You're halfway through a project and the plan calls for a 4mm drill bit.
You open your toolbox - all imperial sizes.
You Google "4mm drill in imperial", and a dozen size charts pop up… but none actually tell you what to do.
Sound familiar?
Let's fix that.
Here's a clear, no-nonsense guide to what 4mm really means in imperial terms, when it's okay to substitute, and when you should just go metric from the start.
🔹 The Quick Answer: What's the Imperial Equivalent of 4mm?
The closest imperial equivalent is 5/32 inch (≈0.1575"), which is only slightly larger than 4mm (0.1570").
That's a difference of just 0.0127mm, so for non-critical applications - like wood, aluminum, or general-purpose drilling - 5/32" will do the job perfectly fine.
But for precision fits - such as bearings, dowel pins, or alignment holes - that extra fraction matters. Even a 0.07mm oversize can cause wobble or fit issues.
👉 Rule of thumb:
For tight or precision fits → Use a true 4mm drill bit
For general-purpose or clearance holes → 5/32" works fine
🔹 The Bigger Question: Do You Really Need a Metric Drill Bit?
A popular Reddit post summed it up perfectly. One user asked:
"Why can't I just use an imperial size? Metric bits are impossible to find at Lowe's or Home Depot."
And the top-voted reply?
"Just buy metric drill bits."
Here's why that advice makes sense.
If your equipment, drawings, or components are metric, using an imperial substitute introduces cumulative error. One small mismatch per hole might seem harmless, but across assemblies, it leads to alignment issues and part rework.
In our experience at Ceres Tools, many customers who started by "converting" eventually standardized their production around metric sizing. It simplified ERP data, reduced rework, and kept inspection consistent across global projects.
🔹 Where to Buy Good Metric Drill Bits (and Avoid the Cheap Stuff)
That same Reddit thread raised another common frustration:
"Sure, I can find metric bits on Amazon, but they're cheap imports that dull after a few holes."
The truth is - not all Chinese drill bits are low quality.
The difference lies in who makes them and how they're made.
At Ceres Tools, we manufacture industrial-grade metric drill bits for professional distributors in Europe, North America, and the Middle East.
Our products are made from imported high-speed steel (HSS), cobalt, and carbide - precision-ground on CNC lines and ISO 9001 certified for diameter accuracy, hardness, and edge retention.
When you order a 4mm bit from us, it's 4.00 mm sharp, concentric, and reliable - not "roughly 4mm."
🔹 Pro Tip: Drilling Speed and Technique
For a 4 mm (≈0.157″) HSS twist drill working in mild steel, industry-speed guides suggest a spindle speed in the range of ≈2,400–2,500 RPM, depending on material hardness and coolant use.Of course, if you are drilling harder materials, using longer flutes or less rigid setups, you may need to reduce speed accordingly.
🔹 Final Thoughts
If you came here searching "4mm drill in imperial," here's your answer:
It's 5/32" - close enough for general use, but not exact.
If accuracy matters, skip the conversions and go metric.
It'll save you from frustration, wasted parts, and mismatched assemblies down the line.
If you're also working with smaller drill sizes, you might find our 3.5 mm Drill Bit FAQ helpful. It explains how 3.5 mm compares to imperial sizes, what equivalent bits can be used, and how to choose the right one for your job.
At Ceres Tools, we supply certified metric drill bits with OEM/ODM labeling, batch inspection reports, and global logistics support - so you always get the exact size you need, every time.
Whether you're drilling 4mm, 3.5mm, or #29 holes, we've got you covered.
👉 Need help choosing the right drill size?
Contact our technical team or chat with us on whatsapp- we'll help you pick the best tool for your material, machine, and tolerance.