Thread Calculator
Calculates the basic dimensions of any 60° thread from its major diameter and pitch: ISO metric, Unified inch, or a special size that is in no chart. Results appear as you type.
Basic dimensions
| Dimension | mm | in |
|---|
Tap drill by percentage of thread
| % thread | Drill Ø (mm) | Drill Ø (in) |
|---|
These are exact diameters. Pick the nearest drill you have that is not below the 83% size, which is the basic minor diameter.
Formulas
- H = 0.866025 × P, the height of the fundamental triangle
- Pitch diameter d2 = d − 0.649519 × P
- Minor diameter of the internal thread D1 = d − 1.082532 × P
- Minor diameter of the external thread d3 = d − 1.226869 × P (ISO metric, rounded root) or d − 1.190785 × P (Unified UNR)
- Tensile stress area, metric: As = π/4 × ((d2 + d3) ÷ 2)² (ISO 898-1)
- Tensile stress area, inch: As = 0.7854 × (d − 0.9743 ÷ n)², n = threads per inch (ASME B1.1)
- Tap drill = d − (% thread ÷ 100) × 1.299038 × P
- Lead angle = arctan(P ÷ (π × d2))
The calculator gives basic sizes, before tolerances. For a standard thread, the thread page also lists the tolerance limits and the drill sizes that are actually stocked.
Common questions
Does the calculator work for pipe threads?
No. NPT and BSP threads have different proportions and, for NPT, a taper. Use the NPT and BSP charts.
Why is the tap drill larger than the minor diameter?
The basic minor diameter is the smallest the hole may be. Drilling to it means cutting an 83% thread, which loads the tap heavily. The standard drill leaves about 77%.
Can I use it for Whitworth or Acme threads?
No. The formulas are for the 60° profile shared by ISO metric and Unified threads.
Other charts and tools
Sources: ISO 724, ISO 898-1 and ASME B1.1 (formulas). How this site checks its data. Last reviewed 29 September 2026.