${\displaystyle TH{D}_{\text{F}}=20\lg \left(\frac{V_{\text{m}}/V_{\text{f}}}{{\mu }_{\text{ea}}/CCF}\right)}$

where

$V_{\text{m}}=\sqrt{{\displaystyle \sum _{n=1}^{\infty }{V}_n^2}}$, where V_n is the amplitude of the n-th harmonic component of the waveform,
V_f is the amplitude of its fundamental component,
μ_ea is the amplitude permeability,
CCF is the Circuit Correction Factor

NOTE 1 – In practice, $CCF=1/\sqrt{1+{\left(3\omega L_1/R_{\text{s}}\right)}^2}$ stands for the Circuit Correction Factor and is given in the approximation for the third harmonic, which is valid for measurements without applied DC bias. L₁ is the primary inductance. R_s is the total source resistance (50 Ω).

NOTE 2 – THD_F is sometimes expressed in decibels.