Basic Thread Dimensions
| Property | Inch | mm |
|---|---|---|
| Nominal Diameter | 0.25 | 6.35 |
| Pitch (TPI) | 20 | |
| Pitch (Distance) | 0.05 | 1.27 |
| Pitch Diameter | 0.2175 | 5.525 |
| Minor Diameter | 0.1905 | 4.839 |
| Thread Height | 0.0271 | 0.688 |
| Addendum | 0.0162 | 0.411 |
| Crest (External) Root (Internal) | 0.0063 | 0.16 |
| Tap Drill Size (75% depth) | 0.201 | 5.11 |
| Lead Angle (Single Start) | 4.19° | |
| Thread Family | Unified Coarse Pitch (UNC) | |
| Relevant Standards | ASME B1.1-2003 | |
| Equivalent MetricThread | M6 |
For more threading data scroll down, or use the Thread Calculator
Choose the Threading Data you want to explore
Tolerance Classes
Thread Milling Gcode
Turning Gcode
Over Wire
Tap Drill Size
Tolerance Classes
Thread Milling Gcode
Turning Gcode
Over Wire
Tap Drill Size
1/4 - 20 UNC Possible Thread Classes
External Threads
1A, 2A, 3A
Internal Threads
1B, 2B, 3B
1/4 - 20 UNC 1A (External)
* Permisable values for thread Class 1A# Description Value (Inch) Value (mm) es Allowance 0.0011 (Ref) 0.028 (Ref) d Major Diameter 0.2367 - 0.2489 6.012 - 6.322 d2 Pitch Diameter 0.2108 - 0.2164 5.354 - 5.497 d3 Minor Diameter
(UNR Only)0.1893 (Ref) 4.808 (Ref) α Varation of 30° anngle (+/-) 1.1667° P Varation of pitch (+/-) 0.0016 (Ref) 0.041 (Ref)
* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator'
* Back to Thread Class Selection
1/4 - 20 UNC 2A (External)
* Permisable values for thread Class 2A# Description Value (Inch) Value (mm) es Allowance 0.0011 (Ref) 0.028 (Ref) d Major Diameter 0.2408 - 0.2489 6.116 - 6.322 d2 Pitch Diameter 0.2127 - 0.2164 5.403 - 5.497 d3 Minor Diameter
(UNR Only)0.1893 (Ref) 4.808 (Ref) α Varation of 30° anngle (+/-) 1.1667° P Varation of pitch (+/-) 0.0011 (Ref) 0.028 (Ref)
* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator'
* Back to Thread Class Selection
1/4 - 20 UNC 3A (External)
* Permisable values for thread Class 3A# Description Value (Inch) Value (mm) es Allowance 0 (Ref) 0 (Ref) d Major Diameter 0.2419 - 0.25 6.144 - 6.35 d2 Pitch Diameter 0.2147 - 0.2175 5.453 - 5.525 d3 Minor Diameter
(UNR Only)0.1905 (Ref) 4.839 (Ref) α Varation of 30° anngle (+/-) 1.1667° P Varation of pitch (+/-) 0.0008 (Ref) 0.02 (Ref)
* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator'
* Back to Thread Class Selection
1/4 - 20 UNC 1B (Internal)
* Permisable values for thread Class 1B# Description Value (Inch) VValue (mm) D1 Minor Diameter 0.196 - 0.207 4.978 - 5.258 D2 Pitch Diameter 0.2175 - 0.2248 5.525 - 5.71 D Major Diameter 0.2500 (Min) 6.35 (Min) α Varation of 30° anngle (+/-) 1.1667° P Varation of Pitch (+/-) 0.0021 (Ref) 0.053 (Ref)
* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator
* Back to Thread Class Selection
1/4 - 20 UNC 2B (Internal)
* Permisable values for thread Class 2B# Description Value (Inch) VValue (mm) D1 Minor Diameter 0.196 - 0.207 4.978 - 5.258 D2 Pitch Diameter 0.2175 - 0.2224 5.525 - 5.649 D Major Diameter 0.2500 (Min) 6.35 (Min) α Varation of 30° anngle (+/-) 1.1667° P Varation of Pitch (+/-) 0.0014 (Ref) 0.036 (Ref)
* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator
* Back to Thread Class Selection
1/4 - 20 UNC 3B (Internal)
* Permisable values for thread Class 3B# Description Value (Inch) VValue (mm) D1 Minor Diameter 0.1959 - 0.2067 4.976 - 5.25 D2 Pitch Diameter 0.2175 - 0.2211 5.525 - 5.616 D Major Diameter 0.2500 (Min) 6.35 (Min) α Varation of 30° anngle (+/-) 1.1667° P Varation of Pitch (+/-) 0.0011 (Ref) 0.028 (Ref)
* Values are calculated for Length of engagement of 5xPitch. For calculations of different engagement length use the Thread Calculator
* Back to Thread Class Selection
Tap Drill Sizes for 1/4 - 20 UNC
- If not specified, assume a thread depth of 75%.
- You can learn more about thread depth in tapping here.
- To get the tap drill size in other depths, use our Tap Drill Size Calculator .
| Thread Depth | Tap Drill Size (Inch) | Tap Drill Size (mm) |
|---|---|---|
| 55% | 0.214 | 5.44 |
| 60% | 0.211 | 5.36 |
| 65% | 0.208 | 5.28 |
| 70% | 0.205 | 5.19 |
| 75% | 0.201 | 5.11 |
| 80% | 0.198 | 5.03 |
| 85% | 0.195 | 4.95 |
Over Wire Measurements for 1/4 - 20 UNC
* OverWire is a measuring method to obatin the pitch diameter of threads. It is mostly used when a manufacturer does not have a Go/Nogo gauge for a thread. You can learn more about it here
| Inch | mm | |
|---|---|---|
| Permissible Wire Diameter Range | 0.0253 - 0.0505 | 0.642 - 1.283 |
| Ideal Wire Diameter | 0.0289 | 0.733 |
| Possible Wires From Standard Sets | * 0.0262 * 0.0289 | * 0.722 * 0.866 * 1.01 * 1.155 |
Once you know your wire diameter, you need to find out what is the premissable Measurement for your pitch diameter (Depending on the class). Use our Over Wire Calculator to find out!
CNC Gcode Program for Turning of 1/4 - 20 UNC
The below sample Gcode is configured for:
- Fanuc Controller
- External RH Thread
- Right hand tool, machining towards the chuck
- Flank Infeed method with contant volume
- Flank Infeed method with contant volume
- Cutting conditions for steel
- Default number of passes
You can configure all the above parameters and much more in our ThreadTurningGcodeGenerator
%O0100(MACHINING DOCTOR - THREADING CNC PROGRAM)(CONTROLER - FANUC A)(THREAD - )(D=0.25 INCH, P=0.05 INCH, L=0.5 INCH)(APPLICATION - EXTERNAL TURNING, TOWARDS CHUCK, RH TOOL, RH THREAD)(FEED - 17 PASSES, FLANK, CONSTANT VOLUME)(FINISH PASS - 0.0005 INCH)(WITH CHAMFER - NO)G28 U0G28 U0T0100G99 G97 S4584 M03G00 Z5.1G00 X8.4 M08G76 P010060 Q13 R0G76 X5 Z-13 P779 Q186 F1 RG28 U0G28 U0M09M30(PLEASE NOTE THIS GCODE HAS NOT BEEN TESTED ON YOUR MACHINE AND SETUP. WE CANNOT GUARANTEE ITS ACCURACY OR COMPATIBILITY. NOTE THAT YOU ARE USING IT AT YOUR OWN RISK, AND WE SHALL NOT BE HELD LIABLE FOR ANY DAMAGES OR LOSSES ARISING FROM THE USE OF THIS PROGRAM.)%CNC Gcode Program for Thread Milling of 1/4 - 20 UNC
The below sample Gcode is configured for:
- Fanuc Controller
- Internal RH Thread
- Climb Milling
- Cutter diameter: 70% of the thread diameter
- Single tooth Thread Mill cutter
- Cutting conditions for steel
- One radial pass
You can configure all the above parameters and much more in our ThreadMillingGcodeGenerator
%O0101(WWW.MACHININGDOCTOR.COM - THREADMILLING PROGRAM)(CONTROLER - FANUC)(THREAD - MAJOR DIAM.=0.25 INCH, PITCH=0.05 INCH, LENGTH=0.5 INCH)(APPLICATION - INTERNAL, RH THREAD, CLIMB MILLING)(CUTTER DIAM=0.125 [SINGLE TOOTH])(CUTTER COMPENSATION D=0 - TOOLPATH FOR TOOL CENTER)N1 M06 T1G54G90 G40 G17 G94 G21G00 X0 Y0 S9167 M03G43 H1 Z25.4 M08(M-CODE "THROUGH COOLANT ON")(PASS 1 / 1)G90 G01 Z-12.8588 F2032G91 G01 G41 D1 X0.7938 Y-0.7938 F1397G03 X0.7938 Y0.7938 Z0.1588 I0 J0.7938 F350G03 Z1.27 I-1.5875 J0 F699G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 Z1.27 I-1.5875 J0G03 X-0.7938 Y0.7938 Z0.1588 I-0.7938 J0G01 G40 X-0.7938 Y-0.7938 F2032G90 G00 Z25.4M30(PLEASE NOTE THIS GCODE HAS NOT BEEN TESTED ON YOUR MACHINE AND SETUP. WE CANNOT GUARANTEE ITS ACCURACY OR COMPATIBILITY. NOTE THAT YOU ARE USING IT AT YOUR OWN RISK, AND WE SHALL NOT BE HELD LIABLE FOR ANY DAMAGES OR LOSSES ARISING FROM THE USE OF THIS PROGRAM.)%