The formation of α-AlFeSi sludge in AlSi10Mg has been studied by computational thermodynamics based on the CALPHAD method. Both the amount of sludge and the sludge onset temperature T_Sludge have been investigated. We find that the amount of sludge increases linearly with the empirical sludge factor for the studied composition interval, which agrees well with published experimental data. We also find a notable difference between the total amount of α-AlFeSi phase and the amount of pre-eutectic sludge. The T_Sludge follows a similar linear relationship but only when there is no Cr present in the material. However, we propose a modified sludge factor like expression for the temperature onset with a significantly improved predictability compared to previous empirical expressions. With Cr present, a α-AlMSi phase, rich in Cr but poor in Mn, forms at a significantly higher temperature which leads to higher amounts of sludge suggesting Cr to be more detrimental for sludge formation than previously thought. Finally, we also suggest ranges of validity for the linearized sludge factor expression outside which full thermodynamic calculations accounting for all multicomponent effects must be used.