What is the tap drill size for 1/4-28 UNF?

The reference cut-tap drill is #3 (0.2130 in / 5.4102 mm). Confirm the tool manufacturer’s hole range for your material and required thread. Form taps use a different pilot size.

Can I use a 7/32 inch drill for 1/4-28?

7/32 in is 0.21875 in, larger than #3 at 0.2130 in. It leaves a shallower cut thread. It is not an exact substitute: check the tap maker’s hole range, material and required thread before choosing it.

What is the pitch of 1/4-28?

It is 28 threads per inch: a pitch of 0.03571 in (0.90714 mm). Pitch is the axial distance between corresponding points on adjacent threads.

Does this table show 2A or 2B acceptance limits?

No. It shows theoretical basic dimensions. A denotes an external thread and B an internal thread. Use the specified class limits and inspection requirements from the governing standard for acceptance.

Is percent thread the same as length of engagement?

No. Percent thread describes the theoretical thread height left by the pilot hole. Length of engagement is the axial distance over which the internal and external threads overlap. Neither alone establishes the strength of a joint.

1/4-28 UNF Tap Drill Size & Thread Dimensions

Find the 1/4-28 UNF cutting-tap drill, its inch and metric diameter, and the basic dimensions for this 28-TPI thread.

Page last changed 2026-10-19 · based on ASME B1.1-2019

Quick Answer

For a 1/4-28 UNF cutting tap, the reference drill size is #3: 0.2130 in (5.4102 mm). The thread has 28 TPI and a nominal major diameter of 0.2500 in. Form taps need a different pilot hole.

What drill size for 1/4-28 UNF?

#3 (0.2130 in / 5.4102 mm) is the reference size used here for cutting a 1/4-28 thread. A clearance hole lets a screw pass through freely; this smaller pre-tap hole leaves material for the tap to cut.

The quick estimate is D − 1/TPI: 0.2500 − 1/28 = 0.21429 in. Rounding to a stocked drill does not guarantee exactly 75% thread.

Using Guhring’s cut-tap sizing equation, drill = D − (% thread × 0.01299 / TPI), this drill gives approximately 79.8% theoretical thread. A theoretical 75% target gives 0.21521 in before selecting a stocked drill. These percentages describe thread height, not the length of engagement or joint strength.

For comparison, Guhring’s chart lists 5.45 mm in its 75% cut-tap column for 1/4-28. This is a different selection from the reference drill above; follow the target percentage and tooling instructions for your job rather than treating the two as identical.

Cut tap or form tap? A form tap displaces material and generally needs a larger, more closely controlled hole. Use the selected tool’s recommended range for the material and thread requirement. Drill runout and the measured finished hole also affect the result; this calculation is not a thread-gauge acceptance limit.

1/4-28 thread dimension diagram

Original schematic, not to scale. P = 1/28 in; d = 0.2500 in. The basic pitch and minor diameters are in the table below. The pre-tap drill diameter is a process choice, not the basic external minor diameter.

Thread Calculator

Major diameter (inch)

Threads per inch

Basic Dimensions

Dimension^[1]	Symbol^[1]	Inch^[2]	Millimetre^[3]
Major diameter (external) #	`d`	0.2500	6.35
Pitch diameter (basic) #	`d2`	0.2268	5.7608
Minor diameter, external thread #	`d1`	0.2062	5.2371
Minor diameter, internal thread #	`D1`	0.2113	5.368
Pitch #	`P`	0.0357	0.9071
Thread height, external #	`h`	0.0219	0.5565
Thread height, internal #	`hn`	0.0193	0.491
Crest flat #	`f`	0.0045	0.1134
Tensile stress area #	`As`	0.0364	23.4669

How to Read the Designation

1/4-28 UNF specifies a nominal diameter of 0.2500 in and 28 threads per inch. The pitch is 1/28 = 0.03571 in (0.90714 mm).

1/4-20 UNC has the same 0.2500 in nominal diameter but 20 TPI. It needs a different tap and pilot hole.

Class of Fit

The dimensions on this page are the basic dimensions — the theoretical profile with no allowance or tolerance applied. Production threads are specified by class:

Class 1A / 1B — loosest, for quick assembly and threads that may be dirty or damaged. Rarely specified.
Class 2A / 2B — the normal commercial fit, and the default for bolts, screws and nuts. Has a slight allowance on the external thread to prevent binding.
Class 3A / 3B — tight fit with no allowance, for close-tolerance work and high-strength fasteners. Requires better process control.

A and B denote external and internal threads respectively: 2A is an external bolt thread, 2B is an internal nut thread. The basic dimensions on this page are the reference from which the class limits are derived.

← #12-28 All UNF sizes 5/16-24 →

Frequently Asked Questions

What is the tap drill size for 1/4-28 UNF?: The reference cut-tap drill is #3 (0.2130 in / 5.4102 mm). Confirm the tool manufacturer’s hole range for your material and required thread. Form taps use a different pilot size.
Can I use a 7/32 inch drill for 1/4-28?: 7/32 in is 0.21875 in, larger than #3 at 0.2130 in. It leaves a shallower cut thread. It is not an exact substitute: check the tap maker’s hole range, material and required thread before choosing it.
What is the pitch of 1/4-28?: It is 28 threads per inch: a pitch of 0.03571 in (0.90714 mm). Pitch is the axial distance between corresponding points on adjacent threads.
Does this table show 2A or 2B acceptance limits?: No. It shows theoretical basic dimensions. A denotes an external thread and B an internal thread. Use the specified class limits and inspection requirements from the governing standard for acceptance.
Is percent thread the same as length of engagement?: No. Percent thread describes the theoretical thread height left by the pilot hole. Length of engagement is the axial distance over which the internal and external threads overlap. Neither alone establishes the strength of a joint.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	reference	ASME B1.1-2019 — source
[2]	Basic dimensions computed from the ASME B1.1 basic profile	derived	Pitch P = 1/n. Fundamental triangle height H = √3/2 × P. Pitch diameter d2 = d − 0.75H. External minor diameter d1 = d − 2(0.61343P). Internal minor diameter D1 = d − 2(0.54127P). Tensile stress area As = π/4 × (d − 0.9743/n)².
[3]	Millimetre conversion	derived	Inch value × 25.4.
[4]	Standard twist drill size series (number, letter and fractional)	reference	n/a — industry standard drill series — source
[5]	MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	reference	MIL-S-7742D (free public-domain mirror at everyspec.com) — source
[6]	Theoretical cut-tap percentage from Guhring’s sizing equation	derived	% thread = (nominal major diameter − drill diameter) × TPI / 0.01299. The linked Guhring chart supplies the sizing convention; this is not a measured thread percentage.

Data Sources

Standard	Revision	What it covers on this page
ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	ASME B1.1-2019	thread form geometry, the designation system and class-of-fit definitions
MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	MIL-S-7742D (free public-domain mirror at everyspec.com)	the selection of nominal diameters and pitches in the standard series

Verification scope

2026-10-15 — Drill decimal equivalents, cut/form-tap distinction and the theoretical percentage convention; not a review of the full ASME standard. Evidence

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
All inch dimensions and the tensile stress area	Computed from the ASME B1.1 basic profile formulas, not transcribed from a table — see the Value Sources table above for the exact expressions.
Millimetre values	Inch value × 25.4, rounded to 4 decimal places.
Theoretical percent thread and 75% pilot example	Rearranged Guhring cut-tap equation: % = (D − drill) × TPI / 0.01299. Values use the nominal drill diameter; actual hole size is not measured here.

Every dimension on this page is generated from the standard's own formulas, so the values are self-consistent by construction. Basic dimensions do not include class-of-fit allowances or tolerances — for a threaded assembly, specify the class and use the limit dimensions from ASME B1.1. The reference drill is a lookup starting point for cut tapping, not an exact 75% recommendation. The theoretical percent calculation and tool-selection guidance were added on 2026-10-15; this is not a new review of the full ASME standard.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "1/4-28 UNF Tap Drill Size & Thread Dimensions." https://refmanual.com/threads/unf/1-4-28/ (accessed 2026-10-19). Data from ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019.

BibTeX

@misc{unf-1-4-28,
  title        = {1/4-28 UNF Tap Drill Size & Thread Dimensions},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/threads/unf/1-4-28/}},
  note         = {Accessed: 2026-10-19},
  source       = {ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - 1/4-28 UNF Tap Drill Size & Thread Dimensions
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/threads/unf/1-4-28/
PY  - 2026
DA  - 2026-10-19
N1  - Source: ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.