Enjoy 10% off your first order with code FIRST10 — max $500 discount. Start an Instant Quote

Reference Chart

Metric vs Imperial Thread Comparison Chart

Side-by-side comparison of metric and imperial (unified) thread sizes with equivalent diameters, pitches, and closest matches. Use this chart to find the nearest metric equivalent for any UNC or UNF thread.

Imperial Thread ▲	TPI ▲	Major Dia (in) ▲	Closest Metric ▲	Pitch (mm) ▲	Major Dia (mm) ▲	Notes ▲
#2-56 UNC	56	0.0860	M2.5 x 0.45	0.45	2.50	M2.5 is slightly larger; closest standard metric
#4-40 UNC	40	0.1120	M3 x 0.5	0.50	3.00	M3 is slightly larger (0.118" vs 0.112")
#6-32 UNC	32	0.1380	M3.5 x 0.6	0.60	3.50	Very close match in diameter
#8-32 UNC	32	0.1640	M4 x 0.7	0.70	4.00	M4 slightly smaller (0.157" vs 0.164")
#10-24 UNC	24	0.1900	M5 x 0.8	0.80	5.00	M5 is noticeably larger (0.197")
#12-24 UNC	24	0.2160	M5.5 x 0.5	0.50	5.50	M5.5 is uncommon; M5 or M6 usually substituted
1/4"-20 UNC	20	0.2500	M6 x 1.0	1.00	6.00	Most common comparison pair; M6 is slightly smaller
5/16"-18 UNC	18	0.3125	M8 x 1.25	1.25	8.00	M8 is slightly larger (0.315")
3/8"-16 UNC	16	0.3750	M10 x 1.5	1.50	10.00	M10 is significantly larger (0.394")
7/16"-14 UNC	14	0.4375	M11 x 1.5	1.50	11.00	M11 is rare; M10 or M12 typically used
1/2"-13 UNC	13	0.5000	M12 x 1.75	1.75	12.00	Close match; M12 is slightly smaller (0.472")
9/16"-12 UNC	12	0.5625	M14 x 2.0	2.00	14.00	M14 is slightly smaller (0.551")
5/8"-11 UNC	11	0.6250	M16 x 2.0	2.00	16.00	M16 is slightly larger (0.630")
3/4"-10 UNC	10	0.7500	M20 x 2.5	2.50	20.00	M20 is significantly larger (0.787")
7/8"-9 UNC	9	0.8750	M22 x 2.5	2.50	22.00	M22 is close (0.866")
1"-8 UNC	8	1.0000	M24 x 3.0	3.00	24.00	M24 slightly smaller (0.945"); M27 is larger
1-1/8"-7 UNC	7	1.1250	M27 x 3.0	3.00	27.00	M27 is slightly larger (1.063"); closest standard
1-1/4"-7 UNC	7	1.2500	M30 x 3.5	3.50	30.00	M30 is slightly smaller (1.181")
#4-48 UNF	48	0.1120	M3 x 0.35	0.35	3.00	Fine metric pitch; close diameter match
#6-40 UNF	40	0.1380	M3.5 x 0.35	0.35	3.50	Fine pitch M3.5; very close match
#8-36 UNF	36	0.1640	M4 x 0.5	0.50	4.00	M4 fine pitch; close diameter
#10-32 UNF	32	0.1900	M5 x 0.5	0.50	5.00	M5 fine; diameter difference ~0.007"
1/4"-28 UNF	28	0.2500	M6 x 0.75	0.75	6.00	Fine pitch pair; not interchangeable
5/16"-24 UNF	24	0.3125	M8 x 1.0	1.00	8.00	M8 fine; slightly larger diameter
3/8"-24 UNF	24	0.3750	M10 x 1.0	1.00	10.00	M10 fine; significantly larger
7/16"-20 UNF	20	0.4375	M11 x 1.0	1.00	11.00	M11 fine is rare; M12 x 1.25 more common
1/2"-20 UNF	20	0.5000	M12 x 1.25	1.25	12.00	M12 fine; close match for precision work
5/8"-18 UNF	18	0.6250	M16 x 1.5	1.50	16.00	M16 fine; slightly larger diameter
3/4"-16 UNF	16	0.7500	M20 x 1.5	1.50	20.00	M20 fine; larger but closest standard

No threads match your search. Try a different term or filter.

Notes on This Chart

Metric and imperial threads are NOT interchangeable even when diameters are close. Thread pitch and form differ.
"Closest Metric" shows the nearest standard metric size by major diameter. Always verify fit before substitution.
Imperial major diameters are per ASME B1.1. Metric diameters are per ISO 261.
For numbered screws (#2 through #12), the major diameter in inches = 0.060 + (screw number x 0.013).
TPI = Threads Per Inch. To convert to metric pitch: pitch (mm) = 25.4 / TPI.

Need metric or imperial threads machined?

Upload your CAD file for an instant quote. Free DFM review included with every order.

Upload CAD for Instant Quote

Frequently Asked Questions

Can I substitute a metric bolt for an imperial bolt?

In most cases, no. Even when the major diameters are very close, the thread pitches are different. A metric bolt will not properly engage imperial threads and vice versa. The only safe approach is to replace both the bolt and the nut (or tapped hole) with the same system. Forcing a cross-thread fit can strip threads and cause joint failure.

What is the difference between UNC and UNF threads?

UNC (Unified National Coarse) has fewer threads per inch and is the most common thread type in North America. UNF (Unified National Fine) has more threads per inch, giving it a tighter fit, better resistance to vibration loosening, and more tensile strength. UNC is easier to assemble, while UNF is preferred in precision applications like aerospace.

How do I measure thread pitch on an existing bolt?

For imperial threads, use a thread pitch gauge and count the number of threads per inch (TPI). For metric threads, measure the distance in millimeters between two adjacent thread crests. You can also use calipers to measure the major diameter and cross-reference with a thread chart to identify the size.

Why do some metric and imperial threads have nearly identical diameters?

Metric and imperial systems were developed independently, but basic engineering needs result in similar size progressions. For example, 1/4 inch (6.35 mm) is close to M6 (6.0 mm). Despite similar diameters, the thread form and pitch differ, so they are not interchangeable. Always verify thread compatibility before assembly.

What does TPI mean vs. pitch in mm?

TPI stands for Threads Per Inch, used in the imperial system. It tells you how many complete threads fit in one inch. Pitch in millimeters is used in the metric system and measures the distance between two adjacent thread crests. To convert: pitch (mm) = 25.4 / TPI. For example, 20 TPI = 1.27 mm pitch.

Explore

More Resources

Thread Chart | UNC Thread Dimensions Fastener Torque Chart | Bolt Tightening Specs Anodizing Color & Thickness Chart CNC Material Machinability Chart Tap & Drill Size Chart | UNC, UNF & Metric Thread Sizes Material Hardness Chart | Brinell, Rockwell & Vickers Comparison ISO 2768 Tolerance Chart | General Tolerances for Linear & Angular Material Properties Chart | Strength, Density & Thermal Data

Ready to Start?

Get Your Parts Made Right

Upload your CAD file and get instant pricing. Prototypes ship in as few as 3 days.

Get an Instant Quote Email Us

No minimums · 100% Made in USA · Never brokered · Ships anywhere in the US

Ready to machine your part?

Upload CAD for Instant Quote