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Invoice-level attribution comes first; turnover-based allocation applies only to the common credit left after direct classifications. Under India’s Central Goods and Services Tax (CGST) framework, Rule 42 requires a registered person to remove credit exclusively linked to non-business use, exempt supplies and blocked credits, then identify credit exclusively linked to taxable or zero-rated supplies. The residual common credit is apportioned to exempt supplies using the exempt-supply-to-total-turnover ratio. Applying the ratio to the entire invoice pool skips the statutory sequence and can overstate or understate eligible input tax credit (ITC).
The calculations below follow the CGST Rules text amended through 1 January 2022. That PDF is not a consolidated 2026 edition, and the CBIC Act and Rules pages should be checked for amendments effective in the tax period being closed.
What is the difference between invoice-level attribution and turnover-based allocation?
Invoice-level attribution identifies the actual use of a particular input or input service. Turnover-based allocation uses a statutory fraction to apportion only the residual common credit that cannot be linked exclusively to one use.
| Issue | Invoice-level attribution | Turnover-based allocation |
|---|---|---|
| What it examines | The purpose of each invoice or clearly segregated amount | The proportion of exempt supplies in total turnover |
| Where it sits in Rule 42 | Used to identify T1, T2, T3 and T4 before the common-credit calculation | Used for D1 after common credit C2 has been determined |
| Typical evidence | Invoice descriptions, contracts, cost-centre or project mapping, and usage records | Period turnover records supporting exempt supplies (E) and total turnover (F) |
| Main risk | Misclassifying a mixed or shared cost as exclusive | Using an incomplete turnover denominator or applying the ratio to credit that should have been directly attributed |
Section 17 of the CGST Act supplies the governing principle: when goods or services are used partly for non-business purposes or partly for exempt supplies, credit is restricted to the portion attributable to business and taxable supplies, including zero-rated supplies. Rule 42 operationalises that principle for inputs and input services.
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A practical decision test
Ask three questions for every material invoice. First, can the cost be connected exclusively to non-business activity, exempt output, blocked credit, or taxable and zero-rated output? Second, is that conclusion supported by the invoice and operational records rather than a label in the accounting system? Third, if the answer is no, is the cost genuinely shared so that it belongs in C2? This test keeps a convenient turnover percentage from replacing the statutory classification exercise.
Where one invoice contains separable components, retain the line-level or allocation working that supports the split. If the amount cannot be reliably separated, classify the undivided balance as common instead of forcing an exclusive result merely to reduce the eventual reversal.
How do I calculate common ITC under Rule 42?
Perform the steps in order for each tax period and separately by tax type. The symbols below are the statutory ones used in Rule 42.
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- Determine T. Start with the total input-tax amount for the period covered by the Rule 42 computation.
- Remove T1. Identify credit attributable exclusively to non-business purposes.
- Remove T2. Identify credit attributable exclusively to exempt supplies.
- Remove T3. Remove blocked credit under section 17(5).
- Calculate C1. C1 = T − (T1 + T2 + T3).
- Identify T4. From C1, identify credit attributable exclusively to taxable supplies, including zero-rated supplies. This is an invoice-level classification, not a turnover allocation.
- Calculate common credit C2. C2 = C1 − T4. C2 is the pool that remains shared between taxable and exempt supplies (and may also contain the mixed non-business component addressed by D2).
- Calculate D1 for exempt supplies. D1 = (E ÷ F) × C2, where E is aggregate exempt supplies for the period and F is total turnover for the period. If current-period turnover or the required information is unavailable, Rule 42 provides a last-available-period fallback.
- Calculate D2 for mixed non-business use. Where common inputs or services are partly for non-business use, D2 is 5% of C2, subject to the rule’s proviso for amounts identified and segregated at invoice level.
- Determine C3. C3 = C2 − (D1 + D2). C3 is the eligible balance from the common pool after the Rule 42 reductions.
The rule requires separate computation by tax type and an annual final calculation. Any difference between periodic amounts and the annual result is adjusted through output liability or credit in the manner prescribed for the relevant period.
Illustrative calculation using the statutory sequence
The following numbers are hypothetical and demonstrate the mechanics rather than a taxpayer’s result. Assume T is ₹100,000; T1 is ₹5,000; T2 is ₹10,000; T3 is ₹15,000; and T4 is ₹30,000.
- C1 = ₹100,000 − (₹5,000 + ₹10,000 + ₹15,000) = ₹70,000.
- C2 = ₹70,000 − ₹30,000 = ₹40,000.
- If exempt supplies E are ₹200,000 and total turnover F is ₹1,000,000, D1 = (₹200,000 ÷ ₹1,000,000) × ₹40,000 = ₹8,000.
- If the mixed non-business component is subject to the five-percent rule, D2 = 5% × ₹40,000 = ₹2,000.
- C3 = ₹40,000 − (₹8,000 + ₹2,000) = ₹30,000.
If a portion of mixed non-business use has been identified and segregated at invoice level in accordance with the proviso, that amount is not simply left inside the undifferentiated pool for a second five-percent charge. The records must show what was identified and how it was treated.
How the sequence changes the result
Suppose the same ₹100,000 of starting credit were allocated by applying the 20% exempt ratio directly to T. That shortcut would produce a ₹20,000 reduction before considering blocked credit, T1, T2 or T4. The prescribed sequence instead produces D1 of ₹8,000 in the illustration because the ratio is applied to ₹40,000 of C2. The figures demonstrate why a lower or higher outcome is not, by itself, evidence that either method is correct; the question is whether the statutory pool was built correctly.
When should I attribute ITC invoice by invoice?
Use invoice-level attribution whenever the document and underlying use establish that the credit belongs exclusively to one statutory category. This prevents the E/F fraction from distorting credit that is already identifiable.
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Credit exclusively for non-business use (T1)
Exclude the amount before calculating common credit. A purchase used solely outside the registered person’s business cannot be rescued by a favorable turnover ratio.
Credit exclusively for exempt supplies (T2)
Credit tied solely to exempt output belongs in T2 and is removed before D1. Do not count it again in the common pool.
Blocked credit (T3)
Section 17(5) restrictions are a separate exclusion. Even if an invoice relates to a taxable business activity, a blocked category remains outside eligible credit.
Credit exclusively for taxable or zero-rated supplies (T4)
Credit used solely for taxable supplies, including zero-rated supplies, is identified after C1 and removed from the common pool. It remains eligible rather than being reduced by the exempt-turnover fraction.
Mixed invoices and shared services
Invoices supporting several business lines, premises or output categories may not permit exclusive attribution. Unless the amount can be reliably segregated, it remains in C2 and is subject to the Rule 42 calculations applicable to common use.
What evidence makes an exclusive classification credible?
Retain more than the tax invoice. A purchase order or service contract can show the agreed deliverable; a project code or cost-centre record can show where it was consumed; usage logs, deployment records or a written business-purpose note can explain why the cost is exclusive. For a shared service, preserve the basis used to conclude that no reliable split was possible. The same evidence should be available to a reviewer who did not make the original coding decision.
When does the exempt-turnover ratio apply to common credit?
The E/F ratio applies to C2, the residual common credit after the direct exclusions and the T4 classification. E is aggregate exempt supplies in the period; F is the registered person’s total turnover in that period, with the specified last-available-period fallback when current information is unavailable.
This is an allocation of one registered person’s shared credit between exempt and other supplies. It is not permission to apply a percentage to T before identifying direct-use invoices. The quality of the result therefore depends on both sides of the fraction: exempt-supply classification in E and a complete, consistently defined denominator in F.
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What belongs in the working papers
- A period-wise schedule of exempt supplies used in E.
- The total-turnover computation supporting F, including the period used when the fallback applies.
- A reconciliation from T through T1, T2, T3, C1, T4, C2, D1, D2 and C3.
- Separate schedules for the relevant tax types.
- Explanations for changes in turnover data between provisional and annual calculations.
How should the denominator be controlled?
Freeze the turnover extract used in each period and identify its source ledger or return schedule. Document whether the figures are current-period amounts or the permitted last-available-period figures. When a later period supplies better information, retain both the provisional calculation and the replacement annual calculation rather than overwriting the original file. This creates an audit trail for why D1 changed.
How does the five-percent non-business component work?
Rule 42 specifies D2 as 5% of C2 where common inputs or input services are partly used for non-business purposes, subject to its proviso for amounts identified and segregated at invoice level. The five percent is a statutory calculation parameter, not an industry average, survey result or measured error rate.
Apply the provision only after C2 is known. A business should distinguish invoices demonstrably used for non-business purposes from genuinely shared invoices, document any segregation, and preserve the reasoning used for the selected treatment. Treating every mixed-looking invoice as automatically subject to five percent can produce a different result from one supported by invoice-level identification.
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Periodic calculations use the information available for each tax period, but Rule 42 also calls for an annual final calculation. The annual result can differ when turnover figures are finalised, invoices are reclassified, or the provisional common-credit pool changes.
At year-end, recalculate the prescribed components using the annual figures, compare the result with the amounts dealt with during the year, and make the required adjustment through output liability or credit as prescribed. Keep a bridge that shows every change; an unexplained year-end adjustment is difficult to defend even when the final arithmetic is correct.
Annual close checklist
- Lock the final invoice population and remove duplicates or cancelled documents.
- Reconfirm T1, T2, T3 and T4 classifications, especially invoices re-coded during the year.
- Recompute E and F using the annual information and note any fallback used during interim periods.
- Recalculate D1, D2 and C3 separately by tax type.
- Compare annual amounts with the total of periodic amounts and quantify each variance.
- Post the prescribed liability or credit adjustment and retain approval of the reconciliation.
How are common capital goods treated?
Common capital goods are governed by Rule 43, not the Rule 42 monthly input-and-input-service pool. Rule 43 tracks common capital-goods credit over a deemed five-year, or 60-month, useful-life mechanism and applies the exempt-supply-to-total-turnover ratio to the relevant residual credit amount.
Do not put a capital-good invoice into the Rule 42 C2 schedule merely because it is shared. Maintain a separate Rule 43 register showing the credit, the month placed in use, the remaining useful-life period and the exempt-supply attribution calculated under that rule. The five-year period is a statutory deemed life in this mechanism, not a depreciation estimate selected by the taxpayer.
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No. Input Service Distributor (ISD) provisions can use recipient-turnover proportions to distribute input-service credit among recipients. Rule 42, by contrast, apportions one registered person’s residual common credit between taxable and exempt supplies.
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These mechanisms answer different questions and require different data: ISD distribution concerns which recipient receives credit, while Rule 42 concerns how a recipient’s own credit is classified and restricted. Check the current section 20 text before implementing detailed ISD calculations, because the applicable statutory wording must match the tax period.
Which workflow is more reliable for a finance or GST team?
The strongest workflow combines both methods in sequence rather than choosing one as an alternative to the other.
- Build the invoice register. Capture supplier, invoice date, tax type, amount, business purpose, project or cost centre, and output-supply mapping.
- Apply direct-use flags. Mark T1, T2, T3 and T4 only where the supporting information establishes exclusive attribution.
- Lock the common pool. Reconcile C1 and T4 so that C2 contains only genuinely residual credit.
- Load turnover data. Document E and F for the period and identify whether the last-available-period fallback was used.
- Calculate D1 and D2. Retain the formula, source schedules and any invoice-level segregation relevant to the D2 proviso.
- Review exceptions. Investigate negative or unusual movements, sudden changes in the exempt ratio, and reclassifications between periods.
- Perform the annual reconciliation. Compare provisional totals with the annual final calculation and post the prescribed adjustment.
- Maintain a separate capital-goods schedule. Keep Rule 43 calculations outside the Rule 42 pool.
For high-volume environments, accounting or GST software can reduce spreadsheet handling, but the statutory classification still needs human review. A system that merely multiplies total ITC by an exempt-turnover percentage does not reproduce the Rule 42 sequence.
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Spreadsheet or software?
A spreadsheet can work when invoice volume is manageable and each formula, source extract and reviewer sign-off is controlled. Larger teams may prefer GST accounting or return-preparation software with invoice tagging, turnover imports, tax-type separation and annual reconciliation. No product should be assumed to support the required logic without checking its current features. In either setup, preserve an export of the underlying invoice list and the calculation version used for filing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What mistakes most often change the result?
- Applying E/F to T. The ratio belongs to C2, not the starting total.
- Skipping T4. Exclusively taxable or zero-rated credit should not be diluted as common credit.
- Ignoring blocked credit. Section 17(5) exclusions are made before the common-pool calculation.
- Using an unsupported denominator. F must reflect the rule’s total-turnover requirement and the permitted fallback where applicable.
- Double-counting mixed non-business use. Check the five-percent proviso against amounts already identified and segregated at invoice level.
- Combining capital goods with inputs and services. Rule 43 has its own useful-life and residual-credit treatment.
- Skipping the annual true-up. Monthly or periodic figures are not the final answer when the annual calculation changes the result.
- Confusing ISD distribution with apportionment. Recipient allocation and exempt-supply restriction are separate statutory operations.
- Changing classifications without a version trail. A reclassification should show the original treatment, the new evidence and the period in which the correction was made.
- Rounding too early. Keep the underlying amounts and ratio calculation precise in the working paper, then apply the prescribed reporting treatment to the final result.
What should be verified before filing or advising?
The CBIC Rule 42 and Rule 43 text reviewed for these mechanics is amended through 1 January 2022, while the current online rules page and the Act page may reflect different consolidation dates. Before filing, confirm amendments, notifications, circulars and return instructions in force for the relevant tax period. This article explains the statutory sequence; it is not a taxpayer-specific legal opinion or computation.
What reference material or tools are worth keeping?
A current printed GST bare act or rules reference can be useful when reviewers need the statutory wording beside the calculation, but check that the edition reflects amendments applicable to the period. The CBIC publication remains the authority. For recurring, high-volume work, software in the GST accounting or return-preparation category may reduce manual invoice and turnover handling; verify the product’s current capabilities and program terms rather than assuming that every system performs Rule 42 and Rule 43 separately.
Frequently Asked Questions
What records best support an invoice-level ITC classification?
Keep the tax invoice together with the purchase order or contract, a business-purpose note, the project or cost-centre assignment, and evidence connecting the cost to the relevant output supply. A short written rationale for an exclusive or shared-use decision makes later reclassification and annual reconciliation more traceable.
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Use turnover information that meets the Rule 42 requirement for the period, or apply the rule’s last-available-period fallback when current information is unavailable. Retain the source ledger or return schedules and replace provisional figures in the annual calculation when final data is available.
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Does a zero-rated supply increase the exempt proportion?
Zero-rated supplies are included with taxable supplies when identifying credit exclusively attributable to those supplies. They are not treated as exempt merely because the tax charged on the outward supply is zero.
Why does a capital-good register need a separate useful-life schedule?
Rule 43 spreads common capital-goods credit across a deemed five-year (60-month) period and applies its own residual-credit calculation. A one-month Rule 42 pool cannot show the remaining life and monthly attribution required for that treatment.
Should every mixed-use invoice receive the five-percent reduction?
No. First determine whether any non-business portion can be identified and segregated at invoice level under the Rule 42 proviso. The five-percent D2 calculation applies to the common pool after C2 is determined, and the working papers should explain the treatment of segregated amounts.
What if current-period turnover is not available when the calculation is prepared?
Use the last-available-period fallback provided by Rule 42, identify the period used and retain the supporting turnover schedule. Revisit the figure in the annual final calculation when current information is complete.
Can software decide whether an invoice is T1, T2, T3 or T4?
Software can capture tags, documents and formulas, but the exclusive-use conclusion requires review of the underlying purpose and evidence. A tool that only applies an exempt-turnover percentage to total ITC does not implement the statutory sequence.
How should an annual difference be explained to a reviewer?
Prepare a bridge from the periodic totals to the annual result, showing changes in invoice classifications, E or F turnover figures, D1, D2 and the resulting C3. Record the prescribed output-liability or credit adjustment and retain reviewer approval.
The Bottom Line
Use both methods, in order: classify directly attributable and blocked credit at invoice level, calculate C2, then apply the E/F ratio and any Rule 42 five-percent component to the residual pool. Complete the annual true-up, keep Rule 43 capital goods separate, and do not confuse ISD recipient distribution with Rule 42 apportionment.
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