Thread Calculator
Calculate thread dimensions including pitch diameter, minor diameter, and thread depth for Unified and Metric thread standards.
Introduction
The Thread Calculator returns the working geometry of Unified (UNC/UNF) and ISO metric threads: pitch, pitch diameter, external minor diameter, and single-side thread depth. These are the numbers a machinist actually needs at the lathe or the inspection bench — pitch diameter for go/no-go and three-wire checks, minor diameter for pre-turn stock and tap-drill cross-checks, and thread depth for total infeed when single-point threading. Select a standard size from the list and every derived dimension appears instantly, computed from the same H-profile coefficients used in the thread standards rather than from rounded handbook values.
How it Works
For Unified sizes the calculator first converts TPI to pitch (p = 1/TPI inches), then applies the 60° thread-form coefficients: pitch diameter = nominal − 0.6495 × p, external minor diameter = nominal − 1.2269 × p, and thread depth per side = 0.6134 × p, which is the lathe infeed from crest to root. Metric sizes use identical coefficients with pitch already in millimeters. For a 1/2-13 UNC thread, p = 0.0769 in, so the pitch diameter computes to 0.4500 in and total single-side depth to 0.0472 in.
Usage Scenarios
- Single-point threading on a lathe: setting total compound infeed from the 0.6134 × p depth figure instead of guessing and chasing the fit with a nut.
- Programming a thread mill: the CAM post needs pitch diameter and minor diameter to generate the helical pass and verify the pre-drilled bore.
- Inspecting with thread wires: comparing the measured pitch-diameter equivalent against the calculator's nominal pitch diameter before applying class tolerances.
- Reverse-engineering an unknown fastener: measuring OD and TPI, then matching the computed pitch and minor diameters against the candidate standard sizes.
- Preparing stock for thread rolling, where blank diameter sits near pitch diameter rather than major diameter.
FAQ
Where does the 0.6495 coefficient come from?
It derives from the 60° V-thread geometry: the basic pitch diameter sits below the major diameter by a fixed fraction of the fundamental triangle height, and expressing that in terms of pitch gives major − 0.6495 × p.
Are these basic dimensions or toleranced limits?
Basic (nominal) dimensions. Class allowances and tolerances — 2A/2B for Unified, 6g/6H for metric — are applied on top of these basics per the relevant standard.
Why is thread depth quoted per side?
Because it equals (major − minor)/2 = 0.6134 × p, which is the radial infeed a lathe operator feeds from first scratch to full depth when single-point threading.
Can I use the minor diameter as a tap drill size?
It is a close lower bound. Practical tap drills target roughly 75% thread engagement, so cross-check against the Tap Drill Chart, which lists the standard drill for every size.