Aethalometers measure black carbon mass concentrations using light attenuation through a filter capturing particles. This work compares five micro-aethalometers using known mass concentrations of laboratory-generated soot. Uncertainties were found to scale with mass concentration, and an expression is given for the uncertainty as a function of mass concentration, sampling interval, and flow rate. An open-source algorithm is provided for the reanalysis of aethalometer data.