The assessment of uncertainty in free release measurements by integral gamma scanning method is described and applied to the measurement of homogeneous and heterogeneous waste drums. It is based on the propagation of distributions using the Monte Carlo method. In addition, two techniques for the uncertainty reduction are also described. The first technique makes use of containers constructed from two concentric cylinders and the second technique is based on the measurement of a group of waste drums. It is proved that the uncertainty of clearance measurements can be reduced using both techniques. •Uncertainty assessment in free release measurements is described.•Uncertainty assessment is based on the propagation of distributions.•Two techniques for uncertainty reduction are also described.•It is proved that the uncertainty can be reduced using both techniques.