A sand boil is a cone of granular material deposited on the ground surface around concentrated points of vertical seepage. Sand boils often form downstream of embankments during flood events, creating unfiltered seepage exits that may lead to initiation of backward erosion piping (BEP) in the foundation materials. Further progression of the backward erosion process may undermine the embankment, ultimately leading to collapse and breach. As the progression of BEP is dependent on the horizontal hydraulic gradient in the foundation, it is also impacted by the head loss occurring through the sand boil. In this study, the head loss occurring in sand boils along the Mississippi River and Waal River were measured and compared to predictions from a simple theoretical model. The measured head loss compared favourably with the predictions, indicating that the head loss is not constant but rather a function of grain size, sand boil throat diameter and flow rate.