The penalty you'd owe for paying late.

Both penalise missed advance tax, but they cover different periods. 234C is per-instalment interest for the 3 months immediately following a missed quarterly date (only 1 month for Q4). 234B is interest from 1 April (after FY-end) until you actually pay — applies only if you paid less than 90% of your assessed tax by 31 March. You can owe both at once: 234C for each quarter you missed plus 234B for the balance you carry past FY-end.

No. Section 211(b) gives 44ADA and 44AD assessees a single instalment due 15 March, not four. So 234C only applies if you paid less than 100% of your assessed tax by 15 March — and only 1 month of interest, not 3. This is the “presumptive simplification” most freelancer tax tools miss.

For non-44ADA assessees, Q1 (15 June) requires 15% paid and Q2 (15 September) requires 45% paid. But 234C only kicks in if you paid less than 12% (80% of 15%) by Q1 or less than 36% (80% of 45%) by Q2. So if you paid exactly 12% by 15 June, you skip the Q1 penalty even though the official target was 15%. Q3 and Q4 have no such tolerance — thresholds are exact at 75% and 100%.

They’re separate. 234B is interest (1% per month) on unpaid advance tax — keeps accruing as long as the tax is unpaid. 234F is a flat fee for filing your ITR late: ₹5,000 if filed after the due date (₹1,000 if total income ≤ ₹5L). You can owe both if you both underpaid advance tax AND filed late.

No. 234C is determined by what you paid by each instalment date — not when you file. If you paid zero advance tax through the year and then dump it all in your ITR filed by 31 March, you’ve still missed every instalment date and still owe 234C on each shortfall. The only escape is paying advance tax on time through the year.