NZ GST calculator

Add GST or find the GST already included

This calculator handles the arithmetic for a standard-rated New Zealand transaction at 15% GST. Choose add when the entered amount excludes GST. Choose remove when the amount already includes GST. The output always shows the exclusive amount, GST component and inclusive total together. That makes it possible to check an invoice or quote without switching between separate tools or guessing whether a price was entered on the correct basis.

The mode matters more than the size of the number. Fifteen percent of an exclusive price is the tax added to that price. Fifteen percent of an inclusive price is not the tax embedded in it, because the inclusive price already contains the tax. If you are unsure whether a supplier's figure includes GST, confirm that point before comparing quotes. Correct arithmetic applied to the wrong price basis still gives the wrong commercial comparison.

The formula explained

To add GST, multiply the exclusive amount by 0.15 to obtain the tax and by 1.15 to obtain the total. To remove GST, divide the inclusive total by 1.15. The difference between that total and the exclusive amount is the embedded tax. Equivalently, the embedded GST is three twenty-thirds of the inclusive price. Both versions describe the same calculation and should reconcile apart from displayed rounding.

For $100 excluding GST, the tax is $15 and the customer-facing total is $115. Entering $115 in remove mode returns $100 excluding GST and $15 tax. A common mistake is to multiply $115 by 15%, producing $17.25. That is tax on a different exclusive base, not the GST included in $115. Testing this small example is a useful way to verify that a spreadsheet or invoice template uses the intended basis.

Comparing supplier prices

Suppose one quote says $2,300 including GST and another says $2,000 plus GST. Removing GST from the first gives $2,000, so the quoted base prices are the same. This does not establish that the offers are otherwise equivalent. Scope, delivery, installation, payment terms and exclusions may still differ. Put each quote on the same tax basis first, then compare those commercial details rather than assuming the lower-looking headline is cheaper.

For a consumer paying the full invoice, the inclusive cost is usually the relevant cash requirement. For a GST-registered business assessing a potentially claimable purchase, the exclusive amount may be useful for a separate comparison. Whether an input tax deduction is allowed is not determined by this calculator. Private use, exempt activities, documentation and the nature of the supply can change the treatment.

Checking invoice rounding

The calculator rounds displayed amounts to cents. If you calculate GST separately on many small line items, their rounded taxes may not sum to exactly the tax calculated on the unrounded invoice subtotal. For example, repeated fractional-cent amounts can accumulate into a cent-level difference. Keep your invoice method consistent and retain the underlying values when reconciling a system rather than manually altering a line only to force a match.

For a single total, check that exclusive price plus GST equals the inclusive result within ordinary cent rounding. For a mixed invoice, separate the items by their actual treatment before calculating. Applying 15% to an entire invoice containing exempt or zero-rated supplies can overstate tax. This tool has one rate and one amount, so it deliberately does not classify or allocate a mixed basket of goods and services.

What the result does not establish

GST registration, the place of supply and the nature of the transaction determine whether standard-rate GST applies. Zero-rated supplies, exempt supplies and some property transactions require different treatment. The calculator cannot infer that treatment from an amount. An output of $150 GST on a $1,000 exclusive input means only that 15% was applied; it does not establish an obligation to charge or a right to claim that $150.

This is also not a GST return calculator. A return can involve sales, purchases, adjustments and accounting-basis rules across a period. Do not treat the tax component of one sale as the total amount payable to IRD for that period. Keep the transaction-level calculation with the relevant invoice and use the appropriate records or accounting system for the return itself.

A practical checking sequence

Begin by reading the quote or invoice wording. Record whether its amount is inclusive or exclusive and whether all items are standard-rated. Select the matching mode and enter the original amount without subtracting anything first. Compare the three outputs against the supplier's figures. If the difference is substantial, check the mode before assuming the rate or supplier calculation is wrong.

If a credit, discount or partial refund is involved, identify the amount to which GST should apply before using the tool. The current input expects a non-negative transaction amount; it does not model accounting entries or tax-period corrections. For a disputed classification, consult IRD's guidance or an accountant. The useful outcome is a reproducible arithmetic check with a clear price basis, not an unsupported conclusion about tax liability.

Frequently asked questions

Sources

• IRD charging GST