Identify screw threads from measurements — free online thread identifier for metric, UNC, UNF, BSP, NPT, ACME and 11 other thread standards.

Units
Thread Type
Major Diameter (mm)
mm
Pitch (mm)
mm
Threads per Inch (TPI)
TPI
Tolerance ±5%
  1. 1
    Select units

    Choose mm or Inch depending on how you are measuring.

  2. 2
    Select thread type

    Use External for bolts/studs, Internal for nuts/tapped holes.

  3. 3
    Measure the diameter

    For external threads, measure the outer (major) diameter with calipers. For internal threads, measure the minor (inner) diameter of the bore.

  4. 4
    Measure the pitch or TPI

    Use a thread pitch gauge or count the number of thread crests over 25.4 mm (1 inch) to find TPI. Alternatively enter the pitch directly in mm or inches.

  5. 5
    Tap Identify Thread

    Results are ranked by match quality. Excellent Good Acceptable Poor

About This Thread Identifier Tool

ThreadChaser is a free online tool to identify screw threads from measurements. Measure your bolt, screw, or threaded hole with digital calipers and a thread pitch gauge, enter the dimensions, and the tool instantly identifies the matching thread standard from a database of over 300 thread sizes across 16 international standards. It is the fastest way to identify a thread from measurements without needing a full physical set of thread gauges.

Unlike static thread charts, ThreadChaser accepts your actual measurements and scores every possible match by how closely the diameter and pitch align — so even if your calipers are slightly off, you still get the right result. Results are ranked Excellent, Good, Acceptable, or Poor, with full dimensional data for each match including major diameter, pitch diameter, minor diameter, and the recommended tapping drill size.

Supported Thread Standards

Metric Coarse (ISO 724) — the most common thread worldwide, used on most modern bolts and machinery.
Metric Fine (MF) — finer pitch metric threads used in precision applications and automotive components.
UNC (Unified National Coarse) — the standard coarse thread in North America, used on most inch-measure fasteners.
UNF (Unified National Fine) — finer pitch unified thread, common in automotive and aerospace.
UNEF (Unified Extra Fine) — very fine unified threads for thin-walled components.
BSP (British Standard Pipe, 55°) — standard pipe thread in the UK, Europe, and Australia.
BSW (British Standard Whitworth) — the original British standard thread, found on older machinery and classic vehicles.
BSF (British Standard Fine) — fine-pitch Whitworth threads, common on pre-1970s British vehicles.
BA (British Association) — very small threads used in instruments, electrical equipment, and model engineering.
NPT (National Pipe Taper, 60°) — American tapered pipe thread standard, not interchangeable with BSP.
ANC / ANF (Army-Navy Coarse / Fine) — older American aerospace and military thread standards.
ACME — trapezoidal threads used in lead screws, jacks, and linear motion applications.
Buttress — asymmetric threads designed to withstand high axial force in one direction.
Square — square-section threads used in vices and heavy-duty power transmission.
Knuckle (DIN 405) — rounded-profile threads used in bottle caps, fire hose couplings, and industrial connectors.

How to Identify a Thread From Measurements

The most reliable way to identify an unknown thread without a gauge is to measure two things: the major diameter (the outer diameter of a bolt, or the inner diameter of a nut) and the pitch (the distance between thread crests in mm, or thread count in TPI). Together these two measurements uniquely identify virtually every common thread type. A good set of digital calipers accurate to 0.01 mm is all you need for most identifications.

If you don't have a pitch gauge, count the number of complete thread crests over exactly one inch (25.4 mm) — this gives you the TPI directly. For metric threads, count over 10 mm and multiply by 10 to get pitch in mm. Enter these values into the form above and hit Identify Thread.

Metric vs Imperial Thread Identification

A common source of confusion is whether an unknown fastener is metric or imperial. The quickest test: measure the major diameter and check if it falls on a clean metric number (e.g. 6.0, 8.0, 10.0 mm) or a clean fraction (e.g. 1/4", 3/8", 1/2"). If neither fits, it may be a Whitworth (BSW/BSF) or BA thread. BSP and NPT pipe threads add another layer of complexity — they are nominally sized by bore diameter, not the thread OD, which means a ½" BSP fitting has a thread major diameter of approximately 20.9 mm. ThreadChaser handles all of these automatically.

Identify Internal Threads (Nuts and Tapped Holes)

Switch to Internal mode to identify threads in nuts, tapped holes, and threaded bores. Measure the minor (inner) diameter of the hole and the pitch, then enter them into the tool. ThreadChaser matches against all internal thread standards and also displays the tapping drill size — the drill bit needed to create the pilot hole before tapping.

Frequently Asked Questions

Measure the major diameter of the bolt (or minor diameter of the hole) with calipers, then measure the pitch — either with a pitch gauge or by counting crests per inch (TPI). Enter both values into ThreadChaser and it will instantly match your dimensions against all known thread standards.
Place the bolt against a ruler and count the number of thread crests over exactly 1 inch — this gives you TPI. For metric threads, count crests over 10 mm and multiply by 10 to get pitch in mm. Alternatively, press the thread into a soft material and measure the impression.
Metric threads (M series) are measured in millimetres — pitch is the distance between crests in mm (e.g. M10×1.5 = 10 mm diameter, 1.5 mm pitch). Imperial threads (UNC, UNF, BSW) are measured in inches with pitch expressed as TPI (e.g. 3/8-16 UNC = 3/8 inch diameter, 16 threads per inch). They are not interchangeable.
No — BSP and NPT are not interchangeable. BSP uses a 55° thread angle (common in the UK, Europe, and Australia). NPT uses a 60° thread angle (North American standard). Even where diameters appear similar, mixing them will not produce a reliable seal.
UNC and UNF have the same 60° thread angle but different pitches. At any given diameter, UNF has more threads per inch than UNC. For example: ½ inch UNC is 13 TPI, while ½ inch UNF is 20 TPI. Measure the TPI carefully — ThreadChaser will distinguish them instantly.
The tapping drill size is the drill bit diameter needed to create a pilot hole before cutting an internal thread with a tap. It is slightly smaller than the thread's major diameter, leaving material for the tap to cut. ThreadChaser displays the recommended tapping drill size for every matched thread standard automatically.
Yes. Using digital calipers you can measure the major diameter and estimate the pitch by counting crests along a known length. Enter these into ThreadChaser and it matches against over 300 thread sizes across 16 international standards — no physical gauge required.

Reference aid only. Always verify critical dimensions directly against the relevant published standard, and use calibrated measurement instruments. Do not solely rely on this website where incorrect thread identification could cause injury, death, or property damage.