How to Use ChatGPT and Claude to Calculate GST Interest: A Step-by-Step Guide for CAs

A general AI assistant can compute GST interest reliably if you paste in the rule, the dates and the amount, and make it show its working. It cannot be trusted to remember the section, the rate or the day-count convention on its own, so you supply those and check the answer against a second source.

This guide covers Section 50 of the CGST Act: interest on late payment of tax (18% a year) and interest on input tax credit that was wrongly availed and utilised (24% a year, computed under Rule 88B). The steps work the same in ChatGPT, Claude or Gemini.

What can ChatGPT or Claude do here, and what should you never delegate?

Good use of the assistant	Never delegate
Applying a formula you pasted to dates and amounts	Deciding which limb of Section 50 applies to a client's facts
Counting days between two dates (then you check)	Deciding whether ITC was "wrongly availed" or "utilised"
Building a working table or a spreadsheet formula	Recalling rates and section numbers from memory
Explaining the working in plain words for a file note	Sending client identifiers to a public tool
Spotting a missing input in your data	The final figure you sign off

The pattern is simple: you paste the rule text, the assistant applies it, you verify.

Step 1: Set up a safe workspace

Before any prompt, remove everything that identifies the taxpayer.

• No client name, GSTIN, PAN, invoice numbers or addresses. Use "Client A" and round or synthetic amounts where the exact figure is not needed for the arithmetic.
• Use a paid or business plan with training on your data switched off, and follow your firm's policy on which tools are permitted.
• Keep one chat per computation so an earlier assumption does not leak into the next case.
• Save the prompt templates below in one place. Our DPDP-safe prompt template library for CA firms covers anonymisation in more depth.

Step 2: Identify which interest you are computing

Decide this yourself before opening the assistant.

Situation	Provision	Rate	Interest runs on	Period
Tax paid late (return filed late or tax paid after the due date)	Section 50(1)	18% p.a.	Tax paid through the cash ledger, not the part covered by ITC	Day after the due date to the date of payment
ITC wrongly availed and utilised	Section 50(3), Rule 88B	24% p.a.	The wrongly utilised ITC	Date of utilisation to the date of reversal (or payment of tax)

On the first row, the net cash-ledger basis is explained in our post on GST and TDS late fee and interest side by side. On the second row, interest attaches only where the credit was both availed wrongly and utilised; credit that was availed wrongly but never used does not attract it. Rule 88B treats the credit as utilised to the extent the electronic credit ledger balance falls below the amount wrongly availed.

Step 3: Paste the rule and your inputs (late payment, 18%)

Copy this prompt, replacing the bracketed items with anonymised facts.

You are helping a chartered accountant compute GST interest. Do not use
any rate or rule from memory. Use ONLY the rule I give you.

RULE: Interest on delayed payment of tax under Section 50(1) CGST Act is
18% per annum on the tax that was payable through the electronic cash
ledger. Interest = tax x 18% x number of days / 365.

DAY COUNT CONVENTION: count from the day AFTER the due date up to and
INCLUDING the date of payment. Use the actual number of days and a
365-day year.

INPUTS:
- Tax payable in cash (after ITC set-off): Rs [amount]
- Due date: [dd Mon yyyy]
- Date of payment: [dd Mon yyyy]

TASK:
1. List the dates you are counting from and to.
2. Count the days month by month and show each month's days.
3. Show the multiplication step by step.
4. Round the final figure to the nearest rupee.
5. List every assumption you made. If any input is missing or
   ambiguous, ask me instead of assuming.

Step 4: Paste the rule and your inputs (ITC, 24%)

You are helping a chartered accountant compute interest on ITC wrongly
availed and utilised. Use ONLY the rule I give you.

RULE: Under Section 50(3) CGST Act read with Rule 88B, interest is
24% per annum on the input tax credit wrongly availed and utilised, for
the period from the date of utilisation to the date of reversal of that
credit (or payment of tax). Interest = amount x 24% x days / 365.

DAY COUNT CONVENTION: exclude the date of utilisation, include the date
of reversal. Show the count both ways (excluding and including the
utilisation date) so I can see the difference.

INPUTS:
- ITC wrongly availed AND utilised: Rs [amount]
- Date of utilisation: [dd Mon yyyy]
- Date of reversal: [dd Mon yyyy]

TASK: Show dates, the month-by-month day count, the multiplication,
the rounded result, and list all assumptions. If the ledger balance or
the utilisation date is unclear, ask me.

Rule 88B speaks of a period "from the date of utilisation", and it does not spell out whether that day itself is counted, which is why the prompt asks for both counts. Decide your firm's convention and record it in the file.

Step 5: Worked example, checked by hand

Example A - late payment, 18%. A client has net cash tax of Rs 2,40,000 for August 2026. The due date is 20 August 2026 and it was paid on 14 September 2026.

• Days: 21 to 31 August is 11 days; 1 to 14 September is 14 days. Total 25 days.
• Interest = 2,40,000 x 18% x 25 / 365. Stepwise: 2,40,000 x 0.18 = 43,200; x 25 = 10,80,000; / 365 = 2,958.90.
• Interest: Rs 2,959.

Example B - wrongly utilised ITC, 24%. Rs 1,00,000 of ITC was wrongly availed, utilised on 10 October 2025 and reversed on 15 January 2026.

• Days excluding 10 October: 11 to 31 October is 21; November 30; December 31; 1 to 15 January is 15. Total 97 days.
• Interest = 1,00,000 x 24% x 97 / 365 = 24,000 x 97 = 23,28,000; / 365 = 6,378.08.
• Interest: Rs 6,378. If you also counted the utilisation day (98 days), it becomes Rs 6,444, a Rs 66 difference from one convention.

Your assistant should land on the same numbers. A good response is illustrative only and varies run to run; what matters is that its day count and multiplication match yours line by line. If it does not, ask it to redo the count month by month, then check again.

Where AI goes wrong

1. Day counts. Models often miscount across month ends, or include both boundary dates. Always demand the month-by-month count and recount one month yourself.
2. Gross versus net tax basis. If you paste the full liability from GSTR-3B, the assistant will charge 18% on the whole amount. Interest under Section 50(1) is on the cash-ledger portion, so give it that figure, and say so in the prompt.
3. Outdated or remembered rates and sections. Models mix up 18%, 24% and older provisions, or cite pre-amendment wording. This is why the rule text comes from you.
4. Silent assumptions. A 360-day year, a 30-day month, or "date of filing" treated as "date of payment". The "list every assumption" line in the prompt exists to expose these.
5. The wrong limb. The assistant will happily compute 24% on ITC that was only availed and never utilised. Whether interest applies at all is your judgement.

Verify in 60 seconds

1. Run the same inputs through CORAA's free GST interest calculator.
2. Compare the day count and the rupee figure with the assistant's answer.
3. If they differ, trust neither until you have counted the days on a calendar.
4. For the return late fee that travels with late filing, use the GST late fee calculator; interest and late fee are separate charges.

ChatGPT, Claude and Gemini: practical tips

• All three can read a small CSV or spreadsheet and show the calculation steps; ask for the formula used in each column so you can copy it into Excel.
• Save the prompts above as a reusable role prompt or custom instructions where your tool offers that (for example, project or custom-instruction settings), so you do not retype the conventions.
• Whichever tool you use, ask it to state what it could not verify. Treat that list as your review checklist.
• Do not rely on any tool's built-in knowledge of the law; the pasted rule is the source.

Frequently asked questions

Can ChatGPT or Claude calculate GST interest accurately?

Yes for the arithmetic, if you give it the rule, rate, dates and amount and demand a shown working. No for deciding which provision applies or recalling the current rate unaided.

Is GST interest charged on the full tax or only on the cash portion?

Under Section 50(1) it is charged on the portion of tax paid through the electronic cash ledger, not on the part set off against ITC. Feed the assistant the cash figure.

From which date do I count the days?

For late payment, from the day after the due date to the date of payment. State this in the prompt so the assistant does not choose its own convention.

When does the 24% rate apply?

Under Section 50(3) where ITC was wrongly availed and utilised, calculated under Rule 88B from the date of utilisation to the date of reversal or payment of tax. ITC that was availed but not utilised does not attract this interest.

Is it safe to paste client data into an AI tool?

Not identifiable data. Use anonymised or synthetic figures and your firm's approved tool with data-training off.

Did the Finance Act 2025 change these rules?

We could not confirm any change to Section 50 in the Finance Act 2025 GST amendments we reviewed, so do not assume one. Check the current text of Section 50 and Rule 88B on the CBIC website before pasting them into a prompt.

Where CORAA fits

The steps above make a general assistant usable for one-off calculations. For audit work that has to be repeatable and traceable, see how CORAA's deterministic core is built so the same inputs produce the same output every time, then start a trial.

Related: GST and TDS late fee and interest side by side.

Last reviewed: 26 September 2026