Three separate sections of the Income Tax Act impose interest for different kinds of delays in your tax compliance, and it's easy to confuse them or miss one entirely when self-assessing your tax position. Section 234A charges interest for filing your return after the due date, calculated on any unpaid tax. Section 234B charges interest if you did not pay at least 90% of your total tax liability as advance tax by 31 March. Section 234C charges interest for deferring your advance tax instalments — even if your total advance tax paid by year-end is sufficient, paying too little in an earlier instalment (June, September, or December) can still attract interest for that specific shortfall.
All three sections apply simple interest at 1% per month (or part of a month — a delay of even a few days counts as a full month), calculated independently, meaning it's entirely possible to owe interest under more than one section simultaneously for the same year.
This calculator estimates your interest liability under all three sections based on your tax details, filing date, and the advance tax instalments you actually paid — giving you a reasonably close approximation of what the tax portal would compute, though minor differences can occur due to rounding conventions.
Yes — 234C looks at whether each individual instalment (June, September, December) met its required percentage, while 234B looks at your total advance tax paid by 31 March. It is possible to be short in an early instalment (234C) yet still meet the 90% threshold by year-end but not avoid the 234C charge.
No — Section 234A interest is calculated only on unpaid tax as of the due date. If you have no outstanding tax liability, filing late does not attract 234A interest (though the Section 234F late fee may still apply).
Under all three sections, any part of a month is treated as a full month for interest calculation purposes — so being even one day late into a new month adds a full month's interest.
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