GNU Astronomy Utilities manual

8.1.4 Magnitude to flux conversion

The total brightness of the object is not specified in units of flux, but in magnitudes. The magnitude scale is a relative measure of brightness. When a reference object has magnitude $m_r$ with flux $F_r$, then the magnitude of an object with a flux of $F$ is calculated by:

$$m-m_r=-2.5\log_{10} \left( F \over F_r \right)$$

The zero-point magnitude ($m_z$) is defined as the magnitude in which $F_r=1$. So when you specify the magnitude of your desired object in the --mcol of your input catalog, then it will have a total flux of:

$$\log_{10}F={m_z-m \over 2.5} \rightarrow F=10^{m_z-m \over 2.5}$$

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GNU Astronomy Utilities manual, July 2015.