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The Apportionment Problem in Mixed-Use ITC
When a business has both taxable and exempt revenues — a hospital with pharmacy sales, an IT company with both zero-rated exports and domestic 18% billing — every shared input generates ITC that cannot be claimed in full. Rules 42 and 43 of the CGST Rules define the apportionment method. For a complete foundation on how ITC flows before reversal, see what ITC in GST is.
Rule 42: Inputs and Input Services
Rule 42 applies to recurring purchases — raw materials, professional services, software subscriptions, office consumables, electricity. The calculation separates all inward supply ITC into four pools:
- T1: Exclusively used for taxable supplies — fully claimable
- T2: Exclusively used for exempt supplies — fully reversed
- T3: Blocked under Section 17(5) — fully reversed (for detail on what Section 17(5) covers, see blocked ITC under Section 17(5))
- T4: Common ITC used for both taxable and exempt, or for non-business purposes
From T4, two reversals are calculated each month:
- D1 = T4 × (aggregate exempt turnover for the period ÷ aggregate total turnover for the period)
- D2 = T4 × 5% (proxy for non-business use)
The balance of T4 after deducting D1 and D2 is the eligible portion of common ITC.
The assignment of each invoice to T1, T2, T3, or T4 requires intent-of-use tracking — the ERP or purchase register must record what each purchase was used for. Without this tagging, everything defaults to T4 and the full apportionment calculation applies, which often understates eligible ITC.
Rule 43: Capital Goods
Capital goods — machinery, servers, vehicles, lab equipment — receive different treatment because their useful life extends beyond a single return period. GST prescribes a 60-month useful life for capital goods. The ITC on capital goods is credited in full in the month of purchase but apportioned across 60 months for reversal purposes.
Each month, 1/60th of the total capital goods ITC is taken as the base. The reversal formula is:
Monthly Rule 43 reversal = (1/60 of capital goods ITC) × (exempt turnover ÷ total turnover for the period)
The challenge arises with shared infrastructure — a server farm running both taxable SaaS products and exempt e-learning services — where purpose apportionment requires a defensible allocation basis.
Rule 42 vs Rule 43: Key Differences
| Attribute | Rule 42 | Rule 43 |
|---|---|---|
| Applicable to | Inputs and input services | Capital goods |
| Reversal period | Monthly (current period ITC) | 60 months (1/60th per month) |
| Reversal calculation base | Common ITC for the period (T4) | 1/60th of total capital goods ITC |
| Exempt proportion formula | D1 = T4 × (exempt ÷ total turnover) | Monthly base × (exempt ÷ total turnover) |
| GSTR-3B table | Table 4(B)(1) | Table 4(B)(1) (same row) |
| Annual reconciliation | GSTR-9 — actual vs provisional ratio | GSTR-9 — cumulative 60-month tracking |
The Reconciliation Difficulty With Common Inputs
The practical difficulty is not the formula — it is the classification. Office electricity at 18% GST serves every function in the building: taxable product development, exempt interest-bearing lending, and the managing director’s non-business use of the office cafeteria. Each rupee of ITC requires a defensible allocation basis.
GSTN does not validate the allocation basis at filing time. Scrutiny arises during audit, when the department checks whether the exempt proportion in GSTR-3B Table 4(B) is consistent with exempt revenue in GSTR-1. Understating the exempt proportion — and therefore the reversal — is treated as excess ITC claiming, attracting recovery with interest at 18% per annum from the original claim date. GST reconciliation software that computes the exempt ratio from GSTR-1 data automatically eliminates the arithmetic errors that accumulate across 12 manual periods.
Annual Reconciliation in GSTR-9
Monthly Rule 42 reversals use that month’s turnover ratio as a proxy. Over a full year, the monthly ratios may not average to the actual annual ratio — seasonal revenue patterns, large one-off exempt transactions, or export supply variations can all cause drift.
GSTR-9 (annual return) requires the taxpayer to compute the final Rule 42 reversal using the actual annual exempt-to-total turnover ratio and compare it against the sum of 12 monthly GSTR-3B reversals. The differential must either be paid (if the annual calculation shows under-reversal) or can be reclaimed (if over-reversed) in the March GSTR-3B or GSTR-9C reconciliation statement.
For capital goods under Rule 43, GSTR-9 requires cumulative reversal tracking across all 60 months. Assets disposed of before the 60-month period require the remaining unclaimed ITC to be reversed in full at disposal.
The GSTR-2B reconciliation process generates the ITC data that feeds the Rule 42 calculation — if the ITC pool is wrong at the start, the reversal is wrong regardless of formula accuracy. For teams managing Rule 42 and 43 across multiple GSTINs, reconciliation software India that maintains per-GSTIN common ITC pools and auto-applies the monthly apportionment substantially reduces audit exposure. The authoritative text of the rules and GSTR-3B filing instructions are published on the GST portal.