Reversal of ITC: Decoding Rule 42 and Rule 43 under GST

The fundamental principle of Input Tax Credit (ITC) under the Goods and Services Tax (GST) is simple: you can only claim credit on purchases used to make taxable sales.

If your business exclusively sells agricultural produce that is wholly exempt from GST, you cannot claim a single rupee of ITC on the rent, electricity, or trucks you buy. Conversely, if you exclusively sell laptops (18% GST), you can claim full ITC on all eligible business expenses.

But what happens if your business does both? What if you run a supermarket that sells both taxable packed goods and exempt fresh vegetables, using the same delivery truck, the same air conditioning, and the same chartered accountant?

You cannot claim 100% of the ITC on those common expenses, nor do you lose 100%. You must perform a legally mandated proportional calculation to reverse the credit attributable to the exempt sales. This calculation is governed by two historically complex rules: Rule 42 (for normal inputs) and Rule 43 (for capital goods).

This guide breaks down the mechanics of Rule 42 and Rule 43 for the 2026 Assessment Year.

Understanding Rule 42: Reversal for Inputs and Input Services

Rule 42 deals with the ITC on regular operating expenses—rent, internet bills, audit fees, packing materials, etc.—that are used commonly for making both taxable and exempt supplies, or for both business and personal use.

The 4-Step Rule 42 Calculation

1. Calculate the Total Common Credit (C2): Identify the total ITC on the invoice in question. Subtract any ITC exclusively meant for personal use, exclusively meant for exempt supplies, or blocked under Section 17(5). Subtract any ITC exclusively meant for taxable supplies. What remains is the "Common Credit".

2. Calculate the Exempt Proportion (D1): This is the core formula: D1 = (Exempt Turnover / Total Turnover of the State) × Common Credit (C2) This figure represents the ITC attributable to your tax-free sales. It is essentially the "unearned" credit.

3. Calculate the Personal Use Proportion (D2): If the common input service (like a mobile phone bill) is also used partly for the director's personal use, flatly calculate 5% of the Common Credit (C2) as D2.

4. The Final Reversal: You must reverse (pay back or reduce from your ledger) the total of D1 + D2. You declare this reversal in Table 4(B)(1) of your monthly GSTR-3B.

Legal Reference

Relevant Law: Section 17(1) and (2) of the CGST Act restrict ITC to business and taxable supplies respectively. Rule 42 prescribes the manner of determination of ITC in respect of inputs and input services and reversal thereof. Rule 43 does the same for capital goods.

Understanding Rule 43: Reversal for Capital Goods

Capital goods are big-ticket, long-term assets like factory machinery, delivery trucks, or heavy computers.

Unlike a monthly electricity bill, a ₹50 Lakh machine provides value over several years. Therefore, if you use a machine to manufacture both taxable and exempt goods, the government doesn't demand you do the complex proportional reversal all at once. Rule 43 spreads the calculation over the presumed useful life of the asset.

The Rule 43 Mechanics

Under GST law, the useful life of any capital good is statutorily presumed to be 5 years (60 months).

1. Claim Full ITC Upfront: When you buy a common machine, you claim the entire ITC in the month of purchase. This helps your immediate working capital.
2. Calculate the Common Credit (A): Identify the ITC on this specific machine.
3. Calculate the Monthly ITC (Tm): Divide the Common Credit (A) by 60. This is the ITC "consumed" by the machine each month.
4. Aggregate Monthly Common Credit (Tr): Add up the (Tm) of all the different common machines you own.
5. The Monthly Reversal (Te): Every single month for the next 5 years, you must calculate: Te = (Exempt Turnover in that Month / Total Turnover in that Month) × Tr

You must reverse this specific "Te" amount every month in your GSTR-3B and pay standard 18% interest on the reversed amount to compensate the government for letting you claim the full ITC upfront on day one.

The Year-End Rule 42 Reconciliation (Crucial Step)

The monthly Rule 42 calculation (D1) is based on that specific month's turnover ratio. However, turnover fluctuates wildly. A supermarket might sell tons of exempt vegetables in winter but predominantly taxable processed foods in summer.

Therefore, the law mandates a true-up reconciliation at the end of the financial year.

Before the Annual Return (GSTR-9) deadline, you must recalculate Rule 42 for the entire financial year using the annual aggregate turnover figures.

• If the annual recalculation shows you reversed too much ITC during the monthly GSTR-3Bs, you can reclaim the difference.
• If the annual recalculation shows you reversed too little, you must immediately reverse the shortfall in your next GSTR-3B and pay 18% penal interest.

Common Mistakes Beginners Make

1. Ignoring Interest on Rule 43 Reversals: Many accountants mathematically calculate the monthly Capital Goods reversal correctly but forget that the portal does not auto-calculate the mandated 18% interest. They must manually add the interest component every month in GSTR-3B.
2. Including Zero-Rated Export Turnover as 'Exempt': Direct exports or supplies to SEZs are "zero-rated" (0% tax), but they are legally treated as Taxable Supplies for ITC purposes. Businesses mistakenly plug export turnover into the "Exempt Turnover" numerator of the Rule 42 formula, unnecessarily losing massive amounts of legitimate ITC.
3. Failing to do the Year-End Re-calculation: Skipping the mandatory annual true-up calculation. When a departmental audit occurs 3 years later, the GST intelligence officers will run the annual formula. If a shortfall is found, the 18% interest compounding over 3 years creates a ruinous tax demand.

Conclusion

Operating a mixed-supply business under GST requires a heavily segmented accounting system. By meticulously tagging incoming vendor invoices into three distinct buckets—"exclusively taxable," "exclusively exempt," and "common"—from day one, and strictly running the Rule 42 annual reconciliation before filing the GSTR-9, businesses can confidently optimize their Input Tax Credit without inviting aggressive audit recovery notices in 2026.