Background clutter can obscure or mimic the desired target and confuse the operator. Thus, the similarity between the background and the target is a typical feature of the background clutter. A clutter metric based on human contrast sensitivity function is proposed. It measures the similarity between the background and the target in the frequency domain, after the differences that cannot be seen by the visual system have been "thrown away" and the visible differences have been weighted according to the sensitivity of the visual system. The correlation between the experimental probability of detection and that predicted by the clutter metric are presented. Experiment results show that the contrast-sensitivity-function-based clutter metric can perform other clutter metrics in open literature in quantifying background clutter.