M6×1: basic pitch diameter 5.350 mm, basic minor diameter 4.917 mm.
- Basic pitch diameter d₂ = D₂ (d − 0.649519 P)
- 5.350 mmFull value ≈ 5.350480947161671 mm
- Basic minor diameter d₁ = D₁ (d − 1.082532 P)
- 4.917 mmFull value ≈ 4.917468245269451 mm
- Fundamental triangle height H (0.866025 P)
- 0.866 mmFull value ≈ 0.8660254037844386 mm
- Basic thread engagement H₁ = 5H/8 (0.541266 P)
- 0.541 mmFull value ≈ 0.5412658773652741 mm
- Flat at the major diameter (P/8)
- 0.125 mm
- Flat at the minor diameter (P/4)
- 0.250 mm
- d₃ for the stress area (d − 1.226869 P)
- 4.773 mmFull value ≈ 4.773130677972046 mm
- Tensile stress area Aₛ = π/4 × ((d₂ + d₃)/2)²
- 20.123 mm²Full value ≈ 20.123375990073452 mm²
Basic sizes of the profile, with no tolerance class. The limits of a 6g bolt or a 6H nut come from ISO 965-1, not from this calculator.
What it computes
An ISO metric thread is named by its nominal (major) diameter and its pitch: M10×1.5 is a 10 mm thread with 1.5 mm from one crest to the next. Every other size of the basic profile follows from those two numbers through ISO 68-1. The calculator works out the sizes a designer, a machinist or an inspector checks most often: the pitch diameter, where thread gauges and the three-wire method measure, and the minor diameter, which sets the root of a bolt and the bore of a nut. It also gives the stress area that ISO 898-1 uses to turn a bolt's tensile strength into a load.
Formulas
The basic profile is built on an equilateral triangle with a 60° included angle and a height H. The crest is cut off by H/8 at the major diameter and the root by H/4 at the minor diameter:
- H = (√3 / 2) P ≈ 0.866025 P.
- Basic height of thread engagement H₁ = 5H/8 ≈ 0.541266 P.
- Basic pitch diameter d₂ = D₂ = d − 0.649519 P.
- Basic minor diameter d₁ = D₁ = d − 1.082532 P.
- Flat at the major diameter P/8; flat at the minor diameter P/4.
- For the stress area: d₃ = d − 1.226869 P, and Aₛ = (π/4) × ((d₂ + d₃) / 2)².
Capital letters denote the internal thread of a nut, lowercase the external thread of a bolt. In the basic profile the two share one set of sizes. The flats are exact fractions of the pitch. Every other figure involves √3 or π, so it is shown rounded, with its full value marked ≈.
Worked example
01 Read the designation
M10×1.5: nominal diameter d = 10 mm, pitch P = 1.5 mm. 1.5 mm is the coarse pitch for M10, so the drawing may write it simply as M10.
02 Work out the pitch diameter
d₂ = 10 − 0.649519 × 1.5 ≈ 9.026 mm, the value ISO 724 lists for M10.
03 Work out the minor diameter
d₁ = 10 − 1.082532 × 1.5 ≈ 8.376 mm, again as ISO 724 lists it.
04 Work out the stress area
d₃ ≈ 8.160 mm, and Aₛ = (π/4) × ((9.026 + 8.160) / 2)² ≈ 57.99 mm². ISO 898-1 rounds it to 58.0 mm².
05 Compare a fine pitch
M10×1.25 gives d₂ ≈ 9.188 mm, d₁ ≈ 8.647 mm and Aₛ ≈ 61.2 mm²: a finer pitch leaves more material at the root.
Limitations
- Basic sizes only. The limits of a 6g bolt or a 6H nut, and the allowance between them, come from ISO 965-1 and are not computed here.
- No pitch table. A designation without a pitch means the coarse pitch from ISO 261; look it up and enter it.
- ISO metric threads with the 60° profile only. Unified inch, pipe, trapezoidal and other thread forms have other profiles.
- No tap drill size, thread length or strength calculation. The stress area is a geometric figure; the load also depends on the property class and the engagement.
When to involve a qualified engineer
Ask the engineer who owns the design before you change a pitch, a tolerance class or a thread length, and whenever a joint carries a safety-relevant load. Ask, too, when a coated or plated thread must still assemble. Coating thickness eats into the allowance, and the plating process sets its own rules.
What the product reads and builds
In the Technical Preview, TruePart reads thread (M) callouts, and the one part family it builds has no threaded feature.
Questions
What pitch does M8 mean without a pitch?
The coarse pitch from ISO 261, 1.25 mm for M8. A fine pitch is always written out, as in M8×1.
Is the pitch diameter halfway between the major and minor diameters?
No. It is where the thread's groove and ridge are equally wide. For M6×1 it is 5.350 mm, below the 5.459 mm midpoint of the 6 mm major and 4.917 mm minor diameters, so closer to the minor diameter.
Why does the stress area use d₃ rather than the minor diameter?
d₃ is the minor diameter of a bolt with a rounded root, slightly below d₁. ISO 898-1 takes the stress area from the mean of d₂ and d₃, because a threaded bolt breaks at a load that matches that area better than the root area alone.
Sources
- ISO 68-1, ISO general purpose screw threads, Basic profile, Part 1: Metric screw threads · ISOThe 60° basic profile, H and the truncations of the crest and root.
- ISO 724, ISO general-purpose metric screw threads, Basic dimensions · ISOBasic pitch and minor diameters the worked examples are checked against.
- ISO 261, ISO general purpose metric screw threads, General plan · ISOThe coarse and fine pitches for each nominal diameter.
- ISO 898-1:2013, Mechanical properties of fasteners made of carbon steel and alloy steel, Part 1: Bolts, screws and studs · ISOThe tensile stress area formula and its tabulated values.
- ISO 965-1, ISO general purpose metric screw threads, Tolerances, Part 1: Principles and basic data · ISOTolerance classes such as 6g and 6H, which this calculator does not apply.
What TruePart does today
- LimitIn the Technical Preview, TruePart reads thread (M) callouts, and the one part family it builds has no threaded feature.