Thread Standards Reference

Comprehensive specifications, tolerances, and reference charts for common threads

Metric Threads
Unified Threads
Pipe Threads
Drill & Tap Charts
Tolerance & Fit
Conversion Tables

Metric Thread Standards (ISO)

Metric Thread Profile

Metric threads conform to the International Organization for Standardization (ISO) standards.

P (Pitch) 60° Metric Thread Profile (ISO)
Metric thread profile with 60° thread angle

Characteristics of Metric Threads:

  • 60° symmetric thread angle
  • Measured in millimeters
  • Designated as "M" followed by the nominal diameter and pitch (e.g., M10×1.5)
  • When no pitch is specified, the coarse thread is assumed (e.g., M10)

Metric Coarse Threads (ISO 724/ISO 262)

Thread Size Pitch (mm) Minor Dia. (mm) Tap Drill (mm) Clearance Drill Hex Size (mm)
M1 0.25 0.693 0.75 1.1 -
M1.2 0.25 0.893 0.95 1.3 -
M1.6 0.35 1.171 1.25 1.7 3.2
M2 0.4 1.509 1.6 2.2 4
M2.5 0.45 1.948 2.05 2.7 5
M3 0.5 2.387 2.5 3.2 5.5
M4 0.7 3.141 3.3 4.3 7
M5 0.8 4.019 4.2 5.3 8
M6 1 4.773 5.0 6.4 10
M8 1.25 6.466 6.8 8.4 13
M10 1.5 8.160 8.5 10.5 17
M12 1.75 9.853 10.2 13.0 19
M14 2 11.546 12.0 15.0 22
M16 2 13.546 14.0 17.0 24
M18 2.5 14.933 15.5 19.0 27
M20 2.5 16.933 17.5 21.0 30
M22 2.5 18.933 19.5 23.0 32
M24 3 20.319 21.0 25.0 36
M27 3 23.319 24.0 28.0 41
M30 3.5 25.706 26.5 31.0 46
M33 3.5 28.706 29.5 34.0 50
M36 4 31.093 32.0 37.0 55

Tap drill sizes provide approximately 75% thread engagement, which is suitable for most applications. For harder materials or higher strength requirements, use a larger drill size. For softer materials, a smaller drill size may be used.

Metric Fine Threads (ISO 724/ISO 262)

Thread Size Pitch (mm) Minor Dia. (mm) Tap Drill (mm) Clearance Drill Hex Size (mm)
M1 × 0.2 0.2 0.755 0.8 1.1 -
M1.6 × 0.2 0.2 1.355 1.4 1.7 3.2
M2 × 0.25 0.25 1.693 1.75 2.2 4
M2.5 × 0.35 0.35 2.071 2.15 2.7 5
M3 × 0.35 0.35 2.571 2.65 3.2 5.5
M4 × 0.5 0.5 3.387 3.5 4.3 7
M5 × 0.5 0.5 4.387 4.5 5.3 8
M6 × 0.75 0.75 5.080 5.25 6.4 10
M8 × 1 1 6.773 7.0 8.4 13
M10 × 1.25 1.25 8.466 8.8 10.5 17
M10 × 1 1 8.773 9.0 10.5 17
M12 × 1.5 1.5 10.160 10.5 13.0 19
M12 × 1.25 1.25 10.466 10.8 13.0 19
M14 × 1.5 1.5 12.160 12.5 15.0 22
M16 × 1.5 1.5 14.160 14.5 17.0 24
M18 × 1.5 1.5 16.160 16.5 19.0 27
M20 × 1.5 1.5 18.160 18.5 21.0 30
M22 × 1.5 1.5 20.160 20.5 23.0 32
M24 × 2 2 21.546 22.0 25.0 36
M27 × 2 2 24.546 25.0 28.0 41
M30 × 2 2 27.546 28.0 31.0 46

Fine threads offer advantages including higher tensile strength, better adjustment precision, and improved vibration resistance. They are commonly used in thin-walled applications, precision instruments, and automotive components.

Unified Thread Standards (UTS)

Unified Thread Profile

The Unified Thread Standard (UTS) is the predominant thread standard used in the United States and Canada.

1/TPI 60° Unified Thread Profile (UTS)
Unified thread profile with 60° thread angle

Characteristics of Unified Threads:

  • 60° symmetric thread angle (same as metric)
  • Measured in inches
  • Thread pitch expressed as threads per inch (TPI)
  • Common designations include UNC (coarse), UNF (fine), and UNEF (extra fine)
  • Specified by diameter and TPI (e.g., 1/4-20 UNC means 1/4 inch diameter with 20 threads per inch)

Unified Coarse (UNC) Thread Series

Thread Size TPI Major Dia. (in) Minor Dia. (in) Tap Drill (in) Tap Drill (mm) Clearance Drill
#0-80 UNF 80 0.0600 0.0447 #53 (0.0595) 1.5 #42 (0.0935)
#1-64 UNC 64 0.0730 0.0538 #53 (0.0595) 1.5 #38 (0.1015)
#2-56 UNC 56 0.0860 0.0641 #50 (0.0700) 1.8 #30 (0.1285)
#3-48 UNC 48 0.0990 0.0734 #47 (0.0785) 2.0 #26 (0.1470)
#4-40 UNC 40 0.1120 0.0813 #43 (0.0890) 2.3 #22 (0.1570)
#5-40 UNC 40 0.1250 0.0943 #38 (0.1015) 2.6 #19 (0.1660)
#6-32 UNC 32 0.1380 0.0997 #36 (0.1065) 2.7 #14 (0.1820)
#8-32 UNC 32 0.1640 0.1257 #29 (0.1360) 3.5 #10 (0.1935)
#10-24 UNC 24 0.1900 0.1389 #25 (0.1495) 3.8 #7 (0.2010)
#12-24 UNC 24 0.2160 0.1649 #16 (0.1770) 4.5 #2 (0.2210)
1/4-20 UNC 20 0.2500 0.1887 #7 (0.2010) 5.1 F (0.2570)
5/16-18 UNC 18 0.3125 0.2443 F (0.2570) 6.5 P (0.3230)
3/8-16 UNC 16 0.3750 0.2983 5/16 (0.3125) 7.9 Q (0.3860)
7/16-14 UNC 14 0.4375 0.3499 U (0.3680) 9.4 29/64 (0.4531)
1/2-13 UNC 13 0.5000 0.4056 27/64 (0.4219) 10.7 33/64 (0.5156)
9/16-12 UNC 12 0.5625 0.4603 31/64 (0.4844) 12.3 37/64 (0.5781)
5/8-11 UNC 11 0.6250 0.5135 17/32 (0.5312) 13.5 41/64 (0.6406)
3/4-10 UNC 10 0.7500 0.6273 21/32 (0.6562) 16.7 49/64 (0.7656)
7/8-9 UNC 9 0.8750 0.7387 49/64 (0.7656) 19.4 57/64 (0.8906)
1-8 UNC 8 1.0000 0.8466 7/8 (0.8750) 22.2 1-1/64 (1.0156)

Unified Coarse (UNC) threads are the most common thread type for general purpose fasteners in the USA and Canada. They are easier to assemble and are resistant to cross-threading and damage.

Unified Fine (UNF) Thread Series

Thread Size TPI Major Dia. (in) Minor Dia. (in) Tap Drill (in) Tap Drill (mm) Clearance Drill
#0-80 UNF 80 0.0600 0.0447 #53 (0.0595) 1.5 #42 (0.0935)
#1-72 UNF 72 0.0730 0.0560 #53 (0.0595) 1.5 #38 (0.1015)
#2-64 UNF 64 0.0860 0.0668 #50 (0.0700) 1.8 #30 (0.1285)
#3-56 UNF 56 0.0990 0.0771 #45 (0.0820) 2.1 #26 (0.1470)
#4-48 UNF 48 0.1120 0.0864 #42 (0.0935) 2.4 #22 (0.1570)
#5-44 UNF 44 0.1250 0.0971 #37 (0.1040) 2.6 #19 (0.1660)
#6-40 UNF 40 0.1380 0.1073 #33 (0.1130) 2.9 #14 (0.1820)
#8-36 UNF 36 0.1640 0.1299 #29 (0.1360) 3.5 #10 (0.1935)
#10-32 UNF 32 0.1900 0.1517 #21 (0.1590) 4.0 #7 (0.2010)
#12-28 UNF 28 0.2160 0.1722 #14 (0.1820) 4.6 #2 (0.2210)
1/4-28 UNF 28 0.2500 0.2062 #3 (0.2130) 5.4 F (0.2570)
5/16-24 UNF 24 0.3125 0.2614 I (0.2720) 6.9 P (0.3230)
3/8-24 UNF 24 0.3750 0.3239 Q (0.3320) 8.4 Q (0.3860)
7/16-20 UNF 20 0.4375 0.3762 25/64 (0.3906) 9.9 29/64 (0.4531)
1/2-20 UNF 20 0.5000 0.4387 29/64 (0.4531) 11.5 33/64 (0.5156)
9/16-18 UNF 18 0.5625 0.4943 1/2 (0.5000) 12.7 37/64 (0.5781)
5/8-18 UNF 18 0.6250 0.5568 9/16 (0.5625) 14.3 41/64 (0.6406)
3/4-16 UNF 16 0.7500 0.6733 11/16 (0.6875) 17.5 49/64 (0.7656)
7/8-14 UNF 14 0.8750 0.7874 13/16 (0.8125) 20.6 57/64 (0.8906)
1-14 UNF 14 1.0000 0.9124 15/16 (0.9375) 23.8 1-1/64 (1.0156)

Unified Fine (UNF) threads have more threads per inch than UNC threads of the same diameter. They provide greater tensile strength, better vibration resistance, and improved adjustment precision. They are commonly used in automotive, aerospace, and precision applications.

Pipe Thread Standards

Pipe Thread Types

Pipe threads are used for fluid and gas connections and differ from standard fastener threads in several ways.

1:16 Taper Tapered Pipe Thread
Tapered pipe thread with 1:16 taper rate

Common Pipe Thread Types:

  • NPT (National Pipe Tapered):
    • US standard with 60° thread angle
    • 1:16 taper (3/4" per foot)
    • Self-sealing through interference fit
  • NPTF (National Pipe Tapered for Fuel):
    • Modified NPT for fuel systems
    • Designed for dryseal (no sealant required)
  • BSPT (British Standard Pipe Tapered):
    • UK/European standard with 55° thread angle
    • 1:16 taper (same as NPT)
    • Incompatible with NPT despite similar dimensions
  • BSPP (British Standard Pipe Parallel):
    • Straight/parallel thread with 55° thread angle
    • Uses sealing ring or washer
    • Also known as "G" threads in ISO standards

NPT and NPTF Thread Dimensions

Nominal Size TPI OD at Gauge Plane (in) Tap Drill Size (in) Tap Drill Size (mm) Thread Pitch (in)
1/16 27 0.3125 17/64 (0.2656) 6.75 0.03704
1/8 27 0.405 11/32 (0.3437) 8.75 0.03704
1/4 18 0.540 7/16 (0.4375) 11.1 0.05556
3/8 18 0.675 37/64 (0.5781) 14.7 0.05556
1/2 14 0.840 23/32 (0.7187) 18.3 0.07143
3/4 14 1.050 29/32 (0.9062) 23.0 0.07143
1 11.5 1.315 1-5/32 (1.1562) 29.4 0.08696
1-1/4 11.5 1.660 1-1/2 (1.5000) 38.1 0.08696
1-1/2 11.5 1.900 1-47/64 (1.7344) 44.1 0.08696
2 11.5 2.375 2-7/32 (2.2188) 56.4 0.08696
2-1/2 8 2.875 2-5/8 (2.6250) 66.7 0.12500
3 8 3.500 3-1/4 (3.2500) 82.6 0.12500
4 8 4.500 4-1/4 (4.2500) 108.0 0.12500

Important Notes for NPT Threading:

  • The nominal size of pipe threads does not directly correspond to any actual physical dimension
  • For proper sealing, pipe threads require a thread sealant (PTFE tape, pipe dope, etc.) unless they are NPTF
  • Hand-tight engagement for NPT is approximately 3-4 turns
  • The taper rate for NPT is 1:16 or 3/4" per foot

BSPT and BSPP Thread Dimensions

Nominal Size Designation TPI OD (in) OD (mm) Tap Drill (mm) Thread Pitch (mm)
1/16 1/16 BSP 28 0.3065 7.79 6.8 0.907
1/8 1/8 BSP 28 0.3830 9.73 8.7 0.907
1/4 1/4 BSP 19 0.5180 13.16 11.8 1.337
3/8 3/8 BSP 19 0.6560 16.66 15.25 1.337
1/2 1/2 BSP 14 0.8250 20.96 19.0 1.814
3/4 3/4 BSP 14 1.0410 26.44 24.5 1.814
1 1 BSP 11 1.3090 33.25 30.9 2.309
1-1/4 1-1/4 BSP 11 1.6500 41.91 39.5 2.309
1-1/2 1-1/2 BSP 11 1.8875 47.80 45.4 2.309
2 2 BSP 11 2.3470 59.61 57.2 2.309
2-1/2 2-1/2 BSP 11 2.8750 73.02 70.6 2.309
3 3 BSP 11 3.5000 88.90 86.5 2.309

BSPT vs. BSPP Differences:

  • BSPT (British Standard Pipe Tapered):
    • Has a 1:16 taper like NPT
    • Uses a 55° thread angle (vs. 60° for NPT)
    • Often designated with "R" in ISO standard (e.g., R 1/2)
    • Requires thread sealant for pressure-tight connections
  • BSPP (British Standard Pipe Parallel):
    • No taper (straight/parallel threads)
    • 55° thread angle
    • Designated with "G" in ISO standard (e.g., G 1/2)
    • Requires washer or O-ring for sealing

Warning: Despite having similar dimensions, NPT and BSP threads are not compatible and should not be interchanged.

Drill & Tap Reference Charts

Tap Drill Calculation

The tap drill size is critical for creating proper internal threads. The formula for calculating tap drill size depends on the desired thread engagement:

Tap Drill Size = Major Diameter - (0.75 × Thread Pitch)

This formula is for 75% thread engagement, which is standard for most applications. For reference:

  • 50% engagement: Tap Drill = Major Diameter - (0.5 × Thread Pitch)
  • 75% engagement: Tap Drill = Major Diameter - (0.75 × Thread Pitch)
  • 85% engagement: Tap Drill = Major Diameter - (0.85 × Thread Pitch)
Major Diameter Tap Drill Size Thread Engagement
Relationship between major diameter, tap drill size, and thread engagement

Recommended Thread Engagement:

  • Soft Materials (aluminum, brass, plastic): 50-65% engagement
  • Medium Materials (mild steel, cast iron): 75% engagement
  • Hard Materials (hardened steel, stainless): 55-65% engagement
  • Critical Applications (high-strength requirements): 80-85% engagement

Tips for Successful Tapping:

  • Use cutting fluid appropriate for the material being tapped
  • For blind holes, drill deeper than the thread depth to provide chip clearance
  • Back out the tap regularly to clear chips, especially in blind holes
  • Use spiral point taps for through holes and spiral flute taps for blind holes
  • For hard materials, consider using a tap with more flutes

Drill Size Conversion Chart

Drill Size Diameter (inches) Diameter (mm) Drill Size Diameter (inches) Diameter (mm)
#80 0.0135 0.343 #30 0.1285 3.264
#79 0.0145 0.368 #29 0.1360 3.454
#78 0.0160 0.406 #28 0.1405 3.569
#77 0.0180 0.457 #27 0.1440 3.658
#76 0.0200 0.508 #26 0.1470 3.734
#75 0.0210 0.533 #25 0.1495 3.797
#74 0.0225 0.572 #24 0.1520 3.861
#73 0.0240 0.610 #23 0.1540 3.912
#72 0.0250 0.635 #22 0.1570 3.988
#71 0.0260 0.660 #21 0.1590 4.039
#70 0.0280 0.711 #20 0.1610 4.089
#69 0.0292 0.742 #19 0.1660 4.216
#68 0.0310 0.787 #18 0.1695 4.305
#67 0.0320 0.813 #17 0.1730 4.394
#66 0.0330 0.838 #16 0.1770 4.496
#65 0.0350 0.889 #15 0.1800 4.572
#64 0.0360 0.914 #14 0.1820 4.623
#63 0.0370 0.940 #13 0.1850 4.699
#62 0.0380 0.965 #12 0.1890 4.801
#61 0.0390 0.991 #11 0.1910 4.851
#60 0.0400 1.016 #10 0.1935 4.915
#59 0.0410 1.041 #9 0.1960 4.978
#58 0.0420 1.067 #8 0.1990 5.055
#57 0.0430 1.092 #7 0.2010 5.105
#56 0.0465 1.181 #6 0.2040 5.182
#55 0.0520 1.321 #5 0.2055 5.220
#54 0.0550 1.397 #4 0.2090 5.309
#53 0.0595 1.511 #3 0.2130 5.410
#52 0.0635 1.613 #2 0.2210 5.613
#51 0.0670 1.702 #1 0.2280 5.791
#50 0.0700 1.778 A 0.2340 5.944
#49 0.0730 1.854 B 0.2380 6.045
#48 0.0760 1.930 C 0.2420 6.147
#47 0.0785 1.994 D 0.2460 6.248
#46 0.0810 2.057 E 0.2500 6.350
#45 0.0820 2.083 F 0.2570 6.528
#44 0.0860 2.184 G 0.2610 6.629
#43 0.0890 2.261 H 0.2660 6.756
#42 0.0935 2.375 I 0.2720 6.909
#41 0.0960 2.438 J 0.2770 7.036
#40 0.0980 2.489 K 0.2810 7.137
#39 0.0995 2.527 L 0.2900 7.366
#38 0.1015 2.578 M 0.2950 7.493
#37 0.1040 2.642 N 0.3020 7.671
#36 0.1065 2.705 O 0.3160 8.026
#35 0.1100 2.794 P 0.3230 8.204
#34 0.1110 2.819 Q 0.3320 8.433
#33 0.1130 2.870 R 0.3390 8.611
#32 0.1160 2.946 S 0.3480 8.839
#31 0.1200 3.048 T 0.3580 9.093

Thread Tolerance & Fit Standards

Thread Fit Classes

Thread tolerances define the acceptable variations in thread dimensions and dictate how tightly or loosely mating threads will fit together.

Unified (ANSI/ASME) Thread Classes:

  • Class 1: Loose fit for easy assembly or where foreign material might be present
  • Class 2: General purpose fit for most commercial and industrial applications
  • Class 3: Close fit for precision applications requiring minimal play

Each class is further divided into external threads (A) and internal threads (B). For example, 2A indicates a Class 2 external thread, while 2B indicates a Class 2 internal thread.

Metric (ISO) Thread Tolerances:

Metric thread tolerances use a combined letter and number system:

  • External threads: lowercase letters (e.g., 6g)
  • Internal threads: uppercase letters (e.g., 6H)

Common ISO tolerance grades:

  • Fine tolerance: 4h6h/4H5H (precision applications)
  • Medium tolerance: 6g/6H (general purpose)
  • Coarse tolerance: 7g8g/7H8H (easy assembly)

Unified Thread Tolerance Classes

Class Application Allowance Typical Uses
1A/1B Loose fit Maximum Easy assembly, plating, coating, dirty environments
2A/2B Medium fit Medium General purpose applications, 90% of commercial fasteners
3A/3B Tight fit Minimum Precision applications, aerospace, high vibration

Note: "A" designates external threads (bolts), and "B" designates internal threads (nuts). A 2A bolt should always be used with a 2B nut for proper fit.

Metric ISO Thread Tolerance System

Designation Thread Type Tolerance Grade Typical Uses
6g External Medium General purpose external threads
6H Internal Medium General purpose internal threads
5g6g External Closer Higher precision applications
4h6h External Precision High-precision components
7g External Loose Plated or coated threads
7H Internal Loose Easy assembly applications
5H Internal Closer Higher precision applications
4H5H Internal Precision High-precision components

The ISO metric thread tolerance system uses a combined position (letter) and grade (number) designation.

  • The tolerance position (letter) indicates the basic position of the tolerance zone relative to the basic size.
  • The tolerance grade (number) indicates the tolerance range - lower numbers indicate tighter tolerances.
  • The standard combination for general use is 6g/6H, which is roughly equivalent to Unified Class 2A/2B.

Conversion Tables

Thread Pitch Conversion

TPI = 25.4 ÷ Pitch(mm)
Pitch(mm) = 25.4 ÷ TPI

These formulas convert between metric pitch (measured in mm) and imperial threads per inch (TPI).

Threads Per Inch (TPI) Metric Pitch (mm) Threads Per Inch (TPI) Metric Pitch (mm)
4 6.35 20 1.27
5 5.08 24 1.06
6 4.23 28 0.91
7 3.63 32 0.79
8 3.18 36 0.71
9 2.82 40 0.64
10 2.54 44 0.58
11 2.31 48 0.53
12 2.12 56 0.45
13 1.95 64 0.40
14 1.81 72 0.35
16 1.59 80 0.32
18 1.41 96 0.26

Approximate Thread Size Equivalents

Warning: These are approximate equivalents for reference only. Metric and imperial threads are not interchangeable even when similar in size.

Metric Size Metric Pitch Closest Imperial Size Imperial TPI Decimal Inch
M3 0.5 #4 40 0.112
M3.5 0.6 #6 32 0.138
M4 0.7 #8 32 0.164
M5 0.8 #10 24 0.190
M6 1.0 1/4" 20 0.250
M8 1.25 5/16" 18 0.313
M10 1.5 3/8" 16 0.375
M12 1.75 1/2" 13 0.500
M14 2.0 9/16" 12 0.563
M16 2.0 5/8" 11 0.625
M20 2.5 3/4" 10 0.750
M24 3.0 1" 8 1.000

Inch to Millimeter Conversion

mm = inches × 25.4
inches = mm ÷ 25.4
Fraction (in) Decimal (in) Millimeters Fraction (in) Decimal (in) Millimeters
1/64 0.0156 0.397 33/64 0.5156 13.097
1/32 0.0313 0.794 17/32 0.5313 13.494
3/64 0.0469 1.191 35/64 0.5469 13.891
1/16 0.0625 1.588 9/16 0.5625 14.288
5/64 0.0781 1.984 37/64 0.5781 14.684
3/32 0.0938 2.381 19/32 0.5938 15.081
7/64 0.1094 2.778 39/64 0.6094 15.478
1/8 0.1250 3.175 5/8 0.6250 15.875
9/64 0.1406 3.572 41/64 0.6406 16.272
5/32 0.1563 3.969 21/32 0.6563 16.669
11/64 0.1719 4.366 43/64 0.6719 17.066
3/16 0.1875 4.763 11/16 0.6875 17.463
13/64 0.2031 5.159 45/64 0.7031 17.859
7/32 0.2188 5.556 23/32 0.7188 18.256
15/64 0.2344 5.953 47/64 0.7344 18.653
1/4 0.2500 6.350 3/4 0.7500 19.050
17/64 0.2656 6.747 49/64 0.7656 19.447
9/32 0.2813 7.144 25/32 0.7813 19.844
19/64 0.2969 7.541 51/64 0.7969 20.241
5/16 0.3125 7.938 13/16 0.8125 20.638
21/64 0.3281 8.334 53/64 0.8281 21.034
11/32 0.3438 8.731 27/32 0.8438 21.431
23/64 0.3594 9.128 55/64 0.8594 21.828
3/8 0.3750 9.525 7/8 0.8750 22.225
25/64 0.3906 9.922 57/64 0.8906 22.622
13/32 0.4063 10.319 29/32 0.9063 23.019
27/64 0.4219 10.716 59/64 0.9219 23.416
7/16 0.4375 11.113 15/16 0.9375 23.813
29/64 0.4531 11.509 61/64 0.9531 24.209
15/32 0.4688 11.906 31/32 0.9688 24.606
31/64 0.4844 12.303 63/64 0.9844 25.003
1/2 0.5000 12.700 1 1.0000 25.400
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