Skip to content
1300 65 35 83

BAS Calculator Australia: Estimate Your GST and Business Activity Statement

Whether you are a sole trader lodging your own BAS or a small business owner keeping track of GST, this free BAS calculator Australia tool gives you a quick estimate of what you owe or what you may get back. Enter your total sales, GST collected on sales, and GST paid on purchases, and the calculator works out your net GST position for the period.

The calculator assumes you are using the simplified BAS reporting method and are registered for GST with the Australian Taxation Office (ATO). It covers the GST component of your Business Activity Statement only. PAYG withholding and PAYG instalments are reported separately on your BAS. If you need to estimate employee tax, try our PAYG calculator.

What Is a Business Activity Statement?

A Business Activity Statement (BAS) is the form you use to report and pay your business tax obligations to the ATO. If you’re registered for GST, you need to lodge a BAS. It covers the GST you’ve collected on sales, the GST credits you’re claiming on purchases, and any PAYG withholding or instalments that apply.

Most small businesses lodge quarterly. Larger businesses with a GST turnover of $20 million or more lodge monthly. If you’ve never lodged before, our guide on how to do a BAS statement and lodge it walks you through the process.

How GST Works on Your BAS

GST (Goods and Services Tax) is a 10% tax on most goods and services sold in Australia. As a GST-registered business, you collect GST on your sales and pay GST on your business purchases. The difference between what you collected and what you paid is what you owe the ATO, or what the ATO owes you.

On a simplified BAS, three labels do the heavy lifting:

  • G1 (Total sales): Your total business income for the period, including GST-inclusive and GST-free sales
  • 1A (GST on sales): The total GST you collected on taxable sales
  • 1B (GST on purchases): The total GST you paid on business purchases (your input tax credits)

The calculation is straightforward: 1A minus 1B equals your net GST. If 1A is larger, you owe the ATO. If 1B is larger, you’re entitled to a GST refund. Not everything attracts GST, though. Some items are GST-free (like most basic food and some medical services), so getting the classification right matters when claiming input tax credits.

How This Calculator Works

The tool estimates your net GST based on the simplified BAS method. Enter your total sales at G1, GST collected at 1A, and GST paid on purchases at 1B. The calculator shows whether you have a net amount payable or a refund due.

This is useful when you’re doing a quick check before lodging, forecasting your GST position mid-quarter, verifying your accounting software figures, or estimating what you’ll owe before your BAS due date.

For a complete BAS that includes PAYG withholding (labels W1 and W2) or PAYG instalments (label T7), you’ll need to calculate those separately. Our PAYG calculator can help with the withholding side. Your actual BAS figures should come from reconciled records in Xero, MYOB, QuickBooks, or whichever software you use. If you’re working from a manual BAS calculation sheet, cross-check against your bank statements and tax invoices.

BAS Lodgement Deadlines

Missing a BAS deadline can result in penalties starting at $330 for every 28 days overdue, plus interest on any unpaid amount. Quarterly BAS due dates for 2026-27 are:

  • Q1 (Jul-Sep 2026): 28 October 2026
  • Q2 (Oct-Dec 2026): 28 February 2027
  • Q3 (Jan-Mar 2027): 28 April 2027
  • Q4 (Apr-Jun 2027): 28 July 2027

Lodging online generally gets you an extra two weeks. Using a registered BAS agent can add around four weeks on top of the standard deadline. For the full breakdown, see our guide on BAS due dates.

Need a Hand With Your BAS?

Calculators give you a quick estimate, but getting your BAS right every quarter means keeping your bookkeeping up to date and your GST classifications correct. That takes time that most small business owners and tradies would rather spend on the job.

At Shoebox Books & Tax, our registered BAS agents handle your BAS preparation, GST calculations, PAYG reporting, and lodgement. We work with Xero, MYOB, and QuickBooks, and our fixed-cost packages mean no surprises.

Book a free consultation and let us take your BAS off your plate.

Call us on 1300 65 35 83

Frequently Asked Questions

Do I need to lodge a BAS if my business had no activity?

Yes. Even if your business had no sales, expenses, or GST activity during the period, you still need to lodge a nil BAS to stay compliant with the ATO. Failing to lodge can trigger penalties regardless of whether you owe anything.

What's the difference between a BAS and an activity statement?

They’re essentially the same form. The ATO uses “activity statement” as the broader term, which can include BAS obligations (GST, PAYG withholding) as well as PAYG instalment obligations. If you’re registered for GST, your activity statement is your BAS. The terms are often used interchangeably.

Can I fix a mistake on a BAS I've already lodged?

Many GST errors can be corrected in a later BAS, provided certain conditions are met. For debit errors (underpaid GST), the correction must generally be within the relevant time limit (typically 18 months for businesses under $20 million turnover) and within the applicable value limit (e.g. up to $12,500). Credit errors (overpaid GST) are not subject to value limits but must be within the period of review (generally 4 years). If these conditions are not met, the original BAS must be revised.

How accurate is this BAS calculator?

It gives a reliable estimate of your net GST based on the simplified BAS method. Your actual BAS figures should always come from reconciled bookkeeping records. This tool is for quick estimates and pre-lodgement checks.

Can Shoebox Books & Tax lodge my BAS for me?

Yes. Our registered BAS agents prepare and lodge BAS for small businesses across Australia. We handle the GST calculations, PAYG reporting, and lodgement so you don’t have to. Fixed-cost packages mean you always know what you’re paying. Get in touch for a free consultation.