BAS GST Calculator (Australia)

What the bas gst calculator does

The bas gst calculator works out the GST section of an Australian Business Activity Statement: the GST you collected on sales at label 1A, the GST credits you can claim at label 1B, and the net GST payable or refundable for the period. Enter your GST-inclusive sales and purchases, tell it which of those carried no GST, and it strips the tax out using the GST fraction. Add your PAYG tax withheld and PAYG income tax instalment and it also returns the net amount for the whole statement.

Most small businesses do this in a spreadsheet once a quarter and get it wrong in one of two ways: they apply 10 per cent to a price that already includes GST, or they include GST-free sales in the taxable total. Both errors inflate label 1A. This tool exists to give you a second opinion before you lodge.

The GST fraction

GST is levied on the value of a supply, but prices are quoted with the tax included. To recover the tax from a GST-inclusive amount you multiply by the rate divided by one plus the rate. At a 10 per cent rate that fraction is one eleventh, so a 110 dollar invoice contains 10 dollars of GST and 100 dollars of value.

Multiplying a GST-inclusive price by the rate itself is the classic mistake. On that same 110 dollar invoice it produces 11 dollars, overstating the GST by a dollar and understating your revenue by the same amount. Across a quarter of trading the error compounds into a real overpayment. The rate is a stored parameter here rather than a number written into the code, so if the rate ever changes the fraction follows automatically.

Working out label 1A

Start with total sales including GST, which is label G1 on the statement. Then subtract the sales that carry no GST. Two categories matter:

• GST-free sales, including most basic food, many health and education supplies, childcare, and exports.
• Input-taxed sales, mainly residential rent and most financial supplies.

Whatever remains is your taxable sales figure, and the GST fraction of that is label 1A. If every sale you make is taxable, the GST-free field stays at zero and the whole of G1 is used.

Working out label 1B

The same logic runs on the purchase side. Start with total purchases including GST, which is label G11, then remove anything you cannot claim a credit for. Common exclusions are purchases from suppliers who are not registered for GST, expenses without a valid tax invoice, purchases that relate to making input-taxed supplies, and the private portion of a mixed-use expense.

The GST fraction of what is left is label 1B. Claiming credits you are not entitled to is one of the more common triggers for ATO attention, so the non-creditable field is worth filling in honestly rather than leaving at zero out of optimism.

Net GST, and what a refund means

Net GST is simply 1A minus 1B. When it is positive you owe that amount to the ATO. When it is negative you are in a refund position, which usually means you bought more than you sold in the period. That is normal for a business in a build phase, one that has just bought major equipment, or an exporter whose sales are GST-free while its inputs are not.

The calculator reports the payable and refundable figures separately as well as the signed net figure, so a refund quarter reads clearly rather than as a negative number you have to interpret.

The net amount for the statement

Most activity statements carry more than GST. If you employ people you report PAYG tax withheld, and if the ATO has put you into the instalment system you report a PAYG income tax instalment as well. Those amounts are added to the net GST to give the total you actually transfer.

Both come straight off your own statement, so they are entered as inputs rather than calculated. The result is the figure that hits your bank account, which is usually the number that matters for cash flow planning rather than the GST component on its own.

Reporting cycles

Businesses report monthly, quarterly, or annually depending on turnover and registration choices. The cycle does not change the arithmetic for a single period, but it does change how a period scales to a year. The tool multiplies the net GST by the number of periods in the year so you can see the annual run rate implied by the quarter or month you have entered.

That annualised figure is a planning number, not a forecast. Seasonal businesses will find it wildly wrong in a peak or trough period, which is itself useful information about how much working capital the GST cycle ties up.

Putting money aside

The practical use of this tool is not lodgement, it is discipline. GST collected on a sale was never your money, and the quarterly bill lands whether or not the cash is still in the account. Running the numbers as sales come in, and moving the net GST into a separate account, is the single habit that keeps a small business out of trouble with the ATO.

If the estimate here does not match your accounting software, the difference is nearly always in the classification of sales or purchases rather than in the arithmetic. Check which sales you treated as GST-free and which purchases you claimed credits on before assuming either figure is wrong.

This is general information only and not financial advice.

About This Bas GST Calculator

Use this bas GST calculator to get an instant, accurate estimate. The figures follow official ATO guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.