SAE and metric are the two fastener standards, and the most important fact is that they aren’t interchangeable — forcing a mismatched bolt and nut strips threads or fails.
Threads
SAE (inch) fasteners are sized by diameter and threads per inch (e.g., 1/4-20). Metric fasteners are sized by diameter and thread pitch in millimeters (e.g., M6×1.0). The thread forms differ, so an SAE nut won’t properly fit a metric bolt even when they seem close.
Grade markings
| System | Strength marking |
|---|---|
| SAE | Radial lines on the head (3 = Grade 5, 6 = Grade 8) |
| Metric | Stamped numbers (8.8, 10.9, 12.9) |
Check the head markings and measure thread pitch with a gauge if unsure. Always pair like with like — SAE bolt with SAE nut, metric with metric — and match the grade so the nut is at least as strong as the bolt.
Frequently asked questions
Can I use a metric nut on an SAE bolt? No — the thread systems differ; it will strip or fail.
How do I tell SAE from metric? Head markings (lines vs numbers) and thread pitch measurement.
Does grade need to match? Use a nut of equal or higher grade than the bolt.
When in doubt, keep a thread pitch gauge in your toolbox: it settles the SAE-versus-metric question in seconds and prevents the cross-threading that ruins both the fastener and the part. Buying an assortment in one system also avoids mixing incompatible hardware on a single project.