We present an overview of spending adequacy among individual K-12 school districts in the U.S. Our results are from a new database of over 12,000 public school districts that allows users to compare each district's actual per-pupil spending levels to estimates of adequate spending levels--i.e., spending required to achieve the common goal of national average math and reading scores. The data are for the 2017-18 school year. Predictably, we find substantial heterogeneity, with many districts spending well above our estimated adequacy targets and many others spending well below, in some cases shockingly below. Districts with negative (i.e., inadequate) funding gaps are especially prevalent in the southeast and southwest, but they are also found throughout the entire U.S., including in states, such as Massachusetts and Connecticut, which include generally high-spending districts. The sum of these negative gaps across all districts (ignoring districts with positive gaps) is $104 billion, and the average negative gap is $4,254 per-pupil. Conversely, even in states where underfunding is widespread and typically severe, there are numerous districts in which resources exceed our adequate spending estimates. Finally, we show that the extent of funding inadequacy increases with district child poverty rates and with the proportion of Black and especially Hispanic (Latinx) students served by districts. These results illustrate that most states are failing in their job of filling the holes between districts' costs and their capacity to pay those costs, as well as how, even in states that are more successful, many districts slip through the cracks. An effort to rectify these discrepancies could consist of a strategic expansion of the federal role in education finance, as well as a recalibration of how states fund their schools. High-quality district adequacy measures can help guide this process by identifying where resources are needed most.