ENGINEERING REFERENCE CHART
Thread Chart
If you machine, specify, or inspect threaded parts, you can look up any common thread size here, identify an unknown thread with the step-by-step guide below, and understand thread classes and callouts well enough to specify them correctly on an engineering drawing. And when you need threaded parts machined, you can upload a CAD file with your thread callouts for an instant CNC quote.
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UNC / UNF / Metric / NPT / BSP
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Tap Drill & Pitch Diameter Reference
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CNC Thread Machining Support
Thread Standards
Common engineering thread systems
UNC
Unified Coarse
UNF
Unified Fine
M
Metric (coarse / fine)
NPT
National Pipe Taper
BSPP
British Std Pipe Parallel
ACME
Trapezoidal thread
Thread size & tap drill reference
UNC · UNF · Metric · NPT · BSP · ACME
THREAD REFERENCE
What Is a Thread Chart?
A thread chart is an engineering reference table that lists the dimensional specifications of standardized screw threads: major diameter, pitch (or threads per inch), pitch diameter, minor diameter, and tap drill size. Every machinist, mechanical designer, and quality inspector uses a thread size chart, whether it is a laminated card at the bench, a page in Machinery’s Handbook, or a browser tab open to an online reference like this one.
Threads are defined by international and national standards. Unified threads (UNC, UNF, UNEF) follow ASME B1.1, metric threads follow ISO 724 and ISO 261, pipe threads follow ASME B1.20.1 (NPT) and ISO 7 / EN 10226 (BSP), and ACME threads follow ASME B1.5. Each standard defines the thread form — the 60° flank angle for unified and metric threads, the 55° flank angle for BSP, and the 29° flank angle for ACME — plus the tolerance classes that control fit.
Unified & Metric
UNC, UNF, and UNEF follow ASME B1.1 with a 60° flank angle; metric threads conform to ISO 724 and ISO 261. These standards define coarse, fine, and extra-fine series for general engineering applications.
Pipe Threads
NPT / NPTF follow ASME B1.20.1 with a tapered 60° thread form; BSPP and BSPT conform to ISO 7 / EN 10226 with a 55° flank angle. These standards ensure leak-tight joints in piping systems.
ACME & Tolerance Classes
ACME threads per ASME B1.5 feature a 29° flank angle for power transmission. All standards include tolerance classes (e.g. 2A, 6H) that define pitch and major diameter limits to control the fit.
THREAD REFERENCE DATA
Thread Chart: UNC, UNF, Metric, NPT & BSP Tap Drill Sizes
Compare thread dimensions across common standards — major diameter, pitch, pitch diameter, minor diameter, and tap drill size — based on ASME B1.1, ISO 724, ASME B1.20.1, and ISO 7 / EN 10226.
Tap drill sizes are recommended for approximate 75% thread engagement. Actual drill size may vary based on material, thread class, and manufacturing method. Always verify with your specific thread standard.
| Thread Standard | Size | Pitch (mm / TPI) | Major Dia. (mm) | Tap Drill (mm) |
|---|---|---|---|---|
| UNC | 1/4-20 | 20 TPI | 6.35 | 5.10 |
| UNF | 1/4-28 | 28 TPI | 6.35 | 5.50 |
| Metric | M6×1.0 | 1.0 mm | 6.00 | 5.00 |
| Metric Fine | M8×0.75 | 0.75 mm | 8.00 | 7.20 |
| NPT | 1/8-27 | 27 TPI | 10.24 | 8.43 |
| BSPP | G 1/4-19 | 19 TPI | 13.16 | 11.00 |
| ACME | 1/2-10 | 10 TPI | 12.70 | 10.20 |
ASME B1.1 & ISO 724
Unified (UNC / UNF / UNEF) and metric (coarse / fine) thread standards define the 60° flank angle, pitch, and major diameter for general-purpose fasteners and precision applications.
ASME B1.20.1 & ISO 7 / EN 10226
NPT / NPTF tapered pipe threads and BSPP / BSPT parallel and tapered pipe threads follow 60° and 55° flank angles respectively, ensuring reliable sealing in piping systems.
IDENTIFICATION GUIDE
How to Identify an Unknown Thread
You have a bolt, fitting, or threaded stud and don’t know the thread. This practical process works for all common thread standards.
01
Measure Major Diameter
Measure outside diameter with calipers. Near a round mm value (5.98 mm ≈ M6) indicates metric. Near a fractional inch (0.248″ ≈ 1/4″) indicates unified or BSP. Record the value.
02
Check Pitch with Gauge
Use a thread pitch gauge. Try imperial leaves if inch was measured, metric if mm. The correct leaf seats fully with no light gap. Record TPI or pitch.
03
Check for Taper
Lay a straight edge along crests. Sloping line indicates tapered thread (NPT, BSPT, NPTF). Parallel crests indicate straight thread (UNC, UNF, metric, BSPP, ACME).
04
Cross-Reference Chart
Match diameter and pitch on this chart. If values fall within standard tolerances (±0.005″ or ±0.1 mm), you have identified the thread.
05
Verify with Known Nut
Thread a known nut smoothly by hand for at least three turns with no play. If it binds after one turn, re-measure. Never force it.
Example: Identifying a 1/2-13 UNC Thread
Threaded rod measures 0.498″ (≈ 1/2″). Pitch gauge reads 13 TPI with no light gap. Straight edge shows parallel crests. Chart confirms 1/2-13 UNC. Known nut threads freely — confirmed.
Major Dia.
0.498″ (≈ 1/2″)
Pitch
13 TPI
Result
1/2-13 UNC ✓
THREAD REFERENCE
Common Thread Standards at a Glance
Quick reference for the most frequently used thread families in CNC machining.
| Standard | Angle | Form | Applications | Key Feature |
|---|---|---|---|---|
| UNC / UNF | 60° | Straight | Fasteners, structural | Coarse / fine series |
| Metric (M) | 60° | Straight | OEM, automotive | ISO coarse / fine pitch |
| NPT / NPTF | 60° | Tapered | Pipe fittings, hydraulic | Dry-seal or sealant |
| BSPP / BSPT | 55° | Parallel / Tapered | European piping | Whitworth form |
| ACME | 29° | Straight | Lead screws, power | Trapezoidal load-bearing |
Thread Formula Cheat Sheet
Quick reference for the most commonly used thread formulas in machining and engineering. All formulas assume standard 75% thread engagement unless otherwise noted.
| Formula | Application |
|---|---|
| Tap drill = major Ø − pitch | Metric thread tap drill (75% engagement) |
| Tap drill = major Ø − (0.974 ÷ TPI) | UNC/UNF tap drill (75% engagement) |
| Pitch (mm) = 25.4 ÷ TPI | Convert imperial TPI to metric pitch |
| TPI = 25.4 ÷ pitch (mm) | Convert metric pitch to imperial TPI |
| Pitch Ø = major Ø − (0.6495 × pitch) | Unified and metric 60° thread forms |
| Thread depth = 0.614 × pitch | Standard thread depth, unified and metric |
| Thread depth = 0.5 × pitch | ACME general-purpose thread depth |
Note: Tap drill formulas provide approximate starting sizes. Always verify with your specific thread standard and adjust for material, thread class, and percent of full thread engagement required. For critical applications, use the recommended tap drill sizes from your thread standard (ASME B1.1, ISO 724, ASME B1.20.1, etc.).
THREAD SPECIFICATION
Thread Terminology: Major, Minor & Pitch Diameters
When you read a thread chart, you will see three diameters repeated for every thread size. Understanding them is key to specifying threads correctly.
The major diameter is the largest diameter — the outside of a bolt or the crest-to-crest of a nut. The minor diameter is the smallest — the root of an external thread or the crest of an internal thread. The pitch diameter sits halfway between them and is the single most important dimension for thread fit, because it is the diameter where the thread thickness equals the groove width. Thread tolerance classes (2A/2B, 6g/6H) control the allowable deviation on pitch diameter.
UNIFIED
ASME Y14.5 Format
1/2-13 UNC-2B — 1/2 is nominal diameter, 13 is TPI, UNC is the thread series, and 2B is the internal thread class. For external threads, the class ends in A: 1/2-13 UNC-2A.
METRIC
ISO 724 Format
M12×1.75-6H for an internal thread (6H tolerance), or M12×1.75-6g for an external thread. If the pitch is omitted — just M12 — the coarse pitch is assumed, but explicitly writing the pitch is good practice for machined parts.
PIPE
NPT & BSP Designation
Pipe threads on drawings: 1/4-18 NPT or R 1/4 for BSPT. Always include the standard designation so the machinist knows whether to use an NPT tap or a BSP tap — the two look similar in the tool crib but produce incompatible threads.
How to Specify Threads per ASME Y14.5
Unified threads follow this format: 1/2-13 UNC-2B where 1/2 is the nominal diameter, 13 is the TPI, UNC is the thread series, and 2B is the internal thread class. For external threads, the class ends in A: 1/2-13 UNC-2A.
Metric threads follow: M12×1.75-6H for an internal thread (6H tolerance), or M12×1.75-6g for an external thread. If the pitch is omitted — just M12 — the coarse pitch is assumed, but explicitly writing the pitch is good practice for machined parts.
Pipe threads on drawings: 1/4-18 NPT or R 1/4 for BSPT. Always include the standard designation so the machinist knows whether to use an NPT tap or a BSP tap — the two look similar in the tool crib but produce incompatible threads.
Pitch Diameter
Critical for thread fit
2A / 2B
Unified tolerance classes
6g / 6H
Metric tolerance classes
THREAD QUESTIONS
Frequently Asked Questions
For M8×1.25 (coarse) at 75% engagement, the tap drill is 6.80 mm. Use a 6.8 mm or 17/64 inch drill bit. For M8×1.0 (fine), the tap drill is 7.00 mm — close to 9/32 inch.
Measure the major diameter with calipers, check the pitch with a thread pitch gauge, check for taper with a straight edge, and cross-reference on this thread chart. If the diameter is near a round mm value, it’s metric. If near a fractional inch, it’s unified or BSP. Verify with a known nut — if it threads on smoothly for three full turns, you have the right identification.
Use UNC (coarse) for general fastening — it assembles faster, tolerates minor thread damage better, and is standard for most machined parts. Use UNF (fine) when you need higher strip strength in thin-walled parts, better vibration resistance, or finer axial adjustment. UNF is standard for aerospace and automotive engine components.
No. Both use a 60° flank angle, but the pitch and diameters differ. Threading them together will cross-thread and damage both parts. Approximate matches like M6 (0.236″) and 1/4-20 (0.250″) are not close enough to assemble correctly even though they share the same coarse-pitch general-purpose role.
NPT uses a 60° flank angle per ASME B1.20.1 and is standard in North America. BSP uses a 55° flank angle per ISO 7 and is standard in Europe, UK, and much of Asia. They are not interchangeable — the 5° flank angle difference plus different TPI means forcing one into the other will cross-thread and leak.
Note: At 1/2 inch size only, both happen to use 14 TPI, but even then the thread angle renders them incompatible.
Class 2A is the standard external thread fit (bolt or screw). Class 2B is the standard internal thread fit (nut or tapped hole). The A and B indicate external vs. internal, not a quality grade — both are standard commercial tolerances. Class 3A/3B is the precision fit with tighter tolerances, used when zero-play alignment is required.
For metric at 75% engagement: tap drill = major diameter − pitch. For unified: tap drill = major diameter − (0.974 ÷ TPI). This yields about 75% of full thread depth, providing 95% of full thread strip strength while reducing tapping torque by roughly 25% compared to a 100% thread.
Threaded Parts, Machined to Spec
Baetro CNC-machines threaded features from M1.6 to M64 and #0 to 1 1/2 inch across all major thread standards. Our instant quote system reads standard thread annotations from your CAD file. Upload your design, select your material and finish, and get threaded parts shipped in 3 to 7 days.