Universal voucher programs for schools are rapidly expanding across the country. Under these programs, states give parents stipends to either homeschool their children or send them to private school. The growing popularity of vouchers raises a host of crucial questions and concerns. Key to informing the debate are questions of public finance and education quality. Is allowing public money to leave the public school system and follow kids to private schools the most effective or equitable way to make sure every child has access to an excellent education? This report focuses on an often underrecognized, but extremely important, source of stress that voucher programs can impose on the ability to provide an excellent public education for all. This is the fiscal externality imposed on school districts, students, and their families when declines in student enrollment numbers intersect with the fixed nature of many school costs. The key contribution of this report is the introduction of a calculator for policymakers, researchers, and advocates to assess the quantitative impact of this fiscal externality for every school district in the country.