BAS & GST Set-Aside Calculator
Not sure what your BAS will come to this quarter — or how much to be putting away so the bill never stings? The trick is simple: divide your quarterly BAS bill by 13 and tuck that much into a separate account each week. Pop your raw sales and purchases in below and the calculator works out the GST labels, adds your PAYG instalment, and hands you the number — so you're ahead of the ATO's bill before it lands.
| 1A — GST on sales | $0 |
| 1B — GST credits on purchases | $0 |
| Net GST | $0 |
| PAYG instalment | $0 |
| Total BAS bill | $0 |
How your BAS is calculated
There are only two sums to know, and having them straight is half the game:
If a price already includes GST, divide by 11. A customer pays you $110 — the GST tucked inside is $10. One eleventh, not 10%: taking 10% of an inclusive figure overshoots by exactly 10%, and it's the easiest slip to make on a hand-prepared BAS.
If your books show amounts before GST, multiply by 10%. Same GST, different arithmetic — which is exactly why both inputs above have an inclusive/exclusive toggle. For quick one-off conversions either way, the GST calculator does the same sum on a single amount.
From there it's just assembly: 1A (GST on your sales) minus 1B (GST credits on your purchases) gives your net GST; add any PAYG instalment and that's the quarter's bill. Divide by 13 for the weekly set-aside, or by 3 for monthly.
Worked example — sole-trader electrician, April–June quarter
Here's how it plays out. Sales for the quarter were $33,000 GST-inclusive, so 1A is $33,000 ÷ 11 = $3,000. Materials and fuel came to $8,800 GST-inclusive, so 1B is $8,800 ÷ 11 = $800. Net GST is $3,000 − $800 = $2,200; the ATO's PAYG instalment is $2,200, which makes the total BAS bill $4,400. Spread over 13 weeks that's $338.46 — tuck away $338 a week and the 28 July bill pays itself.
What to leave out
The calculator only wants the sales and purchases that actually carry GST. A few things that feel like they belong here, but don't:
- Wages — leave them out of the sales and purchases boxes; if you employ staff they're reported at W1 (with the tax withheld at W2), and there are no GST credits on them.
- Superannuation — never appears on a BAS at all; it goes straight to your employees' super funds.
- Purchases from unregistered suppliers — no GST was charged, so there's no credit to claim.
- Bank fees and interest — input-taxed, so no credit there either.
- Stamp duty — a state tax with no GST in it.
And one distinction worth having straight: "no GST on the sale" is actually two different categories, and they treat your purchases differently:
| GST-free sales | Input-taxed sales | |
|---|---|---|
| GST charged on the sale | No | No |
| GST credits on related purchases | Yes — still claimable | No |
| Typical examples | Exports, most basic food, health, education | Residential rent, interest and most financial supplies |
One input note: if you account for GST on a cash basis — the choice available to businesses under $10 million aggregated turnover (ATO, accounting methods) — just enter what actually landed in your account and what you actually paid during the quarter, not what you invoiced.
How much should I set aside for GST and tax?
Here's a kinder way to think about a BAS: it's money that was never really yours, just visiting for thirteen weeks. The plan that survives a slow month is the boring one — total bill ÷ 13, moved into a separate account every week, automatically. Once a quarterly bill is funded weekly, it stops being an event.
Prefer a habit that needs no forecasting at all? Bank 1/11 of every GST-inclusive dollar the day it lands. Whatever the quarter does, the GST side of the bill is fully funded by definition — the PAYG instalment is the only thing left to plan for.
And if you're profitable but not yet on PAYG instalments, the income tax bill is still coming — just later, in one hit at tax time. Roughly 25–30% of profit is a reasonable set-aside estimate for many sole traders; the right figure depends on your total taxable income, so it's worth modelling your marginal rate with the income tax calculator. Contractors can build the whole habit into their pricing with the contractor hourly rate calculator.
When it's due — FY 2026–27
| Quarter | Period | Standard due date | Via BAS/tax agent |
|---|---|---|---|
| Q1 | Jul–Sep | 28 October | Later — confirm with your agent |
| Q2 | Oct–Dec | 28 February | No extension past 28 February |
| Q3 | Jan–Mar | 28 April | Later — confirm with your agent |
| Q4 | Apr–Jun | 28 July | 25 August 2026 |
Two things worth knowing: lodging yourself online (rather than on paper) generally buys you around two extra weeks on the standard dates, and monthly lodgers are due on the 21st of the following month. The Q2 quirk: 28 February already includes the holiday concession, so an agent doesn't add anything there.
Why set aside at all
Because the ATO is currently a very expensive lender. The general interest charge (GIC) sits at 11.43% p.a. for the July–September 2026 quarter, compounding daily, and the rate resets every quarter (ATO GIC rates). Since 1 July 2025, GIC is also no longer tax-deductible — which makes an unpaid BAS effectively an ~11.4% after-tax loan. Not one many businesses would sign up for on purpose.
If the money just isn't there when the date arrives, still lodge on time — lodging and paying are separate things, and lodging keeps failure-to-lodge penalties from stacking on top of the interest. The ATO offers payment plans; interest keeps ticking on the balance and a plan is something you apply for rather than automatic, but it beats going quiet.
About the PAYG instalment box
PAYG instalments are simply prepayments of your own income tax, and they come in two flavours (ATO, PAYG instalments): Option 1 — a fixed amount the ATO works out for you, shown at label T7 on your activity statement; or Option 2 — an ATO-set rate you apply to your actual income each quarter. The number to enter here is on your activity statement in myGov or your accounting software. If your income has dropped since the year the amount was based on, you can vary the instalment down — just do it carefully, because varying too low can attract interest on the shortfall.
Frequently Asked Questions
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