Metric and Imperial Screw Size Chart

About Screw and Tool Specifications

Metric and imperial thread references

The references cover common metric and imperial screw diameters, pitches, tap drills, and clearance drills. Separate ISO 4762 and ASME B18.3 tables list hex keys for standard socket head cap screws without assuming that every screw with the same thread uses the same tool.

Switch category and search values

Choose a metric or imperial thread table, or a head-specific hex-key table, then search the values in that reference. The share button keeps the current page available for quick handoff, while every result remains tied to the named standard.

Not torque or repair guidance

Fastener choice depends on material, load, thread class, coating, tolerance, tool condition, and safety requirements. This reference does not provide torque specs, structural ratings, vehicle repair instructions, or load-bearing approval; confirm critical work with the part maker, equipment manual, or qualified technician.

FAQ

Common questions and answers about this topic.

What is the difference between a tap drill and a clearance drill?

A tap drill leaves enough material for threads to be cut, so it is smaller than the screw diameter. A clearance drill lets the screw pass straight through, so it is larger. For M6 the tap drill is 5.0mm and the clearance drill is 6.6mm.

What drill size do I need for an M3 screw?

For cutting threads in M3, use a 2.5mm tap drill. To let an M3 screw pass through, use a 3.4mm clearance drill. The table lists both values for every size from M1.6 to M12.

When should I use coarse pitch instead of fine pitch?

Metric screws of the same diameter come in two pitches. M8 is 1.25mm coarse and 1.0mm fine. Coarse threads assemble faster and tolerate softer material; fine threads allow finer adjustment and hold better under vibration. The table shows both columns.

What does the number in M3 or M8 refer to?

The number is the nominal outer diameter of the thread in millimetres: M3 is 3mm across and M8 is 8mm. It is not the screw length and not the drill size.

What do the numbers in #10-24 or 1/4"-20 mean?

The first part is the diameter, given as a gauge number or a fractional inch; the second is threads per inch. #10-24 is 0.190" across with 24 threads per inch, and 1/4"-20 is 0.250" with 20 threads per inch.

What is the difference between UNC and UNF?

UNC is the coarse series and UNF the fine series. At the same diameter UNF packs more threads into an inch. #10-24 and #10-32 are both 0.190" across, but carry 24 and 32 threads per inch respectively, and need different tap drills.

Data Sources

This tool's data is compiled from the authoritative sources below.

Data last maintained 2026-08-08