Medicare Advantage (MA) accounted for 34 percent of Medicare program spending in 2019. Microsimulation modeling of Medicare policy reforms requires estimates of spending for all Medicare enrollees, but quality spending data for MA enrollees are not generally available. In this methods brief, we describe the approach we use in the MCARE-SIM model to estimate MA program spending and out-of-pocket spending by MA enrollees at the county level using plan-level and county-level data sources from CMS. These estimates can then be used as inputs to subsequent analyses of changes in Medicare policy.