Question: I filed my GST return and paid my tax liability of ₹85,000 exactly 40 days after the due date. Nothing else was outstanding. Roughly how much interest will I owe for this delay?
Answer: Approximately ₹1,677.

Supporting evidence:
title: taxinformation.cbic.gov.in/section50_v1.00.html
quote: Every person who is liable to pay
tax in accordance with the provisions of this Act or the rules made
thereunder, but fails to pay the tax or any part thereof to the
Government within the period prescribed, shall for the period for which
the tax or any part thereof remains unpaid, pay, on his own, interest at
such rate, not exceeding eighteen per cent., as may be notified by the
Government on the recommendations of the Council
context: tax in accordance with the provisions of this Act or the rules made
thereunder, but fails to pay the tax or any part thereof to the
Government within the period prescribed, shall for the period for which
the tax or any part thereof remains unpaid, pay, on his own, interest at
such rate, not exceeding eighteen per cent., as may be notified by the
Government on the recommendations of the Council:
1
[
Provided
that
the interest on tax payable in respect of supplies made during a tax
period and declared in the return for the said period furnished after
the due date in accordance with the provisions of section 39, except
where such return is furnished after commencement of any proceedings
under
section
73
or
section
74
3
[
or
section 74A]
in respect of the said period, shall be
levied on that portion of the tax that is paid by debiting the
electronic cash ledger.]
(2) The interest under sub-section (1)
shall be calculated, in such manner as may be prescribed, from the day
succeeding the day on which such tax was due to be paid.
2
[(3)
Where the input tax credit has been wrongly availed and utilised, the
registered person shall pay interest on such input tax credit wrongly
availed and utilised, at such rate not exceeding twenty-four per cent.
as may be notified by the Government, on the recommendations of the
Council, and the interest shall be calculated, in such manner as may be
prescribed]
*Enforced w.e.f. 1st July, 2017.

1. Substituted (w.e.f. 1st July, 2017) by s. 112 of The Finance Act, 2021 (No. 13 of 2021) dated 28th March, 2021 for "[Provided that the interest on tax payable in respect of supplies made during a tax period and declared in the return for the said period furnished after the due date in accordance with the provisions of section 39, except where such return is furnished after commencement of any proceedings under section 73 or section 74 in respect of the said period, shall be levied on that portion of the tax that is paid by debiting the electronic cash ledger.]" Brought into force on 1st June, 2021 vide Notification No. 16/2021-Central Tax dated 1st June, 2021.
2. Substituted (w.e.f. 1st July, 2017 ) by s. 111 of The Finance Act 2022 (No. 06 of 2022) - brought into force w.e.f 05-07-2022 vide Notification No. 9/2022-C.T , dated 05-07-2022 .
Inserted
by section 126
of The Finance Act (No. 2) Act, 2024 No. 15 of 2024