Identify a thread by diameter and pitch
Identify an unknown thread from its measured diameter and pitch or TPI. Compare metric, UNC, UNF, BSP and NPT catalog matches.
Your measurements
Measure the pitch and diameter with a caliper, thread pitch gauge or thread counter. The tool will compare against the selected catalog.
Result · Catalog matches
Several threads match the search 100%. Confirm the profile and standard before choosing.
ISO 60° profile: confirm the standard and root on the drawing.
Diámetro mayor: 0 mm de diferencia · Paso: 0 mm de diferencia · Series: Coarse ISO Metric
Aerospace metric variant (MJ); confirm on the drawing.
Diámetro mayor: 0 mm de diferencia · Paso: 0 mm de diferencia · Series: Nominal Aerospace Metric MJ
Profile 47.5° · British Association BA.
Diámetro mayor: 0 mm de diferencia · Paso: 0 mm de diferencia · Series: British Association BA
Other matches (37)
Las medidas están muy cerca de esta designación.
Diámetro mayor: 0.35 mm de diferencia · Paso: 0.023 mm de diferencia · Series: Whitworth BSF 55°
Las medidas están muy cerca de esta designación.
Diámetro mayor: 0.35 mm de diferencia · Paso: 0.023 mm de diferencia · Series: BSC/CEI cycle and motorcycles
Hay coincidencia parcial; confirma con galga o plano.
Diámetro mayor: 0.35 mm de diferencia · Paso: 0.093 mm de diferencia · Series: Unified UNF
Hay coincidencia parcial; confirma con galga o plano.
Diámetro mayor: 0.35 mm de diferencia · Paso: 0.093 mm de diferencia · Series: UNJF aeroespacial
La coincidencia es débil; revisa medición y catálogo.
Diámetro mayor: 0.514 mm de diferencia · Paso: 0.058 mm de diferencia · Series: Unified UNC
La coincidencia es débil; revisa medición y catálogo.
Diámetro mayor: 0.514 mm de diferencia · Paso: 0.058 mm de diferencia · Series: UNJC aeroespacial
La coincidencia es débil; revisa medición y catálogo.
Diámetro mayor: 1 mm de diferencia · Paso: 0 mm de diferencia · Series: Coarse ISO Metric
La coincidencia es débil; revisa medición y catálogo.
Diámetro mayor: 1 mm de diferencia · Paso: 0 mm de diferencia · Series: Nominal Aerospace Metric MJ
La coincidencia es débil; revisa medición y catálogo.
Diámetro mayor: 0.514 mm de diferencia · Paso: 0.093 mm de diferencia · Series: Unified UNF
La coincidencia es débil; revisa medición y catálogo.
Diámetro mayor: 0.514 mm de diferencia · Paso: 0.093 mm de diferencia · Series: UNJF aeroespacial
Workshop tips and common problems
Quick Tips and Common Mistakesal identificar una rosca medida.
How to do it right
Measure the pitch: thread gauge or thread count
A thread gauge gives the pitch instantly by profile comparison. Without one, count the threads over a known length and divide: pitch = distance / number of threads. If you count over 1 inch (25.4 mm), the thread count is directly the TPI (threads per inch) of inch threads.
Profile angle: the key to not confusing families
A profile gauge separates families that share diameter and pitch: ISO metric and UN/UNC/UNF are 60°, Whitworth (BSW/BSF) and BSP are 55°, ACME is 29° and metric trapezoidal (Tr) is 30°. That is what settles an M against a Whitworth when the caliper is not enough.
Major or minor diameter depending on external or internal thread
On a screw or stud the caliper measures the major diameter over the crests. In a nut or tapped hole the major diameter is not accessible: you only measure the hole diameter, which is close to the minor diameter. Tick "internal" so the tool compares against the approximate minor diameter.
Metric vs. UNC/UNF: same profile, different step
Both share the 60° triangular profile, so the difference is the pitch: metric is given in mm/turn (M8 × 1.25) and UNC/UNF in threads per inch (3/8-16 UNC). If the TPI comes out a round number (16, 20, 24), it is almost certainly an inch thread.
Step or advance? Be careful with multi-entry threads
If you follow a single thread around the part you are measuring the lead, not the pitch. On an N-start thread, pitch = lead ÷ N. When the measured pitch is an exact multiple of a catalogue value, suspect a multi-start thread before discarding the match.
Tapered pipe threads (NPT/BSPT) - diameter varies
On NPT, BSPT and NPTF the diameter changes along the taper (≈1:16), so a single caliper reading can point to a neighbouring size depending on where you measure. Always confirm with a pipe gauge and the reference drawing; do not confuse them with the parallel BSP/NPS even when the pitch matches.
If something goes wrong · symptom → cause → solution
⚠ No clear match
Causes: you measured over a burr, dirt or a damaged thread; the catalogue you picked does not match the standard; or you mixed up internal and external threads. Fix: clean and deburr, repeat the measurement on several threads, widen the catalogue to "All" and confirm whether it is male or female.
⚠ Varias roscas encajan igual de bien
Metric and UN share the 60° profile and sometimes have nearly identical diameter and pitch (typically 1/4-20 UNC vs BSW). Fix: measure the profile angle with a gauge (60° vs 55°) or, if you know where the part came from, use the application shortcuts to break the tie.
⚠ The TPI does not give a whole number
If counting threads per inch gives an odd value (e.g. 25.4 threads), it is probably a metric thread measured in inches. Switch the pitch unit to mm: it is more likely to land on a standard metric pitch (0.5 · 0.7 · 0.8 · 1 · 1.25 · 1.5…).
Other threading tools
Tech Guide: How to Identify an Unknown Thread
Reference article for this section. Switch goal above (drill, identify, CNC, method, repair or base plate) to see the guide for that tool.
Identifying a thread with no drawing or marking comes down to two measurements — pitch and diameter — but each hides subtleties: which instrument to use, which diameter is actually measurable depending on male or female, and how not to confuse families with similar profiles.
M8 × 1 and M8 × 1.25 share a nominal diameter but have different pitches. Measure both and confirm the profile with a gauge if several families match.
What instrument to use to measure pitch and diameter
A thread gauge gives the pitch by direct profile comparison and is the fastest method on the shop floor. Without one, count the threads over 25 mm (or 1 inch) with a caliper and divide: pitch = distance / number of threads. Measuring a single thread with a caliper is the least accurate method, because positioning error weighs heavily over such a short distance.
For the diameter, an outside caliper measures the major diameter of a screw or stud directly. In a nut or tapped hole you cannot measure the major diameter with a standard caliper: only the hole diameter is accessible, which approximates the thread's minor diameter.
Major diameter vs. minor diameter: what you can measure on an external or internal thread
On an external thread (screw, stud, threaded shaft) the caliper measures the major diameter over the crests without trouble. On an internal thread (nut, tapped hole) the major diameter is not accessible: what you measure is the hole diameter, close to the theoretical minor diameter, which is derived from the nominal major diameter and the profile angle.
Our thread identifier already handles this: when you set the measured thread type to "internal", it compares your measurement against the approximate minor diameter instead of the catalogue's major diameter.
Most common thread families and how not to confuse them
ISO metric and UNC/UNF share the same 60° triangular profile, so they are told apart by the pitch: metric is given in mm/turn (M8 x 1.25) and UNC/UNF in threads per inch or TPI (3/8-16 UNC). BSP and NPT use a 55°/60° profile specific to pipe; BSP and NPS are parallel, while NPT and BSPT are tapered and must not be confused with the parallel ones even when the pitch matches.
Trapezoidal (30°) and ACME (29°) threads have flat crests and bottoms, very different from the triangular pointed profile; They are used in spindles and transmission mechanisms, not in fixing hardware.
When to distrust a coincidence
A "low" confidence in the result usually means the measured pitch or diameter does not fit any catalogue thread well: review the measurement before accepting the highest-scoring match.
If the best candidate is a tapered thread (NPT, BSPT), remember the major diameter varies with the exact point you measure along the taper, so a single caliper reading can point to a neighbouring size by mistake.
Frequently asked questions: Tech Guide: How to Identify an Unknown Thread
How do I tell an M8 x 1 from an M8 x 1.25?
Both have the same major diameter (8 mm) but a different pitch: M8 x 1 is the fine variant and M8 x 1.25 the coarse/standard one. Count the threads over 25 mm or use a thread gauge to tell them apart reliably; by eye they are almost indistinguishable.
How do I know if a thread is UNC/UNF or metric?
The triangular profile is the same (60°), so the key is the pitch: metric is measured in millimetres per turn and UNC/UNF in threads per inch (TPI). If your instrument gives an exact thread count per inch (for example 16, 20, 24), it is almost certainly an inch thread.
Can I identify an internal thread (nut or threaded hole) with a normal gauge?
You can measure the pitch just as on an external thread, but the major diameter is not accessible with a caliper: you can only measure the hole diameter, which approximates the minor diameter. Our tool takes this difference into account when comparing against the catalogue.
What is the gauge plane on a tapered thread like NPT?
It is the axial reference point where the nominal diameter of a tapered thread is defined. Because the diameter changes continuously along the taper, all tolerances and the tightening criterion (turns by hand and by wrench) are referenced to that plane, not to an arbitrary point on the thread.
Can I calculate NPT or BSP the same as a metric thread?
Not for the tapered variants (NPT, BSPT, NPTF): they depend on the gauge plane, not on a direct diameter formula. BSP and NPS, being parallel, do allow an approximation similar to metric, always validated with a pipe gauge.
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