ISO metric screw threads

The ISO metric thread is the general-purpose thread of bolts, screws, nuts and tapped holes. Its geometry is simple: a 60° thread whose proportions are all fixed by the pitch. From that follow every diameter on a drawing, every tap drill and the strength of the bolt.

Which standard covers what

Many tables in circulation, including Wikipedia’s, still cite the 1998 editions. The numbers on this page match both, except where noted.

The basic profile

The profile is built on an equilateral triangle, so the thread angle is 60°. Its height is

$$H = \frac{\sqrt{3}}{2}\,P = 0.866025\,P$$

where P is the pitch. The sharp triangle is cut off by H/8 at the major diameter, leaving a flat P/8 wide, and by H/4 at the minor diameter, leaving a flat P/4 wide. What remains is the basic overlap of the internal and external thread, 5H/8 = 0.541266 P.

The 2023 edition of ISO 68-1 separates this basic profile from the design profile of real parts. According to Bolt Science, the external thread has a rounded root with a minimum radius of 0.125 P, which now applies to all external threads unless a product standard says otherwise; before, it applied only from property class 8.8 up.

Basic dimensions

ISO 724 gives the diameters as functions of the nominal diameter d and the pitch P:

$$d_2 = D_2 = d - 0.649519\,P \qquad D_1 = d - 1.082532\,P \qquad d_3 = d - 1.226869\,P$$

• d2 = D2, the pitch diameter, lies 3H/8 below the major diameter on each side.
• D1, the basic minor diameter of the internal thread, lies 5H/8 below it.
• d3, the minor diameter of the external thread on the design profile, lies 17H/24 below it. The 2023 edition of ISO 724 added it to the tables.

Example, M10 (P = 1.5 mm): d2 = 9.026 mm, D1 = 8.376 mm, d3 = 8.160 mm. ISO 724 rounds to three decimals.

d = 10d₂ = 9.026d₃ = 8.160P = 1.560°

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Coarse and fine pitches

Each diameter has one coarse pitch. ISO 262 selects one or more fine pitches for fasteners from M8 up; the general plan of ISO 261 lists more. ISO 261 sorts diameters into first, second and third choice; first choice should be used where possible. The coarse series from M1 to M64:

M14, M18, M22 and M60 are second choice; so, according to a single source, are M27, M33, M39, M45 and M52. The fine pitches that ISO 262 selects for bolts and nuts, and the full dimensions of every size, are in the tap drill and thread calculator. That list follows the 1998 edition of ISO 262; we have not been able to check it against the table of the 2023 edition.

Designation and tolerance classes

A coarse thread is written with its diameter only, M10; a fine thread adds the pitch, M10 × 1.25. The tolerance class follows after a dash:

• Internal threads have the tolerance positions G and H; external threads e, f, g and h. H and h have a fundamental deviation of zero.
• A tolerance class combines a grade with a position, for example 6H or 6g. Grade 6 is medium quality with a normal length of engagement.
• If no class is given, medium quality applies: 6H/6g from M1.6 up, 5H/6h up to M1.4.
• When the pitch-diameter and crest-diameter classes differ, the pitch-diameter class comes first: M20 × 2-6H/5g6g.
• The length of engagement is short (S), normal (N) or long (L); N applies when nothing is written. A left-hand thread adds LH, as in M6 × 0.75-5h6h-S-LH.

M10M10 × 1.25M20 × 2-6H/5g6gM6 × 0.75-5h6h-S-LH

1. Mthread typeISO metric thread
2. 20nominal diameterd = 20 mm
3. × 2pitchfine pitch (coarse is 2.5)
4. -6Hinternal thread (nut)6H for pitch and crest diameter
5. /5g6gexternal thread (bolt)5g pitch diameter, 6g crest diameter
6. -Nlength of engagement · not writtennormal; applies when nothing is written

ISO 965-6:2025 gives limits of sizes for the ten classes that ISO 965-1 recommends: 4H, 5H, 6H, 7H and 6G for nuts; 4h, 6h, 6g, 6f and 6e for bolts.

Tensile stress area

The strength of a bolt is calculated on the tensile stress area, defined in ISO 898-1 as the area of a circle whose diameter is the mean of d2 and d3:

$$A_s = \frac{\pi}{4}\left(\frac{d_2 + d_3}{2}\right)^2$$

Aₛ = π/4 · ((d₂ + d₃)/2)² = π/4 · 8.593² = 58.0 mm²

A fine thread has a larger stress area than the coarse thread of the same diameter: M10 × 1.25 has 61.2 mm², M10 × 1 has 64.5 mm². Proof load and breaking load follow from As and the property class; see bolt property classes. For the hole a tap needs, see tap drill sizes.