How many students repeat a grade each year? How do retention rates vary across states and over time? Despite extensive research on the predictors and consequences of grade retention, there is no systematic way to quantify state-level retention rates; even national estimates rely on imperfect proxy measures. We present a conceptually simple method--based on publicly available data that are routinely collected each year--that describes retention rates at the state and national levels. After describing and validating this method, we use it to report first- through eighth-grade public school retention rates for 2002-2003 through 2008-2009 for the entire country and for each state. (Contains 13 notes, 5 figures and 1 table.)