How Wide Is a #4-40 Screw—and Why Might Your Calipers Say Otherwise?

The direct answer: #4-40 diameter and thread specifications

A #4-40 machine screw has a nominal major diameter of 0.112 inch, equivalent to approximately 2.84 to 2.845 mm. The 40 means 40 threads per inch (TPI), producing a pitch of 0.025 inch, or 0.635 mm, between corresponding points on adjacent threads. A fastener size chart lists the #4 diameter, metric conversion, and coarse- and fine-thread options for this numbered size together.

Pitch is the reciprocal of threads per inch:

Pitch = 1 ÷ 40 = 0.025 inch

Converting that pitch to millimeters:

0.025 × 25.4 = 0.635 mm

For a #4 machine screw, #4-40 is commonly listed as the coarse UNC option, while #4-48 is the fine UNF option. Both begin with the same numbered diameter, but their thread spacing differs. They are therefore distinct thread designations, not interchangeable labels.

The critical qualifier is nominal. The 0.112-inch figure identifies the standardized screw size; it does not guarantee that ordinary calipers will display exactly 0.112 inch on every finished external thread. Applicable thread limits can place the crest-to-crest diameter below nominal.

The 0.112-inch figure describes the nominal major diameter of the threaded body. It does not describe:

• Head diameter
• Screw length
• Pitch diameter
• Minor diameter
• Drive size
• Clearance-hole diameter
• An unqualified tap-drill size

Those dimensions must be identified separately. In a label such as #4-40 × 7/8 inch, for example, the first two elements identify the thread, while the final dimension identifies length.

What the #4-40 designation means

The two numbers in #4-40 do not both describe diameter:

• #4 is a numbered diameter used for small Unified inch-series machine screws.
• 40 means 40 threads per inch.

The #4 is not four millimeters, four sixty-fourths of an inch, 4/40 inch, or a measurement of the screw head. It is a standardized numbered size from which the nominal diameter can be calculated.

For numbered Unified screw sizes, the commonly cited relationship is:

Nominal diameter = 0.060 inch + (0.013 inch × screw number)

For a #4 screw:

0.060 + (0.013 × 4) = 0.060 + 0.052 = 0.112 inch

This numbered-diameter formula and its application to a #4 screw are described in an independent discussion of Unified screw nomenclature.

The 40 after the dash is the thread count. It does not specify:

• Length
• Head diameter or style
• Drive type or size
• Material
• Finish
• External thread class

Because there are 40 thread intervals per inch, the distance between corresponding points on adjacent threads is 1/40 inch, or 0.025 inch. For the same nominal diameter, a larger TPI number means more closely spaced threads. That is why #4-48 is finer than #4-40.

Length is stated separately:

A commercial listing for a #4-40 × 7/8-inch pan-head machine screw separately identifies the #4 diameter, 40-thread count, 0.112-inch nominal diameter, and 7/8-inch length.

Diameter and pitch should always be considered together. A measurement near 0.11 inch may indicate a #4 screw, but it cannot by itself distinguish #4-40 from #4-48. Identifying an unknown screw therefore requires both a diameter check and a pitch check.

Nominal diameter versus the diameter a screw actually measures

A nominal diameter is the standardized identifying size used to specify and order the thread. It is the correct answer to “What is the diameter of a #4-40 screw?” but not necessarily the exact number shown by calipers on a particular finished screw.

Finished external threads have dimensional limits. Those limits vary with the specified external thread class, so a class-specific range should not be treated as a universal acceptance limit for every #4-40 product.

The following values come from a manufacturer’s third-party thread chart for Class 2A and Class 3A external threads:

These ranges illustrate why a finished external screw may measure below the 0.112-inch nominal diameter. They do not identify the class of an unknown screw, and they should not be applied without considering the product specification and inspection method.

For example, a screw measuring approximately 0.107 to 0.108 inch across intact thread crests and matching 40 TPI can plausibly be #4-40 because the diameter falls within published external-thread ranges. A separate supplier chart lists a Class 2A #4-40 major-diameter range of 0.1061 to 0.1112 inch, reinforcing that such a reading is not automatically inconsistent with the designation #4-40.

That example is not proof of identity. A complete assessment must still consider:

• Whether the pitch is actually 40 TPI
• Whether the jaws contacted opposing thread crests
• Whether the shaft was square to the jaws
• Whether the measured crests are worn, flattened, dirty, or damaged
• Which external thread class applies
• Whether the measuring tool is appropriate and correctly zeroed
• Whether a product drawing specifies different requirements

The pitch-diameter values in the table are not alternative answers to the outside-diameter question.

The quoted ranges also come from commercial technical charts rather than the controlling standards publication. Where conformance matters, verify:

1. The product’s specified external thread class.
2. The governing standard.
3. The edition identified by the drawing, purchase order, or product documentation.
4. The prescribed inspection method.
5. Any product-specific conditions affecting the applicable limits.

In practical terms, 0.112 inch is the nominal identifying diameter, while the actual crest-to-crest diameter of a finished external #4-40 screw can be somewhat smaller.

How to measure and identify a #4-40 screw

The practical identification method is to check both the external major diameter and the thread pitch. Neither value is sufficient on its own.

1. Clean the threaded section

Remove loose dirt, oil, paint, corrosion particles, thread-locking residue, and other debris that could interfere with jaw contact or prevent a pitch gauge from seating properly.

Do not file or abrade the thread merely to obtain a cleaner reading. Altering the thread changes the feature being measured.

2. Inspect the thread crests

Look for flattened, missing, dented, crossed, or otherwise damaged threads. Avoid relying on an obviously damaged section or on incomplete lead threads at the tip.

If the screw has an intact central section, use that section for the primary diameter check.

3. Zero the calipers

Close the jaws gently and confirm that the instrument reads zero. Wipe the jaw faces before measuring.

4. Measure the external major diameter

External major diameter is measured crest-to-crest across the widest opposing points of the thread. Hold the threaded shaft perpendicular to the flat caliper jaws rather than diagonally through them.

A commercial measurement guide likewise instructs readers to measure across opposing thread crests, keep the screw perpendicular to the jaws, identify TPI, and compare the result with a thread chart when measuring an unknown screw.

For a likely #4 screw, expect a reading near the nominal diameter, but do not require every finished screw to display exactly 0.112 inch.

5. Repeat the diameter check

Take readings at more than one intact location and rotate the screw before measuring again. The purpose is to determine whether the result is reasonably repeatable rather than dependent on one jaw position or one damaged section.

If the readings vary materially, recheck alignment, cleanliness, and crest condition before trying to identify the screw.

6. Determine the thread pitch

Fit a 40-TPI leaf from an inch thread-pitch gauge against the screw. The gauge teeth should align with the thread grooves across multiple complete threads without obvious cumulative gaps.

Compare the fit with another plausible leaf, such as 48 TPI. Because #4-40 and #4-48 share the same nominal #4 diameter, pitch is the feature that distinguishes them.

If no pitch gauge is available, thread crests can be counted over a known distance. This becomes difficult on a short, small fastener because only a limited number of threads may be available for counting. A magnified view or comparison with a correctly identified screw may assist, but a pitch gauge provides a more direct check.

7. Compare diameter and pitch with a thread chart

A diameter consistent with a #4 external thread plus a positive 40-TPI match supports identification as #4-40. A diameter match without a pitch match is incomplete, as is a pitch match without a compatible diameter.

If the receiving nut or tapped hole is documented, its stated designation is more useful than trial fitting. As a cautious practical check, stop if a screw does not turn freely by hand and verify both designations rather than treating resistance as confirmation.

Troubleshooting a reading below nominal

If a suspected #4-40 screw measures below 0.112 inch, check:

• External-thread limits: A finished external thread may legitimately be below nominal.
• Jaw alignment: A tilted or diagonal measurement may not capture the widest opposing crests.
• Contact position: The jaws may not be resting on opposing crest points.
• Crest condition: Wear, flattening, corrosion, or damage may lower the reading.
• Debris or residue: Contamination can make readings inconsistent.
• Incomplete threads: Tip and runout geometry may not represent the full major diameter.
• Pitch identification: A similarly sized screw may use a different TPI.
• Tool suitability: Small dimensional differences should not be overinterpreted with unsuitable equipment.

Calipers and a pitch gauge are effective identification tools. They do not automatically establish an external thread class. That requires an applicable specification and a suitable inspection procedure.

Do not confuse thread diameter with pitch, head diameter, or drill size

The nominal 0.112-inch dimension represents the thread’s major diameter—the largest diameter across opposing thread crests.

Several related dimensions are easy to mistake for that answer.

Major diameter

The major diameter is the widest crest-to-crest diameter of the threaded body. For #4-40, its nominal identifying value is 0.112 inch. A finished external thread may measure less within its applicable limits.

Pitch diameter

Pitch diameter is a theoretical thread-engagement dimension between the major and minor diameters. It is not the outside diameter obtained by placing ordinary calipers over the crests.

Machining Doctor lists a basic pitch diameter of 0.0958 inch for #4-40, distinctly smaller than the 0.112-inch nominal major diameter in its dimensional table.

Minor diameter

The minor diameter lies toward the thread roots and is smaller than the major and pitch diameters. It is part of the thread geometry but is not represented by the #4 nominal outside diameter.

Pitch distance

Pitch is measured along the screw axis between corresponding points on adjacent threads. Diameter is measured across the screw. For 40 TPI, the pitch is 0.025 inch.

Head diameter

Head diameter is independent of thread diameter and varies with the head style and product design. A commercial #4-40 pan-head listing gives a product-specific head-diameter range of 0.205 to 0.219 inch, while separately listing the thread diameter as 0.112 inch for that screw.

That head range is not a universal #4-40 dimension. Flat, oval, round, pan, fillister, and socket heads can have different geometry. Match the head separately when selecting a replacement.

Screw length

Length is also independent of thread size. Two screws can both be #4-40 while having different lengths.

The appropriate length reference can depend on head style because some heads sit above the mounting surface while countersunk heads sit into it. When length is critical, compare like-for-like head styles and use the applicable product drawing or specification.

Tap-drill size

A tap-drill size is the hole diameter used before cutting an internal thread. It is not the screw’s outside diameter.

The Machining Doctor table lists 0.088 inch for a stated 75% thread-depth condition for #4-40 threads. That qualified value is not a universal drilling instruction for every material, tap, engagement requirement, or process.

Do not treat 0.088 inch as:

• The screw’s major diameter
• A universal clearance-hole diameter
• The only possible tap-drill selection
• A substitute for process-specific tapping information

A useful way to keep the dimensions separate is to note both their direction and their reference points:

This avoids substituting a head, pitch, pitch-diameter, minor-diameter, drill, or length measurement for the nominal thread diameter.

Fractional and metric comparisons: what is close but not exact

The nearest commonly listed fractional comparison for a #4 diameter is 7/64 inch, but it is not the exact nominal diameter. A commercial conversion chart lists 7/64 inch as the nearest fraction while retaining 0.112 inch as the #4 decimal diameter value.

The arithmetic is:

7 ÷ 64 = 0.109375 inch

Compared with the nominal #4 diameter:

0.112 − 0.109375 = 0.002625 inch

That makes 7/64 inch useful as a rough fractional comparison, not as a replacement thread designation. A nearby fraction on a conversion chart does not redefine #4-40 as a fractional-size thread.

The nominal metric conversion is:

0.112 inch × 25.4 = 2.8448 mm

Rounded appropriately, the nominal diameter is approximately 2.84 or 2.845 mm. This is only a conversion of the inch-series diameter. It does not turn #4-40 into a metric thread designation.

Some references describe M3 as the closest metric size to #4-40. Dimensional proximity does not establish thread compatibility, and the source describing M3 as the closest size does not identify it as an exact interchangeable thread replacement.

A similar outside diameter cannot establish that:

• The pitches match
• The complete thread designations match
• The threads will remain aligned over the required engagement length
• The resulting fit meets the application specification

For a direct replacement, match the full designation: #4-40. Do not select M3 hardware or an unspecified 7/64-inch item merely because its diameter appears close.

A practical checklist for matching or ordering a replacement

Before ordering a replacement, confirm each characteristic that affects identification or installation:

• Major diameter: Look for a crest-to-crest measurement consistent with a #4 external thread.
• Thread pitch: Confirm 40 TPI with a pitch gauge or another reliable method.
• Length: Match the required length using the appropriate convention for the head style.
• Head style: Identify pan, flat, oval, round, socket, or another required form.
• Drive: Match the required slotted, Phillips, hex-socket, or other drive geometry.
• Thread class: Check the drawing or product documentation if a class is specified.
• Material and finish: Verify these independently; #4-40 does not define them.
• Thread coverage: Determine whether the replacement must be fully or partially threaded.
• Mating component: Confirm that the nut, insert, or tapped hole is designated #4-40.
• Application documentation: Follow any product, assembly, regulatory, or engineering requirements.

Do not order #4-48 solely because it shares the #4 nominal diameter. Its 48-TPI fine thread differs from the 40-TPI thread used by #4-40.

Conversely, do not reject a likely #4-40 screw merely because its measured diameter is slightly below 0.112 inch. Recheck the pitch, jaw alignment, and thread condition before drawing a conclusion.

Whenever possible, identify the mating thread from a drawing, label, bill of materials, product manual, or manufacturer’s parts list. If an apparently close screw resists hand installation, stop and verify the designations rather than relying on force as an identification method.

The #4-40 designation by itself does not determine:

• Head diameter
• Head style
• Drive type or size
• Length
• Material
• Coating or finish
• Strength properties
• External thread class
• Suitability for a particular application

For regulated, structural, safety-critical, or inspection-grade work, use the applicable specification, identified standard edition, product documentation, and qualified professional guidance. Mortar Desk provides general reference information rather than individual contracting or engineering advice, and it recommends comparing cited specifications with the locally applicable edition in its scope and standards policy.

The four figures most readers need are:

• 0.112-inch nominal major diameter
• Approximately 2.84 mm
• 40 threads per inch
• 0.025-inch pitch

A finished external screw may measure slightly under nominal. Check diameter and pitch together, keep head and drill dimensions separate, and consult the governing product specification when tolerance-class acceptance matters.

Frequently asked questions

Is a #4-40 screw exactly 0.112 inch across?

Not necessarily. 0.112 inch is the nominal major diameter, while a finished external thread may measure slightly smaller. A manufacturer’s chart lists a Class 2A external major-diameter range of 0.1061–0.1112 inch and a Class 3A range of 0.1069–0.1120 inch for #4-40 threads.

Measurement technique, thread condition, applicable class, and the governing specification still matter. A caliper reading alone does not prove conformance.

What does the 40 mean in #4-40?

It means 40 threads per inch. A retailer’s #4-40 product specification separately lists a 0.112-inch thread diameter and 40 TPI, illustrating that the two parts of the designation describe diameter size and thread count respectively.

A 40-TPI thread has a pitch of:

1 ÷ 40 = 0.025 inch, or 0.635 mm.

What is the #4-40 screw diameter in millimeters?

The nominal diameter is 2.8448 mm, commonly rounded to 2.84 or 2.845 mm:

0.112 inch × 25.4 = 2.8448 mm

That is a unit conversion of an inch-series nominal diameter, not a metric thread designation.

Is #4-40 a coarse or fine machine-screw thread?

For a #4 screw, #4-40 is commonly identified as the coarse UNC option. The commonly listed fine UNF option is #4-48.

They share the same nominal #4 diameter but have different thread counts and pitches. A #4-48 screw is not a direct #4-40 replacement unless the mating thread or product specification also changes.

Can I replace a #4-40 screw with 7/64-inch or M3 hardware?

Not solely because the diameters are close.

7/64 inch equals 0.109375 inch, making it only a nearby fractional comparison with the 0.112-inch nominal #4 diameter. M3 is a metric designation and requires its own pitch specification.

For a direct replacement, match #4-40 along with the required length, head style, drive, thread class, material, and finish.