First decide whether GST is outside or already inside the price

The Australian GST rate is 10% on most taxable supplies, but the button you choose changes the arithmetic. When a quote is GST exclusive, GST is added to the net value. When a receipt shows a GST-inclusive total, the GST is already part of that number and must be extracted. Using 10% of an inclusive price overstates the tax because the inclusive price contains both the original value and the tax.

Example: adding GST to $250

If $250 is wholly taxable and excludes GST, the GST is $25 and the customer-facing total is $275. The calculation is $250 multiplied by 10%, not $275 divided by 10. The GST component is one-eleventh of the final $275, which also returns $25 and confirms both directions agree.

Example: extracting GST from $550

If $550 is a wholly taxable GST-inclusive price, the GST is $50 and the GST-exclusive value is $500. Multiplying $550 by 10% would incorrectly produce $55. The quick-value button can load $550, but the mode still has to be “Extract GST” because the calculator cannot infer whether a bare number came from a quote, invoice or receipt.

A mixed invoice needs a defensible taxable allocation

Some transactions contain taxable, GST-free or input-taxed components. Applying one-eleventh to the complete inclusive total is only correct when the whole relevant price is taxable. The taxable-proportion field lets you model a known allocation; it does not decide the allocation for you. The basis should reflect the supply and a reasonable valuation method, not whichever percentage produces a preferred GST outcome.

Example: 80% of an inclusive $1,100 is taxable

With Extract GST selected and the taxable proportion set to 80%, the calculator assigns $880 of the inclusive price to the taxable part. One-eleventh of $880 is $80 GST. The remaining GST-exclusive value of the whole transaction is $1,020: $800 taxable value plus $220 non-taxable value. Applying one-eleventh to the full $1,100 would show $100 GST and ignore the $220 allocation.

Do not use the proportion field to imitate a special GST scheme. The margin scheme for property, GST at settlement, financial supplies, second-hand goods, vouchers and adjustments can follow specific statutory rules. A simple percentage split is appropriate only when it represents a supportable allocation of the actual supply.

Why the coloured proportion is useful

The orange bar is a visual check rather than a new calculation. At 100% it fills the track. At 80% it leaves one-fifth neutral. If the bar looks inconsistent with the invoice description, stop and review the allocation before copying the figures. This is especially helpful when several people prepare a quote: the result carries the chosen proportion into the copied text so it is less likely to be forgotten.

Keep net, GST and gross amounts tied to the same rounding rule

The calculator keeps full precision during the formula and displays Australian dollars to two decimal places. An accounting system may round GST by line item, by taxable category or on the invoice total. Those methods can differ by a cent when many small items are involved. Choose one permitted method and apply it consistently; do not independently round the net, GST and gross numbers in ways that stop them adding back together.

For a tax invoice, the legal description and required details matter in addition to arithmetic. The supplier must first be making a taxable supply and be registered or required to register for GST. A number labelled “GST” is not made valid merely because it equals one-eleventh of the total. The customer’s ability to claim a GST credit also depends on the acquisition and record requirements, not solely on the figure this calculator extracts.

Quote the price basis in words as well as numbers

A business-to-business quote may show a subtotal, GST line and inclusive total, while a consumer-facing price will commonly present the amount payable. In either case, write “including GST” or “plus GST” beside the relevant figure rather than relying on the reader to infer the basis from a round number. If a discount is agreed before the taxable supply is made, apply the GST calculation to the price that the customer is required to pay under the final terms. If the price later changes through a refund, rebate or cancellation, preserve the link to the original invoice and process the appropriate adjustment document instead of editing the historical figure without a trail.

Use the result as an arithmetic check, not a GST classification decision

This tool assumes the standard 10% rate and a monetary price. It does not determine whether food, health, education, exports, property rent, financial supplies, digital products or imported services are taxable, GST-free or input taxed. It also does not work out GST registration, cash versus non-cash accounting, attribution to a BAS period, input tax credits, decreasing adjustments or the effect of non-monetary consideration.

If a supplier says “$2,000 plus GST”, Add GST gives $2,200 for a wholly taxable supply. If the same supplier says “$2,200 including GST”, Extract GST gives a $200 component. If a contract merely says “$2,200” without clarifying whether GST is included, the problem is contractual interpretation rather than division. Confirm the price basis before issuing or paying the invoice.

On a BAS, GST on sales and GST credits on purchases are separate totals drawn from the entity’s records. The “GST component” here can help check an individual transaction, but adding every extracted amount without reviewing classifications, adjustments and documentation can be wrong. Preserve the invoice, the allocation method for any mixed supply and the accounting-period context alongside the copied calculation.

Reconcile the transaction in both directions

A useful control is to repeat the calculation from the opposite side. After adding GST to an exclusive value, switch to Extract GST and enter the inclusive result; the original net and GST should return, apart from any displayed-cent rounding. For a mixed supply, keep the same taxable proportion during that check. If the figures fail to return, review whether the starting amount was net or gross, whether the allocation was applied to the same component, and whether a line-level rounding rule intervened. This simple reversal catches many spreadsheet errors before the numbers reach an invoice or BAS workpaper.

Zero is a valid input when checking a no-value line, and a 0% taxable proportion deliberately produces no GST. Negative credit notes and adjustment events need direction and legal context, so this page rejects negative values instead of silently treating them as ordinary sales. Record the original transaction and adjustment separately when the accounting system requires that trail.