How to Measure a Screw or Bolt: Diameter, Pitch and Length

To identify an unknown machine screw or bolt, record four essential details: thread diameter, thread pitch or threads per inch, length, and head style. Then identify the drive and material before ordering a replacement.

Quick answer: Measure the outside of the threads for diameter. For metric threads, measure the distance between adjacent thread crests in millimeters. For inch threads, count threads per inch (TPI). Measure most screws from under the head to the tip, but measure flat or countersunk screws over their full length, including the head.

Do not identify a fastener from one measurement alone. An M3 screw and a #4-40 screw look similar, but their diameters and thread spacing differ and they are not interchangeable.

What you need to measure a screw

For a reliable identification, use:

• digital or dial calipers;
• a metric and inch thread-pitch gauge;
• a ruler with inch and millimeter scales;
• a known nut or threaded gauge plate for final confirmation; and
• good lighting or a magnifier for small machine screws.

Calipers are the most useful starting tool, but they do not certify that a thread is within manufacturing tolerance. Production inspection requires the gauges and acceptance criteria specified by the applicable drawing or standard.

Screw measurement order

Write the result in this order:

1. Fastener type
2. Nominal thread diameter
3. Thread pitch or TPI
4. Nominal length
5. Head style and drive
6. Material, finish and strength designation when known

Examples:

• #4-40 × 3/8 in. Phillips pan-head machine screw
• M3 × 0.5 × 10 mm socket-head cap screw
• 1/4-20 × 1 in. hex bolt

The dimensions do not describe the whole product. Two screws can share a thread size and length but differ in head geometry, drive, material, strength, finish or governing product standard.

Step 1: Measure the thread diameter

Close the caliper jaws lightly across the outermost thread crests. Keep the jaws perpendicular to the screw axis and measure across the widest part of the external thread. This is an approximate major diameter.

Do not measure across a hex head and call that the bolt diameter. The wrench size across flats is a separate dimension and can vary even when the thread diameter is the same.

Compare the reading with a nominal size rather than expecting an exact match. A #4 screw is nominally 0.1120 inch (about 2.84 mm), while M3 is nominally 3.00 mm. Actual readings may be slightly lower because of thread tolerances, wear, damage, plating and caliper position.

Common small screw diameters

These conversions help narrow the possibilities; they do not establish compatibility. Continue by measuring the thread spacing.

Step 2: Determine metric pitch or inch TPI

The safest method is to match a thread-pitch gauge leaf to the screw. The correct leaf contacts the thread flanks evenly without light showing between the gauge and the thread.

How to measure metric thread pitch

Metric pitch is the axial distance from one thread crest to the next, expressed in millimeters. For example, M3 × 0.5 has a 3 mm nominal diameter and a 0.5 mm pitch.

Small errors matter, so measure across several complete pitch intervals:

1. Place one caliper jaw on the center of a thread crest.
2. Count ten complete crest-to-crest intervals.
3. Place the other jaw on the eleventh crest.
4. Divide the measured distance by ten.

If ten intervals measure 5.0 mm, the pitch is approximately 0.5 mm. Measuring from the first crest to the tenth crest covers only nine intervals, which creates an off-by-one error.

How to measure threads per inch

Unified inch threads are commonly specified by threads per inch, or TPI. A #4-40 screw has 40 threads per inch; a 1/4-20 bolt has 20 threads per inch.

Align a thread crest with the zero mark on an inch ruler and count complete thread intervals over one inch. If the threaded section is too short, count over 1/2 inch and multiply by two, or measure a known number of intervals and calculate:

TPI = number of complete thread intervals ÷ measured length in inches

For example, 16 intervals over 1/2 inch equal 32 TPI.

Use the Thread Pitch Chart: UNC, UNF and Metric Thread Sizes to match your diameter and spacing to a standard thread designation.

Step 3: Decide whether the screw is metric or inch

Use both diameter and thread spacing:

• Metric designations use millimeters for diameter and pitch, such as M4 × 0.7.
• Unified inch designations use numbered or fractional diameter plus TPI, such as #8-32 UNC or 1/4-20 UNC.
• UNC means Unified National Coarse; UNF means Unified National Fine.

Do not rely on appearance, country of origin or the tool that fits the drive. Metric and inch hex keys can sometimes feel close enough to turn a worn socket, while the threads remain entirely different.

Similar-looking sizes that do not interchange

A wrong screw may start for one or two turns. That is not proof of a match. Stop if hand resistance rises unexpectedly. Forcing the screw can damage both external and internal threads.

For the most commonly confused electronics size, see M3 vs. #4-40: Why These Threads Are Not Interchangeable.

Step 4: Measure screw or bolt length correctly

The general rule is to measure from the surface where the installed fastener bears to the tip.

Heads that sit above the surface

Measure from directly under the head to the tip for:

• pan-head screws;
• button-head screws;
• socket-head cap screws;
• hex-head bolts;
• truss-head screws; and
• most other non-countersunk heads.

Do not include the head height in the nominal length.

Flat and countersunk heads

Measure a flat-head or countersunk screw from the top of the head to the tip. Its nominal length includes the head because the head is intended to sit flush with the material surface.

Oval countersunk heads

An oval head is partly countersunk and partly raised. Its measurement reference can be less intuitive than a flat head. Use the product drawing or governing standard when an exact replacement is required.

Set screws and headless fasteners

Measure a headless set screw end to end. Point style—cup, cone, flat, oval or dog point—is a separate identification feature.

Shoulder screws

Shoulder screws require additional dimensions, especially shoulder diameter and shoulder length. Do not substitute overall length for the stated shoulder length; consult the product drawing.

Step 5: Identify the head style and drive

The head style describes the external shape and how it seats. The drive describes the tool interface.

Common head styles include:

• socket head;
• button head;
• flat or countersunk head;
• pan head;
• hex head;
• truss head; and
• set screw with no head.

Common drives include hex socket, Torx, Phillips, slotted and external hex. Record both. “Hex screw” is ambiguous because it could mean an external hex head or an internal hex socket.

Head and drive measurements help identify the product standard, but they do not replace thread diameter and pitch.

Step 6: Record material, finish and strength separately

Do not identify material from color alone. Stainless steel, zinc-plated steel, nickel-plated brass and other finishes can look similar in photographs or after service.

Check the original specification, packaging, head markings, magnetic response and service environment. A magnet test alone is not conclusive for every stainless alloy. If strength or corrosion performance matters, obtain traceable product documentation rather than relying on appearance.

Keep these descriptions separate:

• base material, such as carbon steel, stainless steel, brass or nylon;
• finish or coating, such as zinc plating or black oxide; and
• strength designation, such as SAE grade or metric property class.

Step 7: Confirm the identification without damaging the assembly

After narrowing the size:

1. Compare the measurements with a recognized thread chart.
2. Try a known matching nut, threaded standoff or gauge by hand.
3. Verify the head and drive against the product drawing.
4. Confirm material, finish and strength requirements.
5. Stop if the mating part binds or wobbles excessively.

Do not use a power driver or wrench to test an uncertain match. For critical joints, damaged threads, safety-related assemblies or production inspection, use calibrated gauges and the requirements of the applicable engineering drawing and standard.

Worked examples

Example 1: Identifying a small electronics screw

The photographed caliper reads approximately 2.90 mm across the thread crests. Ten complete pitch intervals measure approximately 5.0 mm. The screw has a socket head and measures 10 mm from under the head to the tip.

The likely identification is M3 × 0.5 × 10 mm socket-head cap screw.

Example 2: Distinguishing #4-40 from M3

The outside diameter reads approximately 0.111 inch. Sixteen complete thread intervals span 0.4 inch:

16 ÷ 0.4 = 40 TPI

The likely designation is #4-40, not M3. Confirm with a #4-40 gauge or known nut before ordering.

Example 3: Measuring a flat-head screw

The thread diameter is approximately 4 mm and the pitch is 0.7 mm. Because the head is countersunk, the length is measured from the top of the head to the tip and reads 16 mm.

The size portion of the designation is M4 × 0.7 × 16 mm. The complete description must also state the flat-head geometry, drive, material and applicable product standard.

Common screw-measurement mistakes

• Measuring the head instead of the external thread diameter
• Assuming the caliper reading must equal the nominal diameter exactly
• Measuring only one thread interval on a small screw
• Counting crests instead of complete crest-to-crest intervals
• Treating metric pitch as though it were TPI
• Including a pan, button, socket or hex head in nominal length
• Excluding a flat countersunk head from nominal length
• Assuming a screw is compatible because it starts for one turn
• Identifying stainless steel or plating by color alone
• Omitting head style, drive, material or strength from the final description

Find the corresponding fastener family

Once the thread and length are confirmed, compare the remaining specifications in the relevant family:

• Socket head cap screws
• Button head socket screws
• Flat head socket screws
• Pan head screws
• Hex nuts
• PCB standoffs

Availability shown in the catalog may change. Match the complete product specification—not only the nominal thread—before selection.

Frequently asked questions

Where do you measure a screw from?

Measure most screws from under the head to the tip. Measure flat or countersunk screws from the top of the head to the tip because the head sits flush in the installed surface.

Does screw length include the head?

Usually not. The main exception is a flat or countersunk head, whose nominal length includes the head. Headless set screws are measured end to end, while shoulder screws require the dimensions shown on their product drawing.

How do you measure screw diameter?

Use calipers across the outermost crests of the external thread. This approximates the major diameter. Do not use the head diameter or wrench size as the screw diameter.

How do you measure thread pitch without a gauge?

For a metric thread, measure several complete crest-to-crest intervals in millimeters and divide by the number of intervals. For an inch thread, count intervals over a known inch length and divide by that length to calculate TPI.

How do I know if a screw is metric or standard?

Check both diameter and spacing. Metric threads use nominal diameter and pitch in millimeters, such as M3 × 0.5. Unified inch threads use numbered or fractional diameter and threads per inch, such as #4-40 or 1/4-20.

Is a #4 screw the same as an M3 screw?

No. A #4 screw has a nominal diameter of 0.112 inch, approximately 2.84 mm, while M3 is 3.00 mm. Common #4-40 and M3 × 0.5 threads also have different spacing.

What does M3 × 0.5 × 10 mean?

It means a metric screw with 3 mm nominal thread diameter, 0.5 mm pitch and 10 mm nominal length. Head style, drive, material and product standard must be specified separately.

What does #6-32 × 1/2 mean?

It identifies a #6 nominal diameter, 32 threads per inch and a nominal length of 1/2 inch. The thread series, head, drive, material and finish should also be confirmed.

Can I identify stainless steel from a photo?

No. Lighting, surface condition and plating can make different materials look alike. Verify the material from reliable documentation or appropriate testing when it matters to corrosion resistance or joint performance.

Sources and standards

• ASME B1.1-2024, Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms).
• ISO 261:1998, ISO general purpose metric screw threads — General plan.
• ISO 724:2023, ISO general purpose metric screw threads — Basic dimensions.
• ISO 965-1:2026, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data.
• ISO 888:2012, Fasteners — Bolts, screws and studs — Nominal lengths and thread lengths.

Standards are referenced for terminology and dimensional context. Use the current purchased standard, governing drawing and manufacturer documentation for inspection or controlled engineering work.

Technical note and disclaimer

This guide is intended for preliminary fastener identification and product selection. Caliper readings, ruler measurements and trial fits do not establish conformance to thread tolerances or certify a fastener for a safety-critical application. Follow the engineering drawing, applicable ASME or ISO standard, and qualified engineering guidance for controlled assemblies.