.313″) | 1.25 mm | 18 |
M10 | 10.00 mm | 3/8″ | 9.53 mm (0.375″) | 1.5 mm | 16 |
M12 | 12.00 mm | 1/2″ | 12.70 mm (0.500″) | 1.75 mm | 13 |
Two things to notice in the table. First, the match is never exact: M6 is 0.35 mm smaller than 1/4 inch, M4 is 0.17 mm smaller than #8, M10 is 0.47 mm larger than 3/8 inch. Second, the pitch never lines up either — an M6 coarse pitch of 1.0 mm works out to about 25.4 threads per inch, not the 20 TPI of a 1/4-20 screw.
For length, the conversion is exact and safe to calculate: 1 inch = 25.4 mm. So 1/2″ is 12.7 mm, 3/4″ is 19.05 mm, 1″ is 25.4 mm, 1-1/2″ is 38.1 mm and 2″ is 50.8 mm. Length converts cleanly; diameter and thread do not.
4. Why "close" does not mean interchangeable
A screw and a nut fit only when both the diameter and the thread match. Metric and imperial threads are cut to different standards, so even a pair that looks the same size will jam, cross-thread, or feel like it "almost fits" — and "almost fits" is how threads get stripped.
- M6 × 1.0 vs 1/4-20: the diameters differ by 0.35 mm and the threads differ by more than 5 threads per inch. An M6 screw will start in a 1/4-20 nut and then bind.
- M8 × 1.25 vs 5/16-18: 1.25 mm pitch is about 20.3 TPI, against 18 TPI imperial. Same story, different sizes.
- M10 × 1.5 vs 3/8-16: 1.5 mm pitch is about 16.9 TPI — the closest pairing in the table, and still not a match.
The practical rule is simple: never substitute a metric screw for an imperial one (or the reverse) in a threaded hole or nut. The only safe swap is in a clearance hole, where the screw passes through freely and is held by a nut on the other side — and even then, use a nut from the same system as the screw.
If a screw you already have measures between two sizes, re-measure rather than rounding to the nearest equivalent in the other system. Screws do not come in half sizes in this range, and a measurement between sizes usually means a measuring error, a worn thread, or a size from the other system.
5. Which system should you stock?
It depends on who buys from you:
- Selling mainly outside the US — metric covers almost everything: machinery, electronics, furniture and general repairs are all specified in M sizes.
- Selling into the US, or supplying US-made equipment — expect imperial callouts (#6, #8, #10, 1/4″, 5/16″, 3/8″) and keep the conversion chart above beside the bench.
- Distributors serving both markets — stock metric as the main range and keep the imperial equivalents as a cross-reference, not as a substitute. When a customer gives an imperial size, confirm the metric size by measuring the screw, not by reading the chart alone.
Boltriver's standard range — assortment kits, machine screws, self-tapping screws and micro screws — is metric. If you need a size for US equipment, send the imperial size and a photo with a scale reference, and we confirm the matching metric size before quoting.
6. How to order when you only have the other system's size
Send the same three facts as any other enquiry, in whichever system you have them:
- The size you have — for example "1/4-20 × 1 inch" or "M6 × 1.0 × 20" — plus the head type
- A photo of the screw next to a ruler, so the diameter, length and thread can be checked
- Quantity and delivery country
Do not order by the conversion chart alone. The chart tells you what to look for; the photo and the three numbers tell the supplier what will actually fit. See also: How to Request a Quote for Custom Screws.
Quick checklist before you order across systems
- Is the size you have metric (M + mm) or imperial (# gauge / inches + TPI)?
- Have you checked pitch/TPI, not just diameter?
- Are you replacing a screw in a threaded hole or nut — where only an exact match works?
- Have you converted the length correctly (1″ = 25.4 mm)?
- If in doubt, have you photographed the screw next to a ruler?
Talk to Boltriver about metric sizes
Boltriver supplies metric fastener assortment kits, machine screws, self-tapping screws and micro screws, plus custom screws made to your specifications. Orders start from just 1 piece.
To request a quote, send a kit reference or product photo, the quantity, and the delivery country, and we will confirm kit contents, pricing and availability. If your drawing or sample is in imperial sizes, send it as it is — include the imperial size and a photo with a scale reference, and we will confirm the matching metric size.
