$\color{red}{K-correction }$: it converts measurements of astronomical objects into $\color{red}{their\ respective\ rest\ frames}$. The correction acts on that object’s observed $\color{red}{magnitude}$ (or equivalently, its flux).

Why should appeal to K-correction?

Because astronomical observation often measure through a single filter or bandpass, observers only measure a fraction of the total spectrum, redshifted into the frame of the observer.

Formula

$K_{corr}=(m-M)-5(log_{10}D_L-1)$

Where,

​ $D_L$ is the luminosity distance measured in parsecs

​ The absolute and apparent magnitude required to correct for the redshift effect.

​ $m-M$ called distance modulus

Calculator: “K-corrections calculator”