Thread Size Chart & Calculator

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Thread specification calculator — Metric · UNC · BSP · NPT

Live thread dimensions across four systems — ISO metric coarse, UNC inch, BSP parallel (G, ISO 228) and NPT taper. Pick a series and size; the table below follows your selection.

Thread series

Thread size

Dimension basis

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major Ø d (mm)

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pitch P (mm)

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pitch Ø d₂ (mm)

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minor Ø (mm)

Thread	Pitch / TPI	Major Ø	Pitch Ø	Minor Ø (ext)	Tap drill	Clearance

Coarse-pitch series per ISO 261. Basic diameters per ISO 724: d₂ = d − 0.6495·P, d₃ = d − 1.2269·P (external), D₁ = d − 1.0825·P (internal). Fine-pitch threads (e.g. M8×1) use different pitches — ask us for the full fine series.

References: ISO 261 / ISO 724 / ISO 965 / ISO 273 (metric), ASME B1.1 (UNC), ISO 228-1 (BSP parallel G), ASME B1.20.1 (NPT). See also our thread standards guide.

How the dimensions are computed

Metric values follow the ISO 724 basic profile: pitch Ø d₂ = d − 0.6495·P, external minor d₃ = d − 1.2269·P, internal minor D₁ = d − 1.0825·P. UNC converts TPI to pitch (P = 25.4/TPI) with the same 60° UN form; BSP G uses the 55° Whitworth form (ISO 228-1); NPT diameters are at the gauge plane of the 1:16 taper (ASME B1.20.1).

Choosing between the four systems

Metric ISO is the global machine-thread default; UNC/UNF its North-American counterpart. G (BSP parallel) is the fluid thread of Europe/Asia and seals on a washer or O-ring; NPT is the North-American taper thread sealing on the flanks. Mixing G and NPT is the classic costly mistake — the 55°/60° flank angles and taper do not mate. More background in our thread standards guide.

Threads on turned brass parts

We produce external threads by thread rolling (stronger, smoother flanks) or single-point cutting, and internal threads by tapping or thread milling — metric, G, UNF/UNC and NPT to class 6g/6H as standard on made-to-drawing parts.

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Limits & Fits

Type a size and a class or fit — H7, g6, H7/g6, H7/p6 — and get limit dimensions and deviations.

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Boss hole diameters, heat-set tip temperatures and seating-force guidance for brass threaded inserts.

Alloy Density

Nominal densities of 20 brass, copper and aluminium alloys — CW614N 8.50, CW617N 8.47, C36000 8.49.

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Frequently asked questions

What are the dimensions of an M6 thread?

M6 coarse (pitch 1.0 mm): major Ø 6.000 mm, pitch Ø d₂ 5.350 mm, external minor Ø 4.773 mm, internal minor Ø 4.917 mm. Tap drill 5.0 mm, close clearance hole 6.4 mm.

How are metric thread dimensions calculated?

From ISO 724 basic profile: pitch Ø d₂ = d − 0.6495×P; external minor d₃ = d − 1.2269×P; internal minor D₁ = d − 1.0825×P, where d is the nominal diameter and P the pitch.

What is the difference between BSP and NPT threads?

BSP (G, ISO 228-1) is a parallel 55° thread that seals on a washer or O-ring face. NPT (ASME B1.20.1) is a tapered 60° thread (1:16) that seals on the flanks with sealant. They are not interchangeable.

What is the pitch diameter of a thread?

The diameter at which thread groove and thread width are equal — the functional diameter where mating threads bear. Thread gauging (6g/6H classes per ISO 965) is based on it.

Does this chart cover fine-pitch metric threads?

The table shows the coarse series per ISO 261. Fine pitches (M8×1, M10×1, M12×1.5…) use the same ISO 724 formulas with the finer P — ask us for the full fine-series data.