GST late fee and interest — what do I owe?

Late fee: Section 47, CGST Act 2017 (plus the same under the State Act), with the caps in Notification No. 19/2021-Central Tax (GSTR-3B) and 20/2021-Central Tax (GSTR-1). If you have only IGST supplies, the same total is payable as IGST. Interest: Section 50(1) at 18% a year, charged on the tax paid in cash when a return is filed late (proviso to Section 50(1)); wrongly availed and utilised credit carries interest under Section 50(3). Returns more than three years past their due date cannot be filed (Sections 37(5) and 39(11)). Rules change — check the latest CBIC notification before you pay.