SizingKitThread identification
Thread Pitch Calculator
Measure across the crests with a caliper and read the flanks with a pitch gauge, and this gives back the designation — 47 Unified UNC and UNF sizes and 56 ISO metric ones, searched together on every measurement, with pitch, threads per inch, pitch diameter, both minor diameters, thread height and tensile stress area printed in inches and millimeters at once. It never asks which system the fastener is, because 21 imperial and metric pairs between 4 mm and 20 mm sit inside 0.10 mm of pitch of each other and no gauge separates them. Free, no account, and nothing you measure leaves the tab.
- 100% free
- No signup
- 103 designations
- Both series searched
- 21 cross-thread pairs
What thread is this?
A caliper across the crests and a pitch gauge on the flanks are the whole measurement. Both series are searched every time — you are not asked which system it is, because that is the thing you came here to find out.
A unit you type wins over the one selected. Expect the reading to fall under the nominal size — that is the class allowance, not a worn bolt.
23 thread counts and 21 metric pitches exist across the two series, and a reading is snapped to the nearest of them before the search.
Default 0.30 mm (0.0118 in). This is a search band, not a tolerance: the class limits that put a real thread under its basic size are in ASME B1.1 and ISO 965, this site does not carry them, and the band is set wider than any of them so a plated or worn fastener still matches.
2 candidates — pick one
Designation
5/16-24 UNF
Unified inch · UNF — fine
Under its basic major diameter
0.025 mm
0.0010 in between what you measured and the basic profile
| Basic profile | Inch | Metric |
|---|---|---|
| Major (nominal) diameter | 0.3125 in | 7.938 mm |
| Pitch | 0.04167 in — 24 TPI | 1.0583 mm |
| Pitch (effective) diameter | 0.2854 in | 7.250 mm |
| Minor diameter, internal thread | 0.2674 in | 6.792 mm |
| Minor diameter, external thread | 0.2614 in | 6.639 mm |
| Thread height, external | 0.0256 in | 0.649 mm |
| Tensile stress area | 0.05807 in² | 37.46 mm² |
Helix angle at the pitch diameter is 2.66° on a single start, measured on the 60° included flank both series share. Across all 103 designations it only ever runs between 1.05° and 5.43°, which is why the geometry alone never decides whether a joint stays tight — what governs the preload is the nut factor, on the bolt torque calculator.
Measured 1.0000 mm of pitch — that is 25.40 threads per inch. Searched as 24 TPI on the Unified side (1.0583 mm) and 1 mm on the ISO side, the nearest leaf each series actually has.
Every diameter above is the basic, zero-allowance profile computed from the 60° form with H = 0.8660254 P — nothing here is transcribed. ASME B1.1, Unified Inch Screw Threads (UN and UNR thread form) — the coarse (UNC) and fine (UNF) series and their threads per inch. Basic major diameter of a numbered size is 0.060 + 0.013 n in, by definition. All other diameters are computed from the basic 60° profile: pitch diameter D − 0.64952 P, internal minor D − 1.08253 P, external minor D − 1.22687 P. Tensile stress area is 0.7854 (D − 0.9743/n)², ASME B1.1 Appendix B. The hole this thread is tapped into, and the drill that makes it at any engagement you choose, are on the drill size chart, which is not repeated here.
The 21 pairs a pitch gauge will not separate
Every imperial and metric thread between 4 mm and 20 mm whose pitches differ by less than 0.10 mm and whose diameters differ by less than 1.05 mm. The pitch is what most people check first, and on these rows it is the measurement that cannot decide.
| Unified | Major, mm | Pitch, mm | ISO metric | Major, mm | Pitch, mm | Δ diameter | Δ pitch |
|---|---|---|---|---|---|---|---|
| #8-36 UNF | 4.166 | 0.7056 | M4×0.7 | 4.000 | 0.7000 | 0.166 | 0.006 |
| #8-32 UNC | 4.166 | 0.7937 | M4×0.7 | 4.000 | 0.7000 | 0.166 | 0.094 |
| #8-36 UNF | 4.166 | 0.7056 | M5×0.8 | 5.000 | 0.8000 | -0.834 | -0.094 |
| #8-32 UNC | 4.166 | 0.7937 | M5×0.8 | 5.000 | 0.8000 | -0.834 | -0.006 |
| #10-32 UNF | 4.826 | 0.7937 | M5×0.8 | 5.000 | 0.8000 | -0.174 | -0.006 |
| #10-32 UNF | 4.826 | 0.7937 | M4×0.7 | 4.000 | 0.7000 | 0.826 | 0.094 |
| #12-28 UNF | 5.486 | 0.9071 | M6×1 | 6.000 | 1.0000 | -0.514 | -0.093 |
| #12-24 UNC | 5.486 | 1.0583 | M6×1 | 6.000 | 1.0000 | -0.514 | 0.058 |
| 1/4-28 UNF | 6.350 | 0.9071 | M6×1 | 6.000 | 1.0000 | 0.350 | -0.093 |
| 5/16-24 UNF | 7.938 | 1.0583 | M8×1 | 8.000 | 1.0000 | -0.063 | 0.058 |
| 3/8-24 UNF | 9.525 | 1.0583 | M10×1 | 10.000 | 1.0000 | -0.475 | 0.058 |
| 3/8-16 UNC | 9.525 | 1.5875 | M10×1.5 | 10.000 | 1.5000 | -0.475 | 0.087 |
| 7/16-20 UNF | 11.112 | 1.2700 | M12×1.25 | 12.000 | 1.2500 | -0.888 | 0.020 |
| 7/16-14 UNC | 11.112 | 1.8143 | M12×1.75 | 12.000 | 1.7500 | -0.888 | 0.064 |
| 1/2-20 UNF | 12.700 | 1.2700 | M12×1.25 | 12.000 | 1.2500 | 0.700 | 0.020 |
| 9/16-18 UNF | 14.287 | 1.4111 | M14×1.5 | 14.000 | 1.5000 | 0.287 | -0.089 |
| 5/8-18 UNF | 15.875 | 1.4111 | M16×1.5 | 16.000 | 1.5000 | -0.125 | -0.089 |
| 3/4-16 UNF | 19.050 | 1.5875 | M20×1.5 | 20.000 | 1.5000 | -0.950 | 0.087 |
| 3/4-10 UNC | 19.050 | 2.5400 | M20×2.5 | 20.000 | 2.5000 | -0.950 | 0.040 |
| 3/4-16 UNF | 19.050 | 1.5875 | M18×1.5 | 18.000 | 1.5000 | 1.050 | 0.087 |
| 3/4-10 UNC | 19.050 | 2.5400 | M18×2.5 | 18.000 | 2.5000 | 1.050 | 0.040 |
A negative Δ means the metric thread is the larger of the two. Generated by comparing every row of 47 Unified designations against every row of 56 ISO metric ones, so the count changes only if the standards do.
How to identify a thread you are holding
Two measurements, in this order, and a check before you cut anything.
Caliper the crests
Close the jaws across the outside diameter of the male thread, or use the hole diameter as a lower bound on a female one, and enter it in whichever unit the caliper is showing. The reading will come out under the nominal size — a fastener is cut below basic by its class allowance before it is ever plated or used — so the match is made inside a window rather than on equality.
Get the pitch off a gauge, or off a rule
A pitch gauge leaf gives a thread count or a millimeter pitch directly. Without one, lay a rule along the thread, measure across a run of crests and enter the distance with the number of pitches inside it — count the gaps, not the crests. The reading is then snapped to the nearest leaf either series actually has, and the page prints how far it had to move.
Read both columns before you commit
Candidates come back from the Unified and the ISO series side by side, each with the gap between what you measured and its basic diameter. If two come back, that is the answer rather than a failure — check which one your gauge and your caliper agree on, and look the pair up in the cross-thread table before you start a nut on it.
What the identifier searches
| Unified designations | 47 — UNC and UNF from #0-80 to 1-1/2-6, ASME B1.1. The numbered sizes come from D = 0.060 + 0.013 n by definition, so #10 is 0.1900 in exactly |
|---|---|
| ISO metric designations | 56 — coarse and fine, M1×0.25 to M48×5, pitches from ISO 261 and diameters from ISO 724 |
| Gauge leaves recognized | 23 thread counts (80 down to 6 TPI) and 21 metric pitches (0.25 mm to 5.00 mm). A measurement between two of them is snapped to the nearest and the move is printed |
| Default match window | 0.30 mm, or 0.0118 in, on the major diameter — set wider than any class allowance so a plated or worn fastener still matches, and editable |
| Helix angle across both series | 1.05° at 1-1/2-12 UNF to 5.43° at M1×0.25, measured at the pitch diameter on a single start |
| Closest pair in the two standards | 5/16-24 UNF and M8×1.0 — 0.063 mm apart on diameter, which is 0.0025 in, and 0.058 mm apart on pitch |
| How the diameters are arrived at | Computed, not transcribed: pitch diameter is D − 0.64952 P, internal minor D − 1.08253 P, external minor D − 1.22687 P, all from the 60° basic profile with H = 0.86603 P |
| Where the measurement goes | Nowhere. The tables are in the page, the arithmetic runs in the tab, and your readings are not uploaded or stored |
Frequently asked questions
How do I tell a metric bolt from an imperial one?
Measure both the diameter and the pitch, and treat the diameter as the deciding one — the pitch is what most people check and on the dangerous pairs it is the measurement that cannot decide. 5/16-24 UNF and M8×1.0 are 0.063 mm apart on diameter and 0.058 mm on pitch; #10-32 UNF and M5×0.8 differ by six microns of pitch, which is a hundredth of a gauge leaf, and only the 0.174 mm of diameter tells them apart.
What is the difference between pitch and TPI?
Pitch is the distance from one crest to the next and TPI is how many crests fit in an inch, so they are reciprocals: pitch in millimeters is 25.4 divided by the thread count. 20 TPI is a 1.270 mm pitch, and M8×1.25 is 20.32 TPI — which is exactly why an inch gauge laid on a metric bolt reads almost 20 and encourages the wrong conclusion.
Can I measure the pitch with a rule instead of a thread gauge?
Yes, over a run of threads rather than one. Lay the rule along the crests, span ten pitches and divide, and a half-millimeter reading error becomes 0.05 mm of pitch instead of 0.5 mm. The trap is the count: from the first crest to the eleventh is ten pitches, not eleven, and getting that wrong on a 1.25 mm thread lands you on 1.14 mm, which is not a pitch either series has.
Why does my caliper read less than the nominal diameter?
Because a real thread is cut below the basic profile on purpose. ASME B1.1 gives an external Class 2A thread an allowance and a tolerance below basic, and ISO 965 does the same for 6g, so a new 5/16 in bolt measures under 0.3125 in before plating, coating or wear touch it. This site does not carry those class limit tables, which is why identification is done inside a window that is deliberately wider than any of them rather than by looking for an exact size.
Will an M6 nut start on a 1/4-20 bolt?
No — 1/4 in is 6.350 mm against M6's 6.000 mm, and 20 TPI is a 1.270 mm pitch against 1.000 mm, so the nut will not go on at all. That pair gets named constantly and it is the easy one. The genuinely expensive case is 5/16-18 UNC and M8×1.25: 0.063 mm apart on diameter, which means the nut starts, feels right for a turn and a half and then strips the first threads of whichever part is softer.
What is the pitch diameter and why does a thread micrometer read it?
The pitch diameter is where the width of the thread equals the width of the groove — D minus 0.64952 P on the basic profile — and it is the diameter that actually carries the load, which is why a thread gauge and a thread micrometer are calibrated to it rather than to the crest. For 1/4-20 it is 0.2175 in; for M6×1.0 it is 5.350 mm. A thread can be within tolerance on its major diameter and still be scrap on its pitch diameter.
Which pitch do I use for the tensile stress area?
The pitch of the actual thread, and the formula differs between the two series so the answer is not interchangeable. Unified uses As = 0.7854 × (D − 0.9743/n)², which embeds the Class 2A minimum pitch diameter; ISO uses As = 0.7854 × (D − 0.9382 P)², which is pure basic geometry. Fine threads give roughly 10% more area than coarse at the same nominal size, and that is the whole engineering argument for using them.
About thread designations and the pairs that cross-thread
A designation carries only two pieces of information — a nominal diameter and a pitch — and everything else about the thread follows from the 60° symmetric V that ASME B1.1 and ISO 68-1 both specify. The fundamental triangle has height H = 0.86603 P, and the basic profile is that triangle truncated in two places: the pitch line sits three eighths of H below the sharp crest, a nut’s minor diameter five eighths of H below it, and a screw’s root seventeen twenty-fourths of H below. That is why the pitch, minor and effective diameters on this page are computed rather than looked up, and why 1/4-20 comes out at a 0.2175 in pitch diameter and M6×1.0 at 5.350 mm without either number being typed anywhere. What the basic profile does not include is the allowance that puts a real fastener under it, which lives in the class tables of B1.1 and ISO 965; the limits and fits arithmetic of that kind is on the tolerance calculator.
The reason this page searches both standards on every measurement rather than asking you to pick one is that the two overlap far more closely than their reputation suggests. Between 4 mm and 20 mm there are 21 imperial and metric pairs whose pitches differ by less than a tenth of a millimeter, which is under two leaves of a gauge and invisible across the three or four crests anybody actually counts. #10-32 UNF and M5×0.8 differ by 0.006 mm of pitch. 5/16-24 UNF and M8×1.0 differ by 0.063 mm of diameter and 0.058 mm of pitch, and are the closest pair the two standards contain. 3/4-10 UNC and M20×2.5 agree on pitch to 0.04 mm across a whole inch of diameter. A tool that makes you choose a system first can only confirm the guess you arrived with; it cannot tell you that the other system has a candidate 0.06 mm away.
Two more things are worth knowing before a nut goes on. The lead angle of every thread in both series falls between 1.05° and 5.43°, so the geometry is broadly similar everywhere and it is never what distinguishes one fastener from another — the difference that matters in service is preload, and preload comes from torque through a nut factor that varies with the coating. And the hole is a separate question from the thread: this page deliberately does not reproduce a tap drill table, because the drill depends on the engagement percentage you want rather than on the designation alone, and that argument with all 103 rows behind it is on the drill size chart. Nothing here is a stamped design — it is a measurement read against two published standards, and what the joint has to hold is still for you and a qualified engineer to settle.
Where your caliper reading goes
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The Unified and ISO tables are compiled into the page itself, so the search runs against data already in your browser. No measurement is sent anywhere, and the identifier works with the phone in airplane mode, which is where most of these measurements get taken.