Amount (in billions of U.S. dollars) Americans plan to spend celebrating St. Patrick’s Day this year according to the National Retail Federation. That’s an increase from last year’s $5.3 billion. Over 149 million U.S. adults are expected to spend an average of just under $40 per person, up from last year’s $38. The holiday is most popular among individuals 18-24 years old. And clearly being Irish isn’t necessary.