Bolt & Screw Size Reference Hub
Every specification page on BoltLab: metric lists, imperial UNC, numbered screws, and Spanish perno pages—plus links to the thread identifier, tap drill size tool, and thread types reference. Use this hub as a crawl entry point to the full size cluster.
Metric bolt sizes (English)
M3 through M20 — pitch, tap drill, hex, and closest imperial note.
Imperial bolt sizes (English)
Numbered screw sizes (English)
Metric & UNC sizes (Spanish)
Páginas en español: índice ES. Cada talla métrica enlaza a su par en inglés.
UNC (ES)
Tools & charts
Metric vs UNC bolt sizes
Metric bolts use millimeters and pitch, while UNC uses inches and threads per inch.
They are not interchangeable and require different tools and taps.
Use screw size conversion when you compare stocked inch and metric callouts.
Review metric vs unc thread naming before you mix fasteners from both systems.
How to choose the right bolt size
Choosing the correct bolt size depends on load, material, and thread type. Larger diameters increase strength, while thread pitch affects holding force.
Use the bolt size chart to compare shank and hex dimensions before you standardize hardware.
Use tap drill size guidance when you plan tapped holes.
Use the thread identifier when stamped sizes are missing.