Metric vs Imperial Fasteners in Australia: Conversion Guide
Most Australian construction is metric. But imperial fasteners still show up — imported equipment, older plant, American-specified components, and the occasional “mystery bolt” on site that doesn’t match any M-size you’ve got.
This guide is for the real world: how to tell what you’ve got, how to convert sizes, and how to avoid the classic mistakes that strip threads or round heads.
Metric vs imperial: the fastest way to tell
Metric (M):
- Diameter is in millimetres: M6, M8, M10, M12
- Pitch is in mm between threads (e.g., M10 x 1.5)
Imperial:
- Diameter in inches: 1/4″, 3/8″, 1/2″
- Thread is often in TPI (threads per inch), e.g., UNC/UNF
If you can’t identify it quickly, don’t force it — mismatching threads will feel like they’re “biting” and then suddenly tighten hard. That’s how you destroy both nut and bolt.
Common fastener conversions you’ll actually see on Aussie jobs
These are the “usual suspects” when something imperial sneaks onto a metric site:
- 1/4″ ≈ 6.35 mm (closest metric diameter: M6)
- 3/8″ ≈ 9.53 mm (closest: M10)
- 1/2″ ≈ 12.7 mm (closest: M12)
Important: “closest diameter” does not mean interchangeable — thread form and pitch still matter.
Pitch vs TPI (where most mix-ups happen)
- Metric pitch: distance between thread peaks in mm
- Imperial: number of threads in one inch
That’s why two bolts can “look” similar but not mate properly.
Don’t mix spanners blindly
Spanner sizes are another trap:
- A metric head might accept an imperial spanner “sort of” (and vice versa)
- That’s how corners get rounded, especially on higher-torque work
If it’s a critical fix, use the right tool. If you’re doing repeat work, label bins clearly (metric/imperial) and keep them separated.
What to do when you’re stuck (quick field process)
- Measure diameter (vernier if possible)
- Identify thread type (metric pitch gauge / TPI gauge)
- Test with a known nut (don’t cross-thread — it should spin easily by hand)
- If it’s for structural or safety-critical work: stop guessing and confirm spec
Internal links (Topfix)
- For fastener selection in Australian environments (corrosion, coastal, etc.): https://topfix.com.au/best-construction-fasteners-for-australian-conditions-complete-guide/
- If you’re trying to keep deliveries tight to Sydney sites: https://topfix.com.au/shipping/
- About Topfix (trade supply + sourcing help): https://topfix.com.au/about-us/
