# Thread Calculator

Calculate thread dimensions: pitch, TPI, major diameter, minor diameter, pitch diameter, and tap drill size for unified and metric threads.

## What this calculates

Whether you are selecting a tap drill, checking thread compatibility, or programming a CNC lathe, you need accurate thread dimensions. Enter the major diameter and thread pitch to calculate the pitch diameter, minor diameter, thread height, and recommended tap drill size.

## Inputs

- **Thread System** — options: Unified (UNC/UNF) - inches, Metric (ISO) - millimeters
- **Major Diameter (Nominal)** (in) — min 0.01, max 100 — Outermost diameter of the thread
- **Threads Per Inch (TPI)** (TPI) — min 1, max 200 — For unified threads. Common: 1/4-20, 3/8-16, 1/2-13
- **Pitch** (mm) — min 0.1, max 20 — For metric threads. Distance between thread crests in mm
- **Thread Class/Fit** — options: Class 1 (Loose), Class 2 (Standard), Class 3 (Tight) — Class 2 is most common for general fasteners

## Outputs

- **Pitch** (in) — Distance between thread crests
- **Threads Per Inch** (TPI)
- **Pitch Diameter** (in) — Diameter at the thread midpoint
- **Minor Diameter** (in) — Root diameter of external thread
- **Thread Height (H)** (in) — Theoretical sharp V-thread height
- **Tap Drill Size** (in) — Recommended drill size for ~75% thread

## Details

## Thread Geometry Basics

All standard 60-degree threads (Unified and Metric ISO) use the same basic geometry. The key dimensions are:

- **Major diameter (D):** The outer diameter of the bolt or the inner diameter of the nut thread
- **Pitch diameter:** The diameter where the thread tooth and groove are equal width. This is the most critical dimension for fit.
- **Minor diameter:** The root diameter of an external thread (or the crest diameter of an internal thread)
- **Thread height (H):** The theoretical height of a perfect sharp V-thread: H = pitch x sqrt(3) / 2

### Common Thread Sizes

**Unified (Inch):**
| Size | TPI (Coarse) | TPI (Fine) |
|---|---|---|
| 1/4" | 20 | 28 |
| 5/16" | 18 | 24 |
| 3/8" | 16 | 24 |
| 1/2" | 13 | 20 |
| 3/4" | 10 | 16 |

**Metric:**
| Size | Pitch (Coarse) | Pitch (Fine) |
|---|---|---|
| M6 | 1.0 mm | 0.75 mm |
| M8 | 1.25 mm | 1.0 mm |
| M10 | 1.5 mm | 1.25 mm |
| M12 | 1.75 mm | 1.25 mm |

### Tap Drill Sizing

A 75% thread engagement is the standard target for tapped holes. Going higher than 75% does not add much strength but makes tapping significantly harder. The tap drill formula is approximately: major diameter minus one pitch.

## Frequently Asked Questions

**Q: What is the pitch diameter of a thread?**

A: The pitch diameter is the diameter at the point where the thread tooth width equals the groove width. It is the most important dimension for determining thread fit. For a 60-degree thread, pitch diameter = major diameter - (0.6495 x pitch).

**Q: How do I calculate tap drill size?**

A: For approximately 75% thread engagement, subtract one pitch from the major diameter. For a 1/2-13 UNC bolt: 0.500 - (1/13) = 0.500 - 0.077 = 0.423 inches. The closest standard drill is a 27/64 (0.4219 inches).

**Q: What is the difference between UNC and UNF threads?**

A: UNC (Unified National Coarse) has fewer threads per inch and is the standard for general use. UNF (Unified National Fine) has more TPI, giving a tighter fit and higher tensile strength. UNC is easier to assemble and more tolerant of damage, while UNF is better for vibration resistance.

**Q: How do I convert between TPI and pitch?**

A: Pitch (in inches) = 1 / TPI. Pitch (in mm) = 25.4 / TPI. For example, 13 TPI = 1/13 = 0.0769 inches = 1.954 mm pitch. Going the other way, a 1.5 mm pitch = 25.4 / 1.5 = 16.93 TPI.

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Source: https://vastcalc.com/calculators/construction/thread
Category: Construction
Last updated: 2026-04-08