U.S. wars in Afghanistan, Iraq, Syria and Pakistan have cost American taxpayers $5.6 trillion since they began in 2001, according to a new study, a figure more than three times that of the Pentagon’s own estimates.

The Defense Department earlier this year estimated that the total cost of the conflicts since the 2001 attacks has amounted to about $1.5 trillion.

The new study, by the Watson Institute of International and Public Affairs at Brown University, aims to reflect costs the Pentagon doesn’t include in its own calculations, since war costs aren’t borne by the Defense Department alone. As such, said the study’s author, Neta Crawford, the analysis isn’t an apples-to-apples comparison.