Thread Calculator & Identifier

Thread selection & drawing notes

A thread specification identifies the mating geometry. A usable part drawing also defines its material, finish, usable engagement, and acceptance requirements. The callout builder keeps those decisions separate and leaves unresolved items in the detailed note.

Choose the family before the size

Start with the mating component. Metric M, UNC, UNF, and UNEF are 60° machine-thread families. The diameter alone is not enough: a 1/4-20 UNC screw will not mate with a 1/4-28 UNF hole. Fine pitch does not mean closer fit; pitch and tolerance class are separate choices.

Pipe threads identify a connection system, including taper and sealing features. Self-forming and wood screws need a product specification and substrate information. Do not treat either as an ordinary tapped machine-thread hole.

Fit class is not strength grade

Designation	What it controls	What it does not mean
Metric 6H / 6g	Thread tolerance grade and position. Uppercase is internal; lowercase is external.	Material strength or bolt property class.
Unified 2A / 2B	External A / internal B. Class 2 is general-purpose; 1 is wider, 3 is closer.	A load rating or permission to omit engagement checks.
8.8, 10.9, A2-70; SAE grades	Hardware material/mechanical properties under the applicable product standard.	Thread fit. A tapped aluminium part does not become “class 8.8” because its bolt is 8.8.
NPTF Class 1 / 2	Dryseal product-thread inspection requirements; Class 2 adds crest/root truncation inspection.	Pressure rating. The complete connection needs a rating for its material, seal, and service.

For metric fits, a larger tolerance-grade number generally means a wider tolerance. For Unified fits, Class 3 is closer than Class 2. The numbering systems do not have the same meaning. The builder offers common single-class metric choices; combined pitch/crest classes and numerical limits are outside its current scope.

Thread callout examples

M10 x 1.5-6H THRU identifies an internal metric thread with a 10 mm nominal diameter, 1.5 mm pitch, and 6H tolerance. 1/4-20 UNC-2B identifies a quarter-inch, 20 threads-per-inch, internal Class 2 Unified coarse thread. Right-hand is conventional; a left-hand thread is explicitly marked LH.

For a blind hole, state the minimum usable full-thread depth. Do not equate that to drill depth: chamfer, tap lead/runout, and drill point require additional space. For an external thread, dimension the usable full-thread length and any runout or relief. Locate the feature separately on the part drawing.

Material, hardware grade, finish, coating condition, and inspection belong in the detailed note or the parent drawing. The generated note lists requirements still to be supplied. No strength grade or proof load is inferred from the fit class.

Thread geometry and manufacturing method are separate choices

Turning, tapping, milling, and grinding cut a thread. Rolling an external thread and form tapping an internal thread displace material. A form-tapped hole is not the same as a hole made by driving a thread-forming screw into it.

Choose a manufacturing method in Expert options when it is a drawing requirement. The selected method becomes part of the detailed note; it does not change the standard basic profile or add an assumed strength benefit. Specify any root, surface, heat-treatment, or inspection requirements separately.

Form tapping requires a material that can flow without cracking. Pilot-hole size depends on the tap, material, and required thread fill. Use the tap supplier's guidance rather than a cut-tap drill table. Material family alone does not define formability or proof strength. Changing material in the load task clears the proof-strength input for rechecking.

Pipe size and sealing

NPT and NPTF have 60° profiles and a 1:16 taper on diameter. NPT normally needs thread sealant. NPTF controls the crests and roots for a Dryseal joint; it uses its own inspection system. Do not assume an NPT gage proves NPTF acceptance.

ISO 228 G (BSPP) is parallel, with a 55° profile and a separate seal feature. External G threads use A or B tolerance classes; internal G has no A/B suffix. ISO 7 uses R for tapered external, Rc for tapered internal, and Rp for parallel internal jointing threads. Rp is not interchangeable by designation with G.

“1/4 inch pipe” is a nominal size, not a 6.35 mm outside diameter. Check the mating standard, port drawing, gage plane, seal, wall thickness, and product pressure/temperature rating. A nominal thread callout is not a complete port specification.

Thread-forming and wood screws

Thread-forming screws displace material to create their mating thread. Metal-forming products, including DIN 7500 screws, and plastic-fastening products, such as EJOT DELTA PT, have different pilot-hole and installation requirements. A thread-cutting screw removes material; it is not the same process.

For these families the builder creates a procurement draft: product reference, diameter, length, head/drive, substrate, and finish. Use the supplier's pilot-hole, boss, and installation guidance, then validate the joint in the actual material. Mating-hole geometry depends on the selected product and substrate.

Wood-screw design also depends on species or timber grade, density, penetration, load direction, edge distances, spacing, and moisture/service conditions. For structural work, specify an evaluated product and follow its evaluation and the governing timber-design provisions.

Lead-in chamfers and finished ends

A lead-in chamfer tapers a stud end so the first thread can start into its mate. A dog point is an unthreaded cylindrical pilot; it is not the same feature. The CAD specimen defaults to chamfers at both ends, with the finished angle measured from the thread axis. Overall specimen length includes these chamfers. The full-profile envelope span excludes them, but does not establish usable engagement or the number of complete turns at an end.

For rolled threads, specify the finished end dimensions and let the manufacturing process determine the blank and tooling. Material displacement changes the end shape: CJWinter's rolling guidance gives a 30-degree blank chamfer to the axis producing approximately 45 degrees after rolling, with material-dependent adjustments. That example is not a universal blank rule. ISO 4753 covers metric external fastener ends; this representative model does not implement its dimensional tables or assign an end-type designation.

Inspection and finished condition

Calipers and a pitch gauge can suggest a nominal thread, but do not establish its tolerance class. Specify an acceptance plan and suitable plug/ring gages or measurements for the chosen standard. Account for coating and define whether acceptance is required before or after finishing. The visualizer shows basic geometry, not gage limits.

Current coverage

The specification builder includes working size lists for metric M, UNC, UNF, UNEF, NPT, NPTF, G, and R/Rc/Rp, plus product-based workflows for metal-forming, plastic-forming, and wood screws. Basic geometry is available for every metric and Unified specification choice. Find searches the specification catalog; load selection uses the smaller shared calculation catalog. Neither list is an exhaustive handbook table.

All three tasks show a specification and close-up in the same place. Pipe and product close-ups are labeled schematics; they do not supply manufacturing dimensions. The callout and full detailed note are visible beside their copy icons. Expand dimensions and calculation evidence when needed. The references below remain available in every task.

UNR/UNJ root-controlled forms, ACME and metric Tr power screws, buttress, Whitworth/BSF, NPS, thread-cutting/tapping-screw standards, and multi-start threads need separate rules and are not generated here. Numerical fit limits, gage dimensions, engagement/stripping checks, tap drills, and pipe-port details remain outside the builder.