Input tax adjustment and apportionment

Sales tax looks like a tax on sales, but it is really a tax on value added, and the mechanism that makes it so is input tax adjustment. Understanding it is the difference between paying tax on your whole selling price and paying it only on the margin you add. This guide explains the output-minus-input mechanism, what input tax you cannot claim, how to apportion when purchases serve mixed supplies, and what happens when input tax exceeds output tax.

The output-minus-input mechanism

Every registered person does the same monthly calculation:

1. Total output tax: the sales tax charged to customers on taxable supplies in the period.
2. Total admissible input tax: the sales tax paid to suppliers on purchases in the period, to the extent it is claimable.
3. Net payable: output tax minus admissible input tax. If output exceeds input, the difference is paid; if input exceeds output, the excess generally carries forward.

Because each business in the chain remits tax only on its own added value, the tax accumulates down the supply chain to the final consumer, who bears it in full with no onward sale to offset against. This is why sales tax is described as a consumption tax collected in stages, and why the input-tax claim is the heart of the system rather than an optional extra.

Inadmissible input tax

Not every rupee of input tax can be claimed. The Sales Tax Act makes input tax inadmissible in several situations, the main ones being:

• Input tax on purchases used to make exempt supplies — because no output tax is charged on the exempt sale, no input credit is given on the related purchase.
• Blocked items the Act specifically lists as ineligible regardless of use.
• Input tax not supported by a valid tax invoice meeting the invoice requirements, or claimed outside the time limit the Act allows.
• Input tax on supplies from unregistered persons in the cases the Act restricts.

Claiming inadmissible input tax is the single most common cause of a rejected return or an audit demand, because the return-matching system compares claimed input tax against suppliers' declared output tax. An input claim with no matching supplier declaration is flagged automatically.

Apportionment for mixed supplies

A business that makes both taxable and exempt supplies cannot claim all its input tax, because some of its purchases support the exempt side. Apportionment splits the input tax between the two. The usual method is by the ratio of taxable supplies to total supplies:

• Input tax wholly attributable to taxable supplies: fully claimable.
• Input tax wholly attributable to exempt supplies: not claimable.
• Input tax on common purchases (rent, utilities, overheads serving both): apportioned in the taxable-to-total ratio.

This is closely related to the goods-versus-services split, because a business operating across the federal and provincial systems — see sales tax on services versus goods — may have to apportion across both. Keep the calculation transparent and consistent month to month; an apportionment that swings around without explanation invites scrutiny.

Excess input tax and carry-forward

Two provisions govern everything on this page. Section 7 of the Sales Tax Act 1990 allows the deduction of input tax from output tax for the period; section 8 lists what is inadmissible, including input attributable to exempt supplies. Apportionment exists because section 8 requires the exempt portion to be identified even when a single invoice serves both sides of the business.

The three input pools, and what happens to each

The fourth column is the one that decides whether this is manageable. All three pools have to be coded when the purchase is entered — a ledger with a single "input tax" account cannot be apportioned afterwards without going back through every invoice, which is why mixed-supply businesses that discover this at return time end up applying a blanket ratio and under-claiming.

When admissible input tax exceeds output tax in a period — common for exporters, for businesses in a heavy purchasing phase, or where output is zero-rated — the excess is not lost. It generally carries forward to set against output tax in later periods, and in defined cases (notably zero-rated exports) it can be claimed as a sales tax refund rather than only carried forward. The discipline is to reconcile the carry-forward every month so the running balance is always known and supportable.

Records and reconciliation

Everything in the input-tax system rests on documentation: valid tax invoices, a purchase register that ties to the input claim, and a sales register that ties to the output tax. These are the same records required for sales tax registration and filing, and they should reconcile to the turnover shown in the income tax return. A business that keeps its input-tax claim clean — valid invoices, correct apportionment, honest carry-forward — files returns that match the system and rarely attracts a demand on this ground.

Sources

This guide is written against the official and clearly labelled professional references below. Rates, thresholds and portal procedures change between reviews, so open the primary source before relying on a figure.

• Sales Tax Act 1990, amended through 30 June 2026 (FBR)
• Sales Tax Basics (FBR)
• File a Sales Tax Return (FBR)

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