What is the tap drill size for a 1 1/2-12 thread?

The tap drill for 1 1/2-12 at about 75% thread engagement is 1 (1.0000 in / 25.4 mm). It is found by taking major diameter − pitch (1.5000 − 0.0833 = 1.4167 in) and selecting the nearest standard drill.

What is the major diameter of a 1 1/2-12 thread?

The nominal major diameter is 1.5000 in (38.1 mm). On an external thread this is the outside diameter of the thread; on an internal thread it is the theoretical largest diameter of the thread form.

What is the pitch diameter of a 1 1/2-12 thread?

The basic pitch diameter is 1.4459 in (36.7252 mm). This is the diameter at which the thread width and the groove width are equal, and it is the dimension that actually controls whether the thread assembles — it is what a thread micrometer or thread comparator measures.

How many threads per inch does a 1 1/2-12 have?

12 threads per inch, giving a pitch of 0.0833 in (2.1167 mm). Threads per inch defines the pitch; it does not change with the nominal diameter within the same size designation.

What is the difference between major, pitch and minor diameter?

Major diameter is the largest diameter of the thread — the outside of an external thread. Pitch diameter is measured where the thread and groove widths are equal, and is the dimension that controls fit. Minor diameter is the smallest — the root of an external thread or the bore of an internal thread. Pitch diameter is the critical one: a thread can meet its major and minor limits and still not assemble if the pitch diameter is wrong.

1 1/2-12 Thread Dimensions

Dimensions for the 1 1/2-12 UNF (Unified Fine) thread — pitch, pitch diameter, external and internal minor diameters, tap drill size and tensile stress area.

Data verified 2026-09-29 · based on ASME B1.1-2019

Quick Answer

1 1/2-12 has a pitch of 0.0833 in (12 threads per inch), a basic pitch diameter of 1.4459 in (36.7252 mm) and a tap drill size of 1 at about 75% thread engagement.

Thread Calculator

Major diameter (inch)

Threads per inch

Basic Dimensions

Dimension^[1]	Symbol^[1]	Inch^[2]	Millimetre^[3]
Major diameter (external) #	`d`	1.5000	38.1
Pitch diameter (basic) #	`d2`	1.4459	36.7252
Minor diameter, external thread #	`d1`	1.3978	35.5031
Minor diameter, internal thread #	`D1`	1.4098	35.8086
Pitch #	`P`	0.0833	2.1167
Thread height, external #	`h`	0.0511	1.2984
Thread height, internal #	`hn`	0.0451	1.1457
Crest flat #	`f`	0.0104	0.2646
Tensile stress area #	`As`	1.5810	1020.0108

Tap Drill Size

For a 1 1/2-12 thread at approximately 75% thread engagement, the tap drill is 1 (1.0000 in / 25.4 mm).

Pilot diameter is computed as major diameter − pitch = 1.5000 − 0.0833 = 1.4167 in, then rounded to the nearest standard drill in the number, letter or fractional series. Increasing engagement above 75% raises tapping torque sharply and risks tap breakage; decreasing it weakens the thread. See the tap drill size chart for the full series.

How to Read the Designation

1 1/2-12 encodes three things: the nominal major diameter 1 1/2, the thread pitch as 12 threads per inch, and the series — UNF (Unified Fine). The series letter sets the pitch for that diameter; the same nominal diameter exists in UNC, UNF and UNEF at different pitches.

Numbers below 1/4 in are written as machine screw sizes (#0 through #12) whose major diameter follows d = 0.060 + 0.013 × N. A 1 1/2-12 has a nominal major diameter of 1.5000 in (38.1 mm).

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 below are the reference from which the class limits are derived.

← 1 3/8-12 All UNF sizes

Frequently Asked Questions

What is the tap drill size for a 1 1/2-12 thread?: The tap drill for 1 1/2-12 at about 75% thread engagement is 1 (1.0000 in / 25.4 mm). It is found by taking major diameter − pitch (1.5000 − 0.0833 = 1.4167 in) and selecting the nearest standard drill.
What is the major diameter of a 1 1/2-12 thread?: The nominal major diameter is 1.5000 in (38.1 mm). On an external thread this is the outside diameter of the thread; on an internal thread it is the theoretical largest diameter of the thread form.
What is the pitch diameter of a 1 1/2-12 thread?: The basic pitch diameter is 1.4459 in (36.7252 mm). This is the diameter at which the thread width and the groove width are equal, and it is the dimension that actually controls whether the thread assembles — it is what a thread micrometer or thread comparator measures.
How many threads per inch does a 1 1/2-12 have?: 12 threads per inch, giving a pitch of 0.0833 in (2.1167 mm). Threads per inch defines the pitch; it does not change with the nominal diameter within the same size designation.
What is the difference between major, pitch and minor diameter?: Major diameter is the largest diameter of the thread — the outside of an external thread. Pitch diameter is measured where the thread and groove widths are equal, and is the dimension that controls fit. Minor diameter is the smallest — the root of an external thread or the bore of an internal thread. Pitch diameter is the critical one: a thread can meet its major and minor limits and still not assemble if the pitch diameter is wrong.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	standard	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)	standard	n/a — industry standard drill series — source
[5]	MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	standard	MIL-S-7742D (free public-domain mirror at everyspec.com) — source

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

Cross-checked against:

Values recomputed from the ASME B1.1 basic profile formulas and compared against published fastener dimension tables for this size
Tap drill matched against the standard twist drill series rather than transcribed

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.

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.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. 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

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "1 1/2-12 Thread Dimensions." https://refmanual.com/threads/unf/1-1-2-12/ (accessed 2026-09-29). Data from ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019.

BibTeX

@misc{unf-1-1-2-12,
  title        = {1 1/2-12 Thread Dimensions},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/threads/unf/1-1-2-12/}},
  note         = {Accessed: 2026-09-29},
  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 1/2-12 Thread Dimensions
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/threads/unf/1-1-2-12/
PY  - 2026
DA  - 2026-09-29
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.