Undersaving generates multiple sources of risk for consumer detriment. Cognitive biases, such as present bias and exponential growth bias, have been proposed to explain under-saving. We present novel evidence for another explanation: ‘Undersum’ bias describes a systematic tendency for consumers to underestimate the accumulation of monetary amounts, even in the absence of compound interest. We discuss recent lab and field experiments demonstrating its implications for saving. Undersum bias may demotivate saving as consumers fail to appreciate the benefits of starting to save early in life, and lead consumers to underestimate future expenditure and inaccurately judge the cumulative risk of financial shock.