BAS Calculator Quarterly Australia

Every quarter, thousands of Australian small business owners sit down to work out what they owe the ATO on their Business Activity Statement. This free BAS calculator quarterly Australia tool does the arithmetic for you. Enter your GST-inclusive sales and purchases, tick a box if you have a PAYG instalment, and see your net GST payable (or refundable) and your total BAS amount straight away.

Disclaimer: This tool provides general information only and does not constitute tax advice. Consult a registered tax agent for advice specific to your circumstances.

This calculator has no year-specific rates: it uses the 10% GST rate and the PAYG instalment rate you enter.

How quarterly BAS GST is calculated

The ATO requires GST-registered businesses to report GST collected on sales and GST paid on purchases separately. The difference is your net GST obligation.

The core formula uses the divide-by-11 rule. Because GST is exactly one-eleventh of a GST-inclusive price (10% on top of the base, which equals 1/11 of the inclusive total), you extract the GST component by dividing the GST-inclusive amount by 11.

Worked example:

Suppose your quarterly figures are:

• Total GST-inclusive sales: $110,000
• Total GST-inclusive business purchases: $33,000

Then:

• GST on sales (1A) = $110,000 ÷ 11 = $10,000.00
• GST credits on purchases (1B) = $33,000 ÷ 11 = $3,000.00
• Net GST payable = $10,000 − $3,000 = $7,000.00

You owe the ATO $7,000 for that quarter.

Now flip the scenario: if your sales were $22,000 and your purchases were $55,000:

• GST on sales = $22,000 ÷ 11 = $2,000
• GST credits = $55,000 ÷ 11 = $5,000
• Net GST = $2,000 − $5,000 = Refund of $3,000.00

A negative net GST means the ATO owes you a refund. This is common for new businesses in a fit-out phase, or businesses with large capital purchases in a given quarter.

Only include purchases that contain GST and are used for business purposes. Personal expenses, wages, and purchases from non-GST-registered suppliers do not generate GST credits.

When PAYG instalments apply

PAYG (Pay As You Go) instalments are prepayments of income tax made throughout the year so you do not face a large tax bill at lodgement time. If you are a sole trader, the ATO automatically enters you into the PAYG instalment system when your latest tax return shows instalment income (gross business and investment income) of $4,000 or more, your latest notice of assessment shows tax payable of $1,000 or more, and your estimated (notional) tax is $500 or more.

The ATO sends you a rate (shown on your BAS as the instalment rate) based on your prior-year tax liability relative to your income. You apply that rate to your gross instalment income for the quarter to calculate the instalment amount. The ATO can instead give you a set instalment amount to pay; this calculator covers the rate option.

Formula: PAYG instalment = gross instalment income × (ATO rate ÷ 100)

For example, if the ATO has set your rate at 4.5% and your gross instalment income for the quarter was $80,000:

PAYG instalment = $80,000 × 4.5% = $3,600

Sole traders and small companies are the most common recipients of ATO-issued instalment rates. You can choose to vary your instalment if your income has changed significantly from the prior year. But if your varied instalments come to less than 85% of the tax you end up owing for the year, the ATO may charge the general interest charge (GIC) on the difference.

Quarterly BAS due dates 2026-27

Most small businesses lodge their BAS quarterly. The standard due dates for the 2026-27 financial year are:

If a due date falls on a weekend or public holiday, you can lodge and pay on the next business day.

Q2 has a later deadline than the other quarters because it already includes a one-month extension for the Christmas and New Year holiday period. For the same reason, the extra time you can get for lodging online or through an agent does not apply to Q2.

If you lodge online yourself, you may be eligible for an extra two weeks to lodge and pay Q1, Q3 and Q4. If a registered tax or BAS agent lodges electronically for you, the 2026-27 lodgment program dates are 25 November 2026 for Q1, 26 May 2027 for Q3 and 25 August 2027 for Q4 (the ATO has yet to confirm the Q4 date), while Q2 stays at 28 February 2027. Your agent can confirm the dates that apply to your account.

Lodging late can lead to a Failure to Lodge (FTL) penalty, so if you think you'll be late, contact the ATO or ask your agent to request more time before the due date.

What happens if you lodge BAS late

Lodging your BAS late can attract a Failure to Lodge (FTL) penalty. The ATO says it generally doesn't apply the penalty for an isolated late lodgment and warns you before it does. The penalty is one penalty unit for every 28 days (or part of 28 days) that the BAS is overdue, up to 5 penalty units. A penalty unit is $364 for lodgments due on or after 1 July 2026 ($330 before then), and larger businesses pay a multiple:

• Small business (turnover under $1 million): $364 for each 28-day period, up to $1,820
• Medium business (turnover $1 million to $20 million): double, so $728 for each 28-day period, up to $3,640
• Large business (turnover $20 million or more): five times, so $1,820 for each 28-day period, up to $9,100

On top of FTL penalties, any outstanding amount (unpaid GST or PAYG) also accrues the general interest charge (GIC). The ATO sets the GIC rate quarterly and calculates it daily on the outstanding balance. For October to December 2026 the GIC annual rate is 11.51% (it was 11.43% for July to September 2026), which adds up quickly on large balances.

The ATO can remit (reduce or cancel) penalties and looks at your compliance history when it decides, so if you have a genuine reason for lodging late, contact the ATO as soon as possible rather than waiting.

Frequently asked questions

How do I calculate GST on a quarterly BAS?

Use the divide-by-11 rule. Divide your total GST-inclusive sales for the quarter by 11 to get the GST on sales (label 1A on your BAS). Divide your total GST-inclusive business purchases by 11 to get your GST credits (label 1B). Subtract 1B from 1A to get your net GST. If the result is positive, you pay that amount to the ATO. If negative, the ATO refunds that amount to you.

What's the difference between GST on sales and GST credits?

GST on sales (1A) is the GST you have collected from your customers on your taxable sales during the quarter. It is money you hold on behalf of the ATO. GST credits (1B), also called input tax credits, are the GST amounts included in the prices you paid for business purchases and expenses, and you can claim that GST back from the ATO. Your net obligation is 1A minus 1B. Keeping accurate records of both (ideally a scanned receipt for every purchase) helps you claim every credit you are entitled to without overstating them.

What is a PAYG instalment and how is it calculated?

A PAYG instalment is a quarterly prepayment of income tax. If you are a sole trader, the ATO enters you automatically once your latest return shows instalment income of $4,000 or more and tax payable of $1,000 or more (and your estimated tax is $500 or more), then gives you a rate on your BAS. Multiply your gross instalment income for the quarter by that rate (as a percentage) to get the instalment amount due. For example, a 4% rate on $50,000 of instalment income produces a $2,000 PAYG instalment. You can vary the instalment if your circumstances have changed materially from the prior year.

When are quarterly BAS due in 2026-27?

The four quarterly BAS due dates for 2026-27 are: Q1 (July to September 2026) on 28 October 2026; Q2 (October to December 2026) on 28 February 2027; Q3 (January to March 2027) on 28 April 2027; and Q4 (April to June 2027) on 28 July 2027. If a registered tax or BAS agent lodges electronically for you, the lodgment program gives later dates for Q1 (25 November 2026), Q3 (26 May 2027) and Q4 (25 August 2027, yet to be confirmed by the ATO). Q2 gets no extra time.

Can I claim GST credits on receipts I've lost?

For purchases over $82.50 (GST-inclusive) you must hold a valid tax invoice to claim the GST credit. If you have lost the receipt, you need to obtain a replacement from the supplier before lodging your BAS. For purchases of $82.50 or under, a receipt or other documentary evidence of the purchase is acceptable. The ATO does not accept claims based on memory or estimates alone. Scanning receipts as soon as you get them, for example with an app like Taxr, means a lost or faded paper copy is no longer a problem.

What happens if I lodge BAS late?

The ATO can apply a Failure to Lodge (FTL) penalty, although it generally warns you first and doesn't usually penalise an isolated late lodgment. For small businesses the penalty is $364 for each 28 days (or part of 28 days) the BAS is overdue, up to $1,820, for BAS due on or after 1 July 2026. Medium businesses pay double and large businesses pay five times as much. Any unpaid tax also accrues the general interest charge (GIC) daily until the debt is cleared. If you have a genuine reason for the late lodgment, contact the ATO promptly. It can remit penalties and considers your compliance history when deciding.

Related tools and reading

• BAS Lodgement Guide for Quarterly Lodgers
• GST for Small Business: When Do You Need to Register?
• GST Receipt Tracking App Australia
• GST Calculator Australia
• Sole Trader Tax Calculator

Keep your BAS records ready with Taxr

The most time-consuming part of preparing a quarterly BAS is not the maths. It is hunting down every receipt and invoice to tally your GST credits. Taxr fixes that. Every time you make a business purchase, open the app and snap a photo of the receipt. In about 5 seconds Taxr's AI reads the vendor, date, total and GST and suggests a category. You check the details and save, and the receipt is stored in your account, ready to export.

When it is time to lodge, export the quarter's receipts to Excel or PDF with the GST on your purchases totalled by category. That gives you or your accountant the GST credits figure (1B), with every receipt listed behind it. Email it to your accountant from the app, or use it to fill in 1B yourself. Taxr records purchases only, so your GST on sales (1A) still comes from your own sales records.

Download Taxr and start free with your next business receipt.

Last reviewed: 30 September 2026 (FY2026-27). Tax law changes, so verify the current GST and PAYG rules with the ATO before lodging.