Metric vs. US standard
How to read a fastener size
Read 1/4-20 x 2 and M8-1.25 without guessing.
Reading a size
Metric vs. imperial, decoded
Every fastener size is three numbers: how thick, how fine the thread, and how long. Once you can read them, the bin labels make sense.
Imperial (US standard)
1/4"-20 x 2"
- 1/4"
- Diameter of the shank
- 20
- Threads per inch — higher number = finer thread
- 2"
- Length under the head (overall for flat heads)
Under 1/4" the diameter is a gauge number instead: #6, #8, #10, #12 — bigger number, thicker screw.
Metric
M6 x 1.0 x 50
- M6
- 6 mm diameter
- 1.0
- Pitch — millimeters between threads. Smaller number = finer thread
- 50
- Length in millimeters
Metric and imperial are never interchangeable. M6 is close to 1/4" but the threads will cross and ruin both parts.
Coarse thread
Fewer, deeper threads (1/4"-20, M6x1.0). Faster to run, more forgiving with dirt and damage, better grip in soft material like wood, plastic, and aluminum.
Fine thread
More, shallower threads (1/4"-28, M6x0.75). Stronger in thin metal, holds tension better under vibration, and lets you adjust more precisely.