Thread Calculator

Thread Calculator for ISO Metric Threads

The Thread Calculator computes basic dimensions for ISO metric 60° threads (M-profile per ISO 68-1). Enter the thread pitch, major diameter, and tolerance class to find the fundamental triangle height, pitch diameter, and minor diameter for both external (bolt) and internal (nut) threads.

Thread Geometry Formulas

ISO metric threads have a 60° V-profile with root and crest truncation. The fundamental triangle height H is the base of all thread dimension calculations:

$$H = P \times \frac{\sqrt{3}}{2}$$ $$\text{Pitch diameter: } d_2 = d - 2 \times \frac{3}{8} \times H$$ $$\text{Minor diameter: } d_1 = d - 2 \times \frac{5}{8} \times H$$

The basic dimensions are the same for both external and internal threads. The difference between a bolt and a nut comes from tolerance class allowances and grades, which determine the actual fit (clearance, transition, or interference).

Tolerance Classes Explained

A tolerance class consists of a grade (number) and a position (letter). The grade controls how much the diameter can vary during manufacturing. The position controls how far the actual thread deviates from the basic profile (fundamental deviation). For external threads, positions e, f, g, and h provide increasing allowance (e = largest gap, h = zero gap). For internal threads, positions G and H define the minimum material condition.

For related thread tools, see the Thread Pitch Calculator for TPI conversion, the Pitch Diameter Calculator for detailed tolerance limits, and the Clearance Hole Calculator for fastener hole sizing.

Frequently Asked Questions

What does M12 × 1.5 mean?

M12 × 1.5 designates an ISO metric thread with a nominal major diameter of 12 mm and a pitch of 1.5 mm. The coarse pitch for M12 is 1.75 mm, so 1.5 mm is a fine pitch variant. The calculator works with any standard or custom pitch.

What is the difference between pitch diameter and major diameter?

Major diameter is the largest diameter of the thread (the crest for external, root for internal). Pitch diameter is the imaginary cylinder where thread ridge width equals groove width. Fit and tolerance measurements primarily reference pitch diameter, not major diameter.

What tolerance class should I use for general engineering?

The most common tolerance class for general-purpose metric threads is 6g for external (bolts) and 6H for internal (nuts). This provides a standard running fit with moderate clearance. Fine mechanisms may use 4g/4H for tighter fits, while coarse construction threads may use 8g/8H.

Does this calculator work for imperial/UNC threads?

No. This tool applies ISO metric 60° thread standards (ISO 68-1, ISO 965) only. UNC, UNF, and other imperial thread series use different profile angles (60° for UN as well, but with different root/flat geometry and tolerance tables).

How do I use thread calculator results for 3D modeling?

Use the major diameter for the external cylinder. The pitch diameter determines the effective fit surface. The minor diameter defines the thread root depth. For 3D models, the fundamental triangle height H defines the sharp-V profile before rounding and truncation.

What is the fundamental deviation for position g?

For position g on external threads (commonly used on bolts), the fundamental deviation es is negative, meaning the actual thread diameters are smaller than the basic profile. This ensures clearance between the bolt and nut threads. The deviation value depends on the pitch.

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