OBJECTIVES: The objective of this study was to design an app to estimate the budget impact of the adoption of Sofosbuvir, a new antiviral hepatitis C (HCV) therapy to a formulary in the U.S. METHODS: The pharmacy budget impact to a formulary was defined as the difference between the cost "with Sofosbuvir" and the cost "without Sofosbuvir" for the treatment of the eligible chronically infected HCV population. The model takes the following into account: number of HCV infected persons, percent chronically infected, percent diagnosed, percent diagnosed and treated with triple therapy, percent covered by insurance, and drug costs. The app may be used to estimate the budget impact of the adoption of Sofosbuvir to either a commercial or Medicare/Medicaid formulary in each state of the U.S. and in the U.S. overall and may be used with iPad, iPhone, other tablets, laptops, and desktops. RESULTS: In 2013 in the U.S., 4,008,572 persons were estimated to be chronically infected with HCV. At a default of 3% of those diagnosed with chronic HCV being treated with triple therapy, and with Sofosbuvir having 15% of the market share the total pharmacy budget impact was estimated at $11.3 million  annually (before adoption: $1,877 million vs. after adoption: $1,888 million) among those commercially insured in the US. This represents a relative increase of 0.6% ($0.005 per member per month (PMPM)). Among those who are Medicaid/Medicare insured the budget impact was estimated at $5.8 million, also a relative increase of 0.6%. CONCLUSIONS: Based on current treatment practices of HCV infected persons, the budget impact of adding Sofosbuvir to a U.S. commercial or government health plan formulary is low ($0.005 PMPM). The impact of increasing HCV screening and treatment rate and the potential reduction of medical costs due to Sofosbuvir’s >90% sustained virological response will require future studies.