Screw & Bolt Size Reference | CS17 Quick Reference

How to Measure a Bolt

1. DiameterMeasure across the outside crests of the threads with calipers. A metric bolt measuring about 10 mm across the thread is nominally M10.

2. LengthFor a typical hex-head bolt, measure from under the head to the end. Countersunk/flat-head fasteners are generally measured overall because the head sits in the material.

3. Thread pitchFor metric threads, measure crest-to-crest in millimetres or use a thread-pitch gauge.

4. Imperial TPIFor inch threads, identify the nominal diameter and count threads per inch (TPI), preferably with a thread gauge.

Reading the Size

M10 × 1.5 × 60
M10 = 10 mm nominal thread diameter; 1.5 = thread pitch in mm; 60 = nominal length in mm.

3/8"-16 × 2"
3/8" = nominal diameter; 16 = threads per inch; 2" = nominal length.

Identifying an Unknown Bolt

1. Measure the diameterMeasure across the outside crests of the threads with calipers. This gives the nominal thread diameter.

2. Check pitch or TPIFor metric threads, measure the distance from one thread crest to the next in millimetres. For imperial threads, identify the number of threads per inch (TPI).

3. Measure the lengthMeasure from the correct point on the head to the tip. Most projecting-head bolts are measured from under the head; countersunk fasteners are generally measured overall.

4. Identify the headNote whether it is hex, socket cap, button, pan, countersunk or another head type. The head style does not by itself determine the thread size.

5. Check markingsLook for property-class, grade or manufacturer markings on the head. These can matter when replacing structural or load-bearing fasteners.

6. Compare the measurementsUse the Metric & Imperial Bolt Diameter Comparison table below to find the closest nominal diameter and matching pitch or TPI.

Example — unknown metric bolt
Measured thread diameter: approximately 10 mm
Measured thread pitch: approximately 1.5 mm
Measured length: 60 mm
Likely size: M10 × 1.5 × 60

Final check: If possible, confirm the thread with a thread-pitch gauge or a known correct nut. Never force a nut or bolt that starts but quickly becomes tight; a similar diameter can still have a different pitch or thread system.

Where Length Is Measured

Fastener head	Typical length measurement
Hex / socket cap / pan head	From bearing surface under the head to the tip
Countersunk / flat head	Overall, including the head
Stud / threaded rod	End to end
Shoulder bolt	Check the specified convention; shoulder and thread lengths may be stated separately

Matching Nuts & Threads

Diameter alone is not enough: pitch/TPI must also match.
Do not force a nut onto a thread that starts but quickly binds.
Metric and imperial threads are not interchangeable even when diameters appear close.
Head/spanner size does not by itself identify thread diameter because standards and fastener types vary.

Strength Markings

Fastener markings can identify material or strength/property class, but the markings depend on the standard and fastener. Do not select a structural or safety-critical replacement solely from diameter and length; match the required specification and grade.

Metric & Imperial Bolt Diameter Comparison

How to use this table: First measure the outside thread diameter. Then measure the metric thread pitch from crest to crest, or identify the imperial threads per inch (TPI). Use both measurements to identify the thread. Rows are ordered by nominal diameter so similar sizes are easy to compare.

Metric size	Diameter	Common metric coarse pitch	Imperial size	Diameter in mm	Common coarse TPI
M3	3.00 mm	0.50 mm
			1/8"	3.175 mm
M4	4.00 mm	0.70 mm
			3/16"	4.7625 mm
M5	5.00 mm	0.80 mm
M6	6.00 mm	1.00 mm
			1/4"	6.350 mm	20 UNC
			5/16"	7.9375 mm	18 UNC
M8	8.00 mm	1.25 mm
			3/8"	9.525 mm	16 UNC
M10	10.00 mm	1.50 mm
			7/16"	11.1125 mm	14 UNC
M12	12.00 mm	1.75 mm
			1/2"	12.700 mm	13 UNC
M14	14.00 mm	2.00 mm
			9/16"	14.2875 mm	12 UNC
			5/8"	15.875 mm	11 UNC
M16	16.00 mm	2.00 mm
			11/16"	17.4625 mm
M18	18.00 mm	2.50 mm
			3/4"	19.050 mm	10 UNC
M20	20.00 mm	2.50 mm
			13/16"	20.6375 mm
M22	22.00 mm	2.50 mm
			7/8"	22.225 mm	9 UNC
			15/16"	23.8125 mm
M24	24.00 mm	3.00 mm
			1"	25.400 mm	8 UNC
M27	27.00 mm	3.00 mm
			1 1/16"	26.9875 mm
			1 1/8"	28.575 mm	7 UNC
M30	30.00 mm	3.50 mm
			1 3/16"	30.1625 mm
			1 1/4"	31.750 mm	7 UNC

Important: Similar diameters do not make metric and imperial fasteners interchangeable. The thread pitch or TPI, thread form, fit and required fastener specification must also match. Blank TPI cells are intentional where the listed fractional diameter is not being presented here as a standard UNC bolt size.