Screw, Bolt & Thread Calculator
Work out the key dimensions of a screw or bolt thread. Metric ISO and Unified inch threads share the 60° V-form, so from the major (nominal) diameter and the pitch you can find the pitch diameter, minor diameter, thread height and depth. Enter a metric pitch in millimetres, a Unified threads-per-inch figure, or pick a standard preset like M6 or 1/4"-20. These are theoretical basic dimensions of the thread form — useful for reference and understanding, but not a replacement for the full engineering specification with its tolerance classes.
How to use this tool
- Choose a standard preset, or select metric or Unified to enter your own.
- For metric, enter the major diameter and pitch in mm; for Unified, the major diameter and threads per inch.
- Read the pitch diameter, minor diameter, thread height and depth.
- Use the minor diameter as an approximate tapping-drill size.
- For manufacturing, always confirm against the published thread standard and tolerance class.
The formula
Metric ISO and Unified inch threads share the 60° V-form, so their key diameters come from the major diameter and pitch by the same relationships. Unified threads quote pitch as threads per inch.
H = (√3 ÷ 2) · P
pitch_diameter = D − 0.6495 · P
minor_diameter = D − 1.0825 · P
P = 25.4 ÷ TPI (Unified)- D
- Major (nominal) diameter
- P
- Thread pitch (mm between crests)
- TPI
- Threads per inch (Unified)
These are theoretical basic dimensions of the thread form. Real parts have tolerance classes and allowances; use the published spec for manufacturing.
Worked examples
M6 coarse
- Given
- M6 × 1.0
- Result
- d₂ ≈ 5.35 mm, d₁ ≈ 4.92 mm
From D = 6 and P = 1: pitch diameter 6 − 0.6495 = 5.35 mm, minor 6 − 1.0825 = 4.92 mm.
Imperial bolt
- Given
- 1/4"-20 UNC
- Result
- P = 1.27 mm
20 threads per inch gives a pitch of 25.4/20 = 1.27 mm, from which the diameters follow.
Tapping drill
- Given
- M8 × 1.25
- Result
- ≈ 6.75 mm drill
The basic minor diameter, about 6.75 mm, is the starting point for choosing a tapping drill.
Frequently asked questions
The major diameter is the largest, across the thread crests — the nominal size, like 6 mm for M6. The minor diameter is the smallest, across the roots. The pitch diameter sits between them, where the thread’s groove and ridge widths are equal; it is the diameter that actually governs how threads fit together.
A good starting point is the thread’s basic minor diameter, which this tool gives — drilling that size leaves the right amount of material for the thread. Standard charts round it to an available drill; for example M6 × 1.0 uses a 5.0 mm drill. Always cross-check a tapping chart for your material.
The pitch is the distance between adjacent thread crests, in millimetres for metric threads. A smaller pitch means finer threads — more of them per length — which give finer adjustment and better hold in thin material, while coarse threads assemble faster and tolerate damage better. Unified threads quote this as threads per inch instead.
No. These are the theoretical basic dimensions of the 60° form, ideal for understanding and rough reference. Real threads carry tolerance classes, allowances and rounded roots defined in standards like ISO 965 and ASME B1.1. For manufacturing, gauging or anything load-bearing, use the published specification.
Yes, for the basic geometry — both use the 60° symmetric V-form, so the pitch and minor diameters come from the same relationships. The difference is how pitch is stated: metric threads give it directly in millimetres, Unified threads give threads per inch, which converts to a pitch of 25.4 ÷ TPI mm.
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