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 hard work for you – enter your GST-inclusive sales and purchases, tick a box if you have a PAYG instalment, and instantly see your net GST payable (or refundable) and your total BAS amount.

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

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. The ATO automatically enrols you in the PAYG instalment system once your business income exceeds a threshold – typically after your first year with a tax liability over approximately $1,000.

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.

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 you vary it and underpay by more than 15%, the ATO may charge shortfall interest.

Quarterly BAS due dates 2025-26

Most small businesses lodge their BAS quarterly. The standard due dates for the 2025-26 financial year are:

Q2 has a later deadline than usual because of Christmas and New Year public holidays. The ATO extends this quarter to 28 February to give businesses more time during the holiday period.

If you lodge through a registered tax agent, your deadlines are typically extended by around four weeks beyond each of the dates above. Your tax agent can confirm the exact extended dates that apply to your account.

Missing a due date without a deferral triggers a Failure to Lodge (FTL) penalty, so if you think you’ll be late it pays to call the ATO or ask your agent to request an extension in advance.

What happens if you lodge BAS late

Lodging your BAS late attracts an automatic Failure to Lodge (FTL) penalty. The penalty units are set by legislation and increase with business size:

• Small business (turnover under $1 million): $330 per penalty unit, up to $1,650 (5 units, applied after 28 days late)
• Medium business (turnover $1 million – $20 million): $660 per penalty unit, up to $3,300
• Large business (turnover over $20 million): $1,650 per penalty unit, with no cap

On top of FTL penalties, any outstanding amount (unpaid GST or PAYG) also accrues General Interest Charge (GIC). The GIC rate is set quarterly by the ATO and is calculated daily on the outstanding balance. As of early 2026, the GIC annual rate is in the range of 11–12%, which adds up quickly on large balances.

The ATO does have a remission discretion for businesses with a good compliance history, 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. Take your total GST-inclusive sales for the quarter and divide by 11 – that is the GST on sales (label 1A on your BAS). Take your total GST-inclusive business purchases and divide by 11 – that is 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 embedded in the prices you paid for business purchases and expenses. You are entitled to recover that GST from the ATO. Your net obligation is 1A minus 1B. Accurate record-keeping of both – ideally with a scanned receipt for every purchase – is essential to maximise your credits and avoid overstatements.

What is a PAYG instalment and how is it calculated?

A PAYG instalment is a quarterly prepayment of income tax. The ATO enrols you automatically once your tax liability reaches a threshold and sends 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 2025-26?

The four quarterly BAS due dates for 2025-26 are: Q1 (Jul–Sep 2025) on 28 October 2025; Q2 (Oct–Dec 2025) on 28 February 2026; Q3 (Jan–Mar 2026) on 28 April 2026; and Q4 (Apr–Jun 2026) on 28 July 2026. Registered tax agents can access extended deadlines of approximately four weeks beyond each of these dates.

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. Digitising receipts as soon as you receive them – using a scanning app like Taxr – eliminates this risk entirely.

What happens if I lodge BAS late?

The ATO issues an automatic Failure to Lodge (FTL) penalty. For small businesses this starts at $330 and can reach $1,650 depending on how late the lodgement is. For medium and large businesses the penalty is higher. Any unpaid tax also accrues General Interest Charge (GIC) on a daily basis until the debt is cleared. If you have a genuine reason for the late lodgement, contact the ATO promptly – they have discretion to remit penalties for taxpayers with a good prior compliance record.

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

Let Taxr handle your BAS records automatically

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. Taxr’s AI instantly extracts the amount, date, and GST component and stores it in your account, categorised and ready to export.

When it is time to lodge, export a BAS-ready report that shows your GST on sales, your GST credits, and all supporting receipts in one place. Hand it to your accountant or use it to fill in your BAS directly – no spreadsheets, no shoebox, no scrambling at quarter-end.

Download Taxr and make every quarterly BAS faster and more accurate.

Last reviewed: 4 May 2026 (FY2025-26). Tax law changes – verify the current GST and PAYG rules with the ATO before lodging.