How do I estimate my income for an ACA subsidy when my earnings are unpredictable?

Marketplace premium tax credits are based on your estimated modified adjusted gross income for the coverage year, not last year's. Make your best honest projection; if you earn more than expected, you may repay some credit at tax time, and if you earn less, you'll get the rest back as a refundable credit. There is no cliff that makes you repay everything as long as you estimate in good faith.

For variable income, update your marketplace application whenever your outlook changes materially so your monthly credit adjusts. Underestimating badly can leave you owing a large repayment. If unsure, estimate slightly high, take a smaller monthly credit, and reconcile on your return via IRS Form 8962.