1/4 - 28 28-UN - Basic Thread Dimensions

Property Inch mm
Nominal Diameter 0.25 6.35
Pitch (TPI)28
Pitch (Distance) 0.0357 0.907
Pitch Diameter 0.2268 5.761
Minor Diameter 0.2075 5.271
Thread Height 0.0193 0.49
Addendum 0.0116 0.295
Crest (External)
Root (Internal)
0.0045 0.114
Tap Drill Size
(75% depth)
0.215 5.47
Lead Angle
(Single Start)
2.87°
Thread Family Unified Constant Pitch 28 TPI (28-UN)
Relevant Standards ASME B1.1-2003
Equivalent Metric ThreadM6

For more threading data scroll down, or use the Thread Calculator

Table of Content


1/4 - 28 28-UN Possible Thread Classes

External Threads

Choose tolerance class: 2A, 3A

Metric ISO External Thread - Tolerance sketch

Internal Threads

Choose tolerance class: 2B, 3B

Metric ISO Internal Thread - Tolerance sketch

1/4 - 28 28-UN 2A (External)

* Permisable values for thread Class 2A

# Description Value (Inch) Value (mm)
es Allowance 0.001 (Ref) 0.025 (Ref)
d Major Diameter 0.2425 - 0.249 6.16 - 6.325
d2 Pitch Diameter 0.2226 - 0.2258 5.654 - 5.735
d3 Minor Diameter
(UNR Only)
0.2065 (Ref) 5.245 (Ref)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of pitch (+/-) 0.0009 (Ref) 0.023 (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 - 28 28-UN 3A (External)

* Permisable values for thread Class 3A

# Description Value (Inch) Value (mm)
es Allowance 0 (Ref) 0 (Ref)
d Major Diameter 0.2435 - 0.25 6.185 - 6.35
d2 Pitch Diameter 0.2244 - 0.2268 5.7 - 5.761
d3 Minor Diameter
(UNR Only)
0.2075 (Ref) 5.271 (Ref)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of pitch (+/-) 0.0007 (Ref) 0.018 (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 - 28 28-UN 2B (Internal)

* Permisable values for thread Class 2B

# Description Value (Inch) Value (mm)
D1 Minor Diameter 0.211 - 0.22 5.359 - 5.588
D2 Pitch Diameter 0.2268 - 0.231 5.761 - 5.867
D Major Diameter 0.2500 (Min) 6.35 (Min)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of Pitch (+/-) 0.0012 (Ref) 0.03 (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 - 28 28-UN 3B (Internal)

* Permisable values for thread Class 3B

# Description Value (Inch) Value (mm)
D1 Minor Diameter 0.2113 - 0.219 5.367 - 5.563
D2 Pitch Diameter 0.2268 - 0.2299 5.761 - 5.839
D Major Diameter 0.2500 (Min) 6.35 (Min)
α Varation of 30° anngle (+/-) 1.3333°
P Varation of Pitch (+/-) 0.0009 (Ref) 0.023 (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 - 28 28-UN

  • 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% 0.224 5.70
60% 0.222 5.64
65% 0.220 5.58
70% 0.218 5.52
75% 0.215 5.47
80% 0.213 5.41
85% 0.211 5.35

Over Wire Measurements for 1/4 - 28 28-UN


* 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 0.018 - 0.0361 0.458 - 0.916
Ideal Wire Diameter 0.0206 0.524
Possible Wires
From Standard Sets
* 0.018
* 0.0193
* 0.0206
* 0.0214
* 0.0222
* 0.0241
* 0.0262
* 0.0289
* 0.462
* 0.52
* 0.577
* 0.722
* 0.866

Once you knoe 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!


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