Thread Pitch Diameter Calculator (Three-Wire Method)

Inputs

Measurement over wires (M)

Micrometer reading over the three wires (two wires one side, one the other).

Wire diameter (W)

Diameter of the measuring wires. Best results use the best wire size shown below.

Thread pitch (P)

For inch threads, pitch = 25.4 / TPI.

Default result

Pitch diameter(E)

11.308mm

Pass

simple (uncorrected)

Simple (uncorrected) pitch diameter: E = M − 3W + 0.86603·P.

Also computed

Best wire size(W_best)1.128mm

Wire that contacts the flanks on the pitch line: W = 0.57735·P.

Method notes 4 notes

Pitch (effective) diameter E = M − 3·W + 0.86603·P for 60° UN/metric threads, where 0.86603 = 3·cot(60°)/2.
Three-wire method: two wires sit in adjacent grooves on one side and one wire on the opposite side; M is measured over all three with a micrometer.
Best wire size W = 0.57735·P = P / (2·cos30°) — this wire contacts each flank exactly on the pitch line.
This is the simple (uncorrected) pitch diameter; a small correction Δ for lead/flank-angle error and measuring force is ignored.