{"id":"estimated-tax-penalty","summary":"IRS underpayment interest per quarter at the published rates (Form 2210 chart method).","method":"POST","inputSchema":{"type":"object","properties":{"requiredAnnual":{"type":"number","description":"Required annual estimated payment"},"paidPerQuarter":{"type":"array","items":{"type":"number"},"description":"Four amounts paid by Apr 15, Jun 15, Sep 15 and Jan 15"}},"required":["requiredAnnual","paidPerQuarter"]},"exampleInput":{"requiredAnnual":20000,"paidPerQuarter":[0,5000,5000,5000]},"exampleOutput":{"totalPenalty":339.45205479452056,"periods":[{"dueDate":"2026-04-15","underpayment":5000,"days":61,"interest":50.136986301369866},{"dueDate":"2026-06-15","underpayment":5000,"days":92,"interest":86.02739726027397},{"dueDate":"2026-09-15","underpayment":5000,"days":122,"interest":116.98630136986303},{"dueDate":"2027-01-15","underpayment":5000,"days":90,"interest":86.3013698630137}],"requiredPerQuarter":5000,"totalUnderpaid":5000,"taxYear":2026,"rates":[{"from":"2025-01-01","pct":7},{"from":"2026-04-01","pct":6},{"from":"2026-07-01","pct":7}]}}