M2 vs M3 vs M4: Which Screw Size Should You Choose?
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Choosing between M2, M3, and M4 screws is one of the most common decisions in electronics assembly, 3D printing, and light fabrication. Get it wrong and you're either stripping threads or over-engineering a joint. This guide breaks down the differences so you can pick the right size the first time.
What Does the 'M' Stand For?
The 'M' prefix stands for Metric. The number that follows is the nominal outer diameter of the thread in millimetres. So an M3 screw has a 3 mm thread diameter. Simple — but the implications for strength, clearance holes, and compatible hardware are significant.
Quick Comparison: M2 vs M3 vs M4
| Property | M2 | M3 | M4 |
|---|---|---|---|
| Thread diameter | 2 mm | 3 mm | 4 mm |
| Standard pitch | 0.4 mm | 0.5 mm | 0.7 mm |
| Clearance hole (drill) | 2.2 mm | 3.2 mm | 4.3 mm |
| Typical hex key (Allen) | 1.5 mm | 2.5 mm | 3 mm |
| Typical torque (steel) | 0.2–0.3 Nm | 0.5–0.8 Nm | 1.2–1.8 Nm |
| Common use cases | Laptops, PCBs, miniature electronics | 3D printers, RC models, Raspberry Pi | Enclosures, brackets, light structural |
When to Use M2
M2 screws are the go-to for compact electronics and precision assemblies where space is at a premium. You'll find them inside laptops, smartphones, and small PCB mounts. Because of their small diameter, they are sensitive to overtorquing — always use a torque-limited screwdriver or go by feel with a manual driver.
- Laptop screen hinges and motherboard mounts
- Miniature camera housings
- Small 3D-printed enclosures for microcontrollers (e.g., Arduino Nano, ESP8266)
When to Use M3
M3 is the workhorse of the maker and 3D printing world. It strikes the best balance between size, strength, and availability. The Voron, Prusa, and most open-source 3D printer frames are built almost entirely with M3 hardware. If you're designing a 3D-printed part and unsure which size to use, M3 is almost always the right default.
- 3D printer frames, hotends, and extruder assemblies
- Raspberry Pi and SBC mounting
- RC car and drone frames
- General electronics enclosures
When to Use M4
M4 steps up to light structural and mechanical applications where M3 would be marginal. It's common in aluminium extrusion builds (like 2020 and 2040 profiles), panel mounting, and anywhere you need a bit more clamping force without going to M5 or M6.
- Aluminium extrusion frames (V-slot, T-slot)
- Panel and bracket mounting
- Heavier 3D printer beds and gantries
- Light-duty hinges and latches
Brass Threaded Inserts: Size Matters Here Too
If you're threading into 3D-printed plastic, you should be using heat-set brass threaded inserts rather than tapping directly into the print. The insert size must match your screw size exactly. Browse our size-specific collections:
Choosing the Right Length
Diameter is only half the decision. For length, a good rule of thumb is that the screw should engage at least 1.5× its diameter in threaded material (e.g., an M3 screw should have at least 4.5 mm of thread engagement). For 3D-printed parts with brass inserts, 6–8 mm of engagement is typical.
Summary: Which Size Is Right for You?
- M2 — Miniature electronics, laptops, precision assemblies. Handle with care.
- M3 — 3D printing, makers, RC, Raspberry Pi. The universal default.
- M4 — Light structural, extrusion frames, heavier enclosures.
Still unsure? contact us — we're happy to help you spec the right hardware for your project.