{{ summaryHeading }} {{ summaryValue }} {{ summaryLine }} {{ rateBadge }} {{ basisBadge }} {{ roundingBadge }}
Match the amount printed on the quote, receipt, or tax invoice.
{{ currencyPrefix }}
{{ amountHelp }}
Verify the transaction date, supply type, registration status, and special rules before relying on a preset.
%
Enter a percentage from 0 through 100; GST-only recovery needs a positive rate.
Use the invoice denomination; the arithmetic does not convert currencies.
units
Enter a whole number of identical units.
Choose only the tax-head format used by the actual invoice.
Match the invoicing system and apply the chosen method consistently.
Nearest cent is the neutral audit default; down/up model explicit source-system rules.
For example, April invoice check or Quote 1042.
This presentation setting does not change the canonical calculation.
LinePer unitInvoiceAudit noteCopy
{{ row.line }}{{ row.unit }}{{ row.invoice }}{{ row.note }}
PriorityCheckCurrent evidenceNext actionCopy
{{ row.priority }}{{ row.check }}{{ row.evidence }}{{ row.action }}
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The amount printed on an invoice can represent three different starting points. It may be the taxable price before goods and services tax (GST), the total after GST is included, or the GST line alone. Choosing the wrong basis gives a mathematically tidy answer to the wrong question.

GST is a consumption tax applied to taxable supplies in several jurisdictions. The arithmetic connects the taxable base, tax amount, and customer total. Tax law decides whether a supply is standard-rated, zero-rated, exempt, outside scope, split between tax heads, or subject to a special scheme.

Taxable amount
The GST-exclusive base to which the rate is applied.
GST-inclusive total
The taxable amount and GST combined.
GST amount
The tax component added to or extracted from a price.
Rounding scope
Whether tax is rounded for each unit before multiplication or after the invoice totals are formed.

Rounding can create a real one-cent difference. Three items priced at 1.28 each with 9% GST produce 0.1152 tax per item. Rounding each item to 0.12 gives 0.36 total GST, while applying 9% to the 3.84 invoice base gives 0.3456 and rounds to 0.35.

Rates, invoice rules, registration status, transaction dates, place of supply, and product classifications change the legal answer. Use the calculation for arithmetic review and education, not as tax, legal, or financial advice. Verify the actual transaction with the relevant authority or a qualified adviser.

How to Use This Tool:

Match the calculation basis to the source document before choosing a rate or rounding method.

  1. Select Taxable amount, GST-inclusive total, or GST amount only, then enter the one-unit amount shown by the quote, receipt, or invoice.
  2. Choose a current jurisdiction preset or enter a custom GST rate. Confirm the transaction date, supply type, registration status, and special rules before relying on a preset.
  3. Enter the quantity and invoice currency. Currency changes labels only; no exchange-rate conversion occurs.
  4. Match the actual invoice's tax-head presentation, rounding scope, and cent policy. Use CGST/SGST or IGST only when the transaction rules require it.
  5. Compare the selected GST total with the alternate rounding scope in GST Review. Reconcile any cent difference against the source system or tax invoice.

Interpreting Results:

The receipt separates the taxable amount, GST, and GST-inclusive total for one unit and for the full quantity. The tax-head rows are a presentation split inside the same GST total; they do not add extra tax.

  • A nonzero Rounding delta means per-unit and invoice-total scopes disagree after cent rounding. Use the scope required by the actual invoicing method.
  • GST share of total is GST divided by the inclusive total. It is not the same percentage as the GST rate applied to the taxable base.
  • The rate-impact comparison holds the taxable base, quantity, and rounding policy constant. It illustrates arithmetic only and does not imply that every displayed rate is legally available.
  • A matching calculation does not prove that GST should have been charged. Classification and jurisdiction must be checked separately.

Technical Details:

The calculation first solves an exact one-unit taxable amount N, GST amount T, and inclusive amount G. Rate r is expressed as a percentage. Quantity and rounding are applied only after that basis conversion.

Formula Core:

Starting from a taxable amount:

T=Nr100,G=N+T

Starting from a GST-inclusive total:

T=Gr100+r,N=G−T

Starting from the GST amount alone requires a positive rate:

N=T100r,G=N+T

For example, a 115.00 New Zealand GST-inclusive total at 15% contains 15.00 GST because 115 × 15/115 = 15. The taxable amount is 100.00.

Rule Core:

SettingExact rule
Invoice-total roundingMultiply exact one-unit net and GST by quantity, round each invoice amount with the selected cent policy, then set gross to the nearest cent of rounded net plus rounded GST.
Per-unit roundingRound one-unit net and GST with the selected policy, set unit gross to the nearest cent of their sum, multiply each rounded unit amount by quantity, then round invoice amounts to the nearest cent.
Nearest centHalf-cent values round upward.
Round downDiscard fractions of a cent toward the lower cent.
Round upAny positive fraction of a cent advances to the next cent.

Supported inputs include nonnegative amounts through 1,000,000,000, rates from 0% through 100%, and whole-number quantities from 1 through 100,000. A GST-only reconstruction with a 0% rate is rejected because the taxable base would require division by zero. Standard display uses two decimal places; detailed display uses four, without changing the cent-rounded canonical amounts.

ChoiceStored rateScope
Singapore standard9%Convenience preset using SGD
Australia standard10%Convenience preset using AUD
New Zealand standard15%Convenience preset using NZD
Custom0% to 100%User-supplied rate; currency remains a label

Combined GST and IGST each present one tax line. CGST/SGST splits the rounded GST total into cents: the first half is rounded down to a whole cent and any odd cent goes to SGST. Selecting a tax head does not determine whether that structure is legally correct for the transaction.

The current presets reflect official standard rates verified on 1 September 2026, but they do not cover zero-rated, exempt, reduced-rate, reverse-charge, mixed-supply, or special-scheme transactions. Always recheck the authority and effective date.

Tax and Accuracy Notes:

Invoice arithmetic is only one part of GST compliance. These calculations do not determine registration, taxability, place of supply, input credits, reporting periods, exchange rates, documentation requirements, or the correct Indian intra-state or inter-state tax head.

  • Match the rate to the transaction date and the law that applies to the actual supply.
  • Match rounding to the invoicing system and apply the chosen policy consistently.
  • Use the invoice denomination as entered. No currency conversion or authority-approved exchange rate is applied.
  • Processing is local to the browser; copied or downloaded results still need normal handling as business records.

Worked Examples:

Three low-price items

At 1.28 per unit, 9% GST, and quantity 3, per-unit nearest-cent rounding produces 0.12 GST per item and 0.36 for the invoice. Invoice-total rounding produces 0.35 GST on the 3.84 taxable total. The 0.01 delta is expected arithmetic, so the source invoice's permitted method decides which result to use.

Extracting New Zealand GST

A GST-inclusive amount of NZ$115 at 15% resolves to NZ$100 taxable plus NZ$15 GST. Multiplying 115 by 15% would incorrectly return 17.25 because the rate applies to the taxable base, not to the already-taxed total.

References: