THREAD REFERENCE TOOL
Thread Pitch Chart
Before you can tap a hole or specify a thread on a drawing, you need to know what thread you’re working with. This thread size and pitch chart gives you pitch and TPI for every common thread across seven standards — search by callout, identify an unknown fastener, or browse the full reference tables below.
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7 thread standards — UNC, UNF, UNEF, Metric, NPT, BSP
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Search by callout or measure OD + pitch to identify unknown threads
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Built‑in TPI ↔ Pitch converter — instant, two‑way
TPI ↔ Pitch Converter
Convert between threads per inch and metric pitch instantly
TPI
Threads per Inch
Pitch
mm per Thread
TPI = 25.4 ÷ Pitch(mm)
Conversion Formula
TPI = 25.4 ÷ Pitch Pitch = 25.4 ÷ TPI | Enter either value
THREAD REFERENCE GUIDE
How to Use This Thread Pitch Chart
Search by callout, identify an unknown fastener, compare standards, and verify against reference tables — all in four simple steps.
1. Know your thread callout? Search for it directly. Type “M10 × 1.5” or “1/4-20” to see pitch, TPI, major diameter, and thread class in a single view.
2. Have an unknown fastener? Switch to thread identification mode. Measure the outside diameter with calipers — record it in both mm and inches. Measure the pitch with a thread pitch gauge — try metric leaves first, then imperial. Enter both measurements and the tool suggests matching thread standards.
Compare Standards
3. Need to compare standards? Use the comparison view. Select two thread standards side‑by‑side — for example, compare a 1/4‑28 UNF with a 1/4‑20 UNC to see exactly how TPI changes within the same major diameter.
Verify with Reference Tables
4. Confirm your thread. Once the tool narrows down your thread, verify against the complete tables below. Every value is sourced from ASME B1.1, ISO 261/724, ASME B1.20.1, and ISO 228 — authoritative standards you can trust.
All Major Thread Standards
This chart covers UNC, UNF, UNEF, Metric Coarse/Fine, NPT, and BSP — everything from standard fasteners to pipe threads. Whether you’re machining, repairing, or designing, you’ll find the exact pitch and TPI data you need.
THREAD REFERENCE DATA
Thread Pitch Reference Tables
Complete thread pitch and TPI data across UNC, UNF, UNEF, Metric coarse/fine, NPT, and BSP standards. Use this reference to identify, specify, or verify any common thread.
TPI ↔ Pitch: TPI = 25.4 ÷ Pitch(mm) | Pitch(mm) = 25.4 ÷ TPI. All values are sourced from ASME B1.1, ISO 261/724, ASME B1.20.1, and ISO 228. Use these tables to confirm thread callouts before drilling, tapping, or ordering fasteners.
UNC / UNF / UNEF TPI Chart
| Thread Size | Major Dia. (in) | UNC (TPI) | UNF (TPI) | UNEF (TPI) | Metric Pitch Eq. (mm) |
|---|---|---|---|---|---|
| #0 | 0.0600 | — | 80 | — | 0.318 |
| #1 | 0.0730 | 64 | 72 | — | 0.353 |
| #2 | 0.0860 | 56 | 64 | — | 0.397 |
| #3 | 0.0990 | 48 | 56 | — | 0.454 |
| #4 | 0.1120 | 40 | 48 | — | 0.529 |
| #5 | 0.1250 | 40 | 44 | — | 0.577 |
| #6 | 0.1380 | 32 | 40 | — | 0.635 |
| #8 | 0.1640 | 32 | 36 | — | 0.706 |
| #10 | 0.1900 | 24 | 32 | — | 0.794 |
| #12 | 0.2160 | 24 | 28 | 32 | 0.907 |
| 1/4″ | 0.2500 | 20 | 28 | 32 | 1.270 |
| 5/16″ | 0.3125 | 18 | 24 | 32 | 1.411 |
| 3/8″ | 0.3750 | 16 | 24 | 32 | 1.588 |
| 7/16″ | 0.4375 | 14 | 20 | 28 | 1.814 |
| 1/2″ | 0.5000 | 13 | 20 | 28 | 1.954 |
| 9/16″ | 0.5625 | 12 | 18 | 24 | 2.117 |
| 5/8″ | 0.6250 | 11 | 18 | 24 | 2.309 |
| 3/4″ | 0.7500 | 10 | 16 | 20 | 2.540 |
| 7/8″ | 0.8750 | 9 | 14 | 20 | 2.822 |
| 1″ | 1.0000 | 8 | 12 | 20 | 3.175 |
Metric Thread Pitch Chart (Coarse & Fine)
| Thread Size | Major Dia. (mm) | Coarse Pitch (mm) | Fine Pitch Options (mm) | TPI Eq. (Coarse) |
|---|---|---|---|---|
| M1.6 | 1.6 | 0.35 | — | 72.6 |
| M2 | 2.0 | 0.40 | — | 63.5 |
| M2.5 | 2.5 | 0.45 | — | 56.4 |
| M3 | 3.0 | 0.50 | — | 50.8 |
| M3.5 | 3.5 | 0.60 | — | 42.3 |
| M4 | 4.0 | 0.70 | — | 36.3 |
| M5 | 5.0 | 0.80 | — | 31.8 |
| M6 | 6.0 | 1.00 | 0.75 | 25.4 |
| M8 | 8.0 | 1.25 | 1.00 | 20.3 |
| M10 | 10.0 | 1.50 | 1.25, 1.00 | 16.9 |
| M12 | 12.0 | 1.75 | 1.50, 1.25 | 14.5 |
| M14 | 14.0 | 2.00 | 1.50 | 12.7 |
| M16 | 16.0 | 2.00 | 1.50 | 12.7 |
| M18 | 18.0 | 2.50 | 1.50 | 10.2 |
| M20 | 20.0 | 2.50 | 1.50 | 10.2 |
| M22 | 22.0 | 2.50 | 1.50 | 10.2 |
| M24 | 24.0 | 3.00 | 2.00 | 8.5 |
| M27 | 27.0 | 3.00 | 2.00 | 8.5 |
| M30 | 30.0 | 3.50 | 2.00 | 7.3 |
| M33 | 33.0 | 3.50 | 2.00 | 7.3 |
| M36 | 36.0 | 4.00 | 3.00 | 6.4 |
| M42 | 42.0 | 4.50 | 3.00 | 5.6 |
| M48 | 48.0 | 5.00 | 3.00 | 5.1 |
NPT & BSP Pipe Thread Pitch Chart
| Pipe Size | NPT (TPI) | BSP (TPI) | BSP Pitch (mm) | Notes |
|---|---|---|---|---|
| 1/8″ | 27 | 28 | 0.907 | Not interchangeable — TPI differs |
| 1/4″ | 18 | 19 | 1.337 | Common instrument fitting size |
| 3/8″ | 18 | 19 | 1.337 | 60° vs 55° thread angle — do not mix |
| 1/2″ | 14 | 14 | 1.814 | Same TPI, different thread form |
| 3/4″ | 14 | 14 | 1.814 | Same TPI, not interchangeable |
| 1″ | 11.5 | 11 | 2.309 | TPI diverges — not interchangeable |
| 1-1/4″ | 11.5 | 11 | 2.309 | — |
| 1-1/2″ | 11.5 | 11 | 2.309 | — |
| 2″ | 11.5 | 11 | 2.309 | — |
ASME B1.1 & ISO 261/724
Unified inch threads follow ASME B1.1. Metric threads follow ISO 261 (coarse) and ISO 724 (fine). UNC is the default general-purpose standard; UNF provides higher tensile strength and vibration resistance.
NPT & BSP Pipe Threads
NPT per ASME B1.20.1 is tapered (1° 47′). BSPT (tapered) per ISO 7 and BSPP (parallel) per ISO 228 are UK/Commonwealth standards. NPT and BSP threads are not interchangeable despite similar TPI at some sizes.
THREAD PITCH GUIDE
Thread Pitch vs. TPI — What’s the Difference?
Thread pitch and TPI describe the same physical dimension — the spacing between adjacent thread peaks — using two different measurement systems. Mixing them up during design or inspection leads to parts that don’t fit.
01
Metric Thread Pitch
Measures the distance from one thread peak to the next in millimeters. A smaller pitch number means a finer thread. M10 × 1.5 has a 1.5 mm gap; M10 × 1.0 (fine) has a 1.0 mm gap — threads are closer together.
02
TPI — Threads per Inch
Counts how many complete thread crests fit within one inch of length. A higher TPI means a finer thread — the opposite of metric. 1/4‑20 UNC has 20 TPI; 1/4‑28 UNF has 28 TPI — more threads per inch, so finer.
03
The Inverse Relationship
In metric, the number decreases as threads get finer. In imperial, the number increases as threads get finer. They’re inverse. Formula: TPI = 25.4 ÷ Pitch(mm). Pitch(mm) = 25.4 ÷ TPI.
Conversion Examples
Apply the formula to common thread callouts. These calculations help you convert between systems quickly.
TPI = 25.4 ÷ Pitch
Use for metric → imperial
M10 × 1.5
TPI = 25.4 ÷ 1.5 = 16.93 TPI
1/4‑20 UNC
Pitch = 25.4 ÷ 20 = 1.27 mm
QUICK IDENTIFICATION
Metric vs. Inch — How to Tell at a Glance
If the major diameter is a clean millimeter number (6, 8, 10, 12 mm), it’s metric. If it’s a fractional inch (1/4, 3/8, 1/2), it’s imperial. When in doubt, measure with calipers and check both tables.
| Metric Thread | Major Dia. | Closest Inch Thread | Major Dia. | Pitch Difference |
|---|---|---|---|---|
| M10 × 1.5 | 10.0 mm | 3/8‑16 UNC | 9.525 mm | 0.475 mm |
| M8 × 1.25 | 8.0 mm | 5/16‑18 UNC | 7.938 mm | 0.062 mm |
| M12 × 1.75 | 12.0 mm | 1/2‑13 UNC | 12.7 mm | 0.7 mm |
These near‑matches are the most common cause of cross‑threaded assemblies in shops that handle both metric and inch fasteners. Always verify the major diameter and pitch before mating parts.
Coarse vs. Fine Threads — When to Use Which
Every thread standard offers coarse and fine variants. The choice affects assembly speed, joint strength, vibration resistance, and whether the part can even be threaded in thin-walled sections. This thread pitch chart shows both — here’s when to use each.
| Thread Type | Pitch Characteristic | Best For | Key Advantage |
|---|---|---|---|
| Coarse Threads UNC / Metric Coarse |
Fewer TPI (imperial) or larger pitch (metric) — deeper, wider thread grooves | General fastening, construction, soft materials (aluminum, cast iron, plastics), debris-prone environments | Faster assembly, tolerates dirt and minor damage, better hold in soft materials |
| Fine Threads UNF / UNEF / Metric Fine |
More TPI (imperial) or smaller pitch (metric) — shallower, narrower thread grooves | Vibration-critical applications (automotive, aerospace), high tensile strength, thin-wall parts, precision adjustments | Vibration resistance, higher tensile strength, finer control, viable in limited wall thickness |
Application examples: Construction bolts → coarse (fast assembly, debris-tolerant). Automotive cylinder head bolts → fine (vibration resistance, higher clamp load). Aerospace fasteners → UNF/UNEF (maximum vibration resistance, thin-wall compatibility). Plastic enclosures → coarse (better engagement in soft material). Optical instrument adjustments → fine (precise linear control). Hydraulic fittings in thin-wall stainless → metric fine / UNF (limited wall thickness, fluid pulsation).
Rule of thumb: UNC and metric coarse are the defaults — use fine only when vibration, tensile strength, or wall thickness demand it. When in doubt, coarse is usually the safer, more forgiving choice.
THREAD ENGINEERING
Multi-Start Threads — Lead vs. Pitch
For most fasteners, one revolution advances the fastener by exactly one pitch — that’s a single-start thread. But when you need fast linear advance per revolution — in lead screws, ball screws, quick-acting closures, and plastic bottle caps — multi-start threads do the job.
In a multi-start thread, the pitch (distance between adjacent crests of the same groove) stays the same. What changes is the lead — the distance the nut actually travels in one revolution. The formula: Lead = Pitch × Number of Starts.
Single-Start
Lead = Pitch × 1
One continuous thread groove spiraling around the shaft. One revolution advances the nut by one pitch distance. This is the standard for all fasteners — bolts, screws, studs. Every thread in the reference tables above is single-start.
Double-Start
Lead = Pitch × 2
Two parallel thread grooves, offset by 180°, running side by side. One revolution advances the nut by two pitch distances. The thread appears to have twice as many starts when viewed from the end.
Triple-Start
Lead = Pitch × 3
Three parallel grooves at 120° spacing. Used where rapid axial movement is needed — high-helix lead screws, quick-release mechanisms, and some medical device adjustments.
Specifying Multi-Start Threads
A “2-start M20 × 2.5” has a 2.5 mm pitch but a 5.0 mm lead — the nut moves 5 mm per turn, not 2.5 mm. Multi-start threads require precise indexing of multiple entry points; a few degrees of angular error and the thread won’t engage properly. Our 5-axis CNC machining centers produce single and multi-start threads with verified lead accuracy.
Lead = Pitch × Starts
Core Formula
5-Axis CNC
Precision Indexing
Verified Lead
Quality Inspection
THREAD PITCH FAQ
Frequently Asked Questions
Thread pitch is the distance from one thread peak to the next, measured in millimeters (e.g., M10 × 1.5 has a 1.5 mm pitch). TPI (threads per inch) counts how many crests fit within one inch (e.g., 1/4‑20 UNC has 20 TPI). They’re inversely related: TPI = 25.4 ÷ pitch(mm), or pitch(mm) = 25.4 ÷ TPI. A 1.5 mm pitch equals 16.93 TPI; 20 TPI equals 1.27 mm pitch.
M10 coarse (standard) pitch is 1.5 mm. Fine-pitch options are 1.25 mm and 1.0 mm. If a drawing or bolt says “M10” without specifying pitch, it’s always coarse — 1.5 mm.
UNC (Unified Coarse) is the standard inch thread for general fastening. UNF (Unified Fine) has more threads per inch — higher tensile strength, better vibration resistance, finer adjustment. Example: 1/4‑20 UNC has 20 TPI; 1/4‑28 UNF has 28 TPI. UNF provides approximately 10% more tensile strength than UNC for the same nominal diameter. Standard in automotive, aerospace, and precision assemblies.
No. Even when diameters appear close — M8 (8.00 mm) vs. 5/16″ (7.94 mm), M10 (10.00 mm) vs. 3/8″ (9.53 mm), M12 (12.00 mm) vs. 1/2″ (12.70 mm) — the thread pitches don’t match. Forcing them together damages both the male and female threads. Always identify the thread standard before assembly using the identification workflow in this thread pitch chart.
NPT (National Pipe Taper) — North American tapered standard, 60° thread angle, 1° 47′ taper. BSPT (British Standard Pipe Taper) — UK/Commonwealth tapered standard, 55° thread angle. Both seal by wedging the tapered male into the tapered female. BSPP (British Standard Pipe Parallel) — straight (non-tapered) thread, 55° angle, seals with a bonded washer or O‑ring. NPT and BSP threads are never interchangeable, even when TPI matches at certain sizes.
Measure the outside diameter with calipers in both mm and inches. Measure the pitch with a thread pitch gauge — try metric leaves if the diameter is a clean mm number, imperial if a clean fractional inch. Compare against the reference tables. Verify by threading a known nut of the suspected standard — a correct match threads smoothly by hand; a mismatch binds or feels loose. Common errors: M8 vs. 5/16″ and M10 vs. 3/8″.
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