The AB magnitude system is an astronomical magnitude system. Unlike many other magnitude systems, it is based on flux measurements that are calibrated in absolute units, namely spectral flux densities.

The monochromatic AB magnitude is defined as the logarithm of a spectral flux density with the usual scaling of astronomical magnitudes and a zero-point of about 3631 janskys (symbol Jy), where 1 Jy = 10⁻²⁶ W Hz⁻¹ m⁻² = 10⁻²³ erg s⁻¹ Hz⁻¹ cm⁻² ("about" because the true definition of the zero point is based on magnitudes as shown below). If the spectral flux density is denoted f_ν, the monochromatic AB magnitude is:

$${\displaystyle m_{\text{AB}}\approx -2.5\log _{10}\left({\frac {f_{\nu }}{\mathrm {3631\,Jy} }}\right),}$$

or, with f_ν still in janskys,

$${\displaystyle m_{\text{AB}}=-2.5\log _{10}f_{\nu }+8.90.}$$

The exact definition is stated relative to the cgs units of erg s⁻¹ cm⁻² Hz⁻¹:

$${\displaystyle m_{\text{AB}}=-2.5\log _{10}f_{\nu }-48.60.}$$

Inverting this leads to the true definition of the numerical value "3631 Jy" often cited: