This paper analyses the errors on the frequency response function measurement of a transfer function due to finite window effects (leakage). First an analysis of the rectangular and the Hanning window is made. It will be shown that the leakage error consists of two components: a transient error due to initial and end condition effects, and an interpolation error due to the combination of neighbouring spectral lines. Starting from these insights an extremely simple expression to calculate the leakage induced bias and variance errors is generated. Eventually, a new 'default' window is proposed with slightly better properties. This allows a reduction in measurement time by 25% if the leakage errors dominate the disturbing output noise. (c) 2005 Elsevier Ltd. All rights reserved.