How Section 234 interest is calculated
When you don't pay your taxes on time — whether that's the return itself, or the advance tax installments through the year — the Income Tax Act charges simple interest at 1% per month (or part of a month) on the shortfall, under three separate sections. This tool computes all three, row by row, exactly the way the tax department does.
Section 234A — interest for late filing
If you file your return after the due date, 234A charges 1% per month on the tax that remains unpaid as of the due date — that is, your total assessed tax minus advance tax (and TDS/TCS) already paid, minus any self-assessment tax you paid before the due date. The clock runs from the day after the due date to the actual date of filing, with any part of a month counted as a full month — so filing even one day into a new month adds a full month's interest.
Section 234B — interest for advance tax shortfall
If the advance tax you paid during the year (again including TDS/TCS) is less than 90% of your total assessed tax, Section 234B kicks in. It charges 1% per month on the full shortfall (assessed tax minus advance tax paid), counted from 1 April of the assessment year to the date you pay self-assessment tax, or the date you file your return if you pay closer to filing. Pay 90% or more of your final liability through the year and this section simply doesn't apply — that's the "90% escape clause" worth planning around if a large capital gain or bonus lands late in the financial year.
Section 234C — interest for deferred installments
Even if you eventually pay 90%+ of your tax through the year, Section 234C checks whether you paid the right amount at the right time — cumulatively 15% by Jun 15, 45% by Sep 15, 75% by Dec 15 and 100% by Mar 15. Fall short at any one checkpoint and interest is charged separately for that installment, for 3 months (1 month for the final, Mar 15 installment). There is a built-in relief, though: if you've paid at least 12% by Jun 15 or 36% by Sep 15, the shortfall at that particular checkpoint is forgiven entirely — this recognises that estimating income (and hence tax) that early in the year is genuinely hard. No such relief exists for the Dec 15 or Mar 15 checkpoints.
Rule 119A — interest is on a rounded base
Before applying the 1% rate, the shortfall amount at each step is always rounded down to the nearest ₹100 (Rule 119A) — so ₹5,180 of shortfall is treated as ₹5,100 for interest purposes, never rounded up. This tool applies that rounding automatically at every row.
Section 234F — the late-filing fee (not interest)
Separate from the 234A interest above, filing after the due date also triggers a flat late-filing fee under Section 234F: ₹5,000 if your total income exceeds ₹5,00,000, or ₹1,000 if it's ₹5,00,000 or below. Unlike 234A/B/C, this is a fixed penalty, not interest that grows with how late you are — file one day late or six months late, the fee is the same. This tool adds it to the grand total automatically once the filing date is past the due date.
New section numbers under the Income Tax Act, 2025
The Income Tax Act, 2025 renumbers several provisions. Section 234B is renumbered as Section 424, and Section 234C becomes Section 425, under the new Act. This tool and its explanations use the familiar 234A/B/C/F numbering — the one still used in FY 2025-26 filings and in almost all search queries — but if you see "Section 424" or "Section 425" referenced elsewhere (in the new Act, or in commentary written after its rollout), know that they refer to the same advance-tax-shortfall and deferred-installment interest computed here.
Presumptive taxpayers (Section 44AD/44ADA)
If you file under presumptive taxation, you're spared the quarterly discipline — the entire year's advance tax is due in a single installment of 100% by Mar 15. Toggle "presumptive taxation" above and this tool checks only that one date for Section 234C, ignoring the Jun/Sep/Dec checkpoints entirely.