According to the invention, a method and system for estimating fresh air flow into a turbocharged engine (105) is provided. A controller (109) arranged to determine an actual fresh air mass flow in subsequent time frames by measuring, in an actual time frame, a pressure drop over a compressor (101) and using a first calculated fresh air mass flow as a starting value for deriving a second fresh air mass flow in said time frame from a compressor model using the measured pressure drop and a compressor rotational speed. In a previous time frame, before said actual time frame, a pressure drop is measured over an air treatment device. A pressure drop is estimated over the air treatment device (103, 104, 106, 108) using the second fresh air mass flow and an estimated flow resistance of the air treatment device. Subsequently, the second fresh air mass flow is corrected by comparing the estimated pressure drop with the measured pressure drop over the air treatment device and using the corrected second fresh air mass flow as an actual fresh air mass flow in said time frame.