America imprisons more people than any other nation in the world, and hands out harsher and longer sentences than most. As a result, its prisons are overcrowded and increasingly expensive, and its penitentiary policies, especially when it comes to the so-called war on drugs, are largely seen as a failure.

While lawmakers from both parties agree that something needs to change–albeit for different reasons–there is little agreement on finding a solution.

The Urban Institute, a social policy think tank, created the Prison Population Forecaster, an interactive tool that lets you go see how different policy combinations would reduce prison populations. In some states, the optimum solution is to slightly reduce sentences for violent offenses, in others it is to limit imprisonment for new crimes. The tool uses data from 15 states that represent nearly 40% of America’s incarcerated.

If nothing changes, the prison population in all 15 states will fall by only 2% by 2021. However, if states reform only drug sentencing laws–an effort already launched by some states and promoted by the Obama administration–the prison population could fall by 7% over that span.

“This forecasting tool paves the way for a more productive conversation about the need for tailored reforms that address the unique drivers of mass incarceration in each jurisdiction,” the Urban Institute writes.