If you lodge quarterly, the arithmetic is identical every quarter. This page shows the whole calculation by hand, so you can do your BAS yourself or check whatever tool you use.
Divide by 11: the only GST maths you need
Every price an Australian business quotes includes GST unless it says otherwise. GST is one eleventh of any GST-inclusive price:
- You invoiced $1,100 including GST. GST = $1,100 ÷ 11 = $100. The ex-GST price is $1,000.
- Going the other way: an ex-GST price of $500 has GST of $500 ÷ 10 = $50, invoiced as $550.
The classic error is dividing by 10 on inclusive amounts. GST is 10% of the ex-GST price, which works out to 1/11 of the inclusive price. Dividing an inclusive figure by 10 overstates your GST by about 10%.
Where each number goes on the BAS
- G1: your total sales, including GST, and including GST-free income.
- 1A: the GST you collected on sales.
- 1B: the GST you paid on business purchases, your credits.
- The result: 1A minus 1B. Positive, you pay. Negative, you get a refund.
A worked quarter
| Item | Incl GST | GST |
|---|---|---|
| Taxable sales | $22,000 | $2,000 |
| GST-free sales | $3,000 | $0 |
| G1, total sales | $25,000 | |
| Taxable purchases | $5,500 | $500 |
| Bank fees, input taxed | $200 | $0 |
| 1A | $2,000 | |
| 1B | $500 | |
| Amount to pay | $1,500 |
Note the trap in that table: G1 includes the GST-free sales, but 1A does not. Mixing those up is one of the most common BAS mistakes.
Cash basis or accruals?
Most sole traders with turnover under $10 million can choose. Cash basis means you count income when it lands in your account and expenses when you pay them. Accruals means counting when invoices are issued. Cash is simpler and matches your bank statement, and your BAS form states which basis you're on. Pick one and stay consistent.
Five mistakes that cost sole traders real money
- Dividing by 10 instead of 11 on GST-inclusive amounts.
- Claiming GST credits on bank fees and interest. Those are input taxed, no GST was ever charged, so there is nothing to claim.
- Claiming GST on wages and super. No GST applies to either.
- Leaving GST-free income out of G1. G1 is all your sales.
- Claiming the full GST on mixed purchases, like a vehicle used partly privately. Claim only the business-use share.
When doing this by hand gets old
By hand, every quarter means sorting transactions, splitting GST line by line, tagging each one to the right quarter, and second-guessing labels the night before the deadline. The ClearQuarter kit does the splitting and quarter tagging automatically: enter income and expenses once, and it produces G1, 1A, 1B and the net figure for every quarter, with a built-in sanity check before you lodge.