This paper proposes a methodology to assess fan energy savings when improving ductwork airtightness. This methodology builds on standard EN 16798-5-1:2017. Unlike the classical "cube law", it considers pressure drops at air terminal devices separately from the pressure drops in the rest of the system. The calculation tool based on this methodology: (a) gives the fan energy use before and after airtightness improvements with various inputs depending on the initial state; (b) indicates whether or not the required airflow rates are met at the air terminal devices; (c) gives a range of energy savings assuming, on the one hand, perfect fan adjustment, no errors in input data and constant fan efficiency, and on the other hand, safe-side estimates of deviations from these assumptions. Using experimental data from an earlier study on a full scale ductwork built in a laboratory, the tool gives results in good agreement with measured energy savings (within 7 percentage points). With ideal pressure adjustment and when required airflow rates were met before tightening, the maximum fan energy savings by tightening the ductwork from 1.5 times Class A to Class C reaches 51% according to the tool versus 46% according to measurement data. (C) 2018 Elsevier B.V. All rights reserved.