Working Out the GST Portion of a Price: The 1/11th Rule Explained

Why the GST amount isn't just "10% of the price"

Most sole traders know that GST in Australia is charged at 10%. What catches people out is that once a price already includes GST, you can't just take 10% of that price to find the tax component. If you do, you'll overstate it every time.

This matters because the number reported to the ATO isn't the price charged. It's the GST component hidden inside that price. Getting the arithmetic wrong, even slightly, compounds across every invoice issued and every BAS period lodged, so it's worth understanding exactly how the calculation works.

The 1/11th rule explained

When a price already includes GST, the GST component is 1/11th of that total price, not 10% of it. This is a direct consequence of how the 10% rate is built into a GST-inclusive price.

Here's the logic. Say the GST-exclusive price of something is $100. Add 10% GST and the GST-inclusive price becomes $110. The GST component is $10. Now look at that $10 as a fraction of the final $110 price: $10 divided by $110 is 1/11, not 1/10. That's the whole rule. Once GST is already folded into a price, dividing that price by 11 gives the GST component. Taking 10% of the GST-inclusive price instead would give $11, which overstates the real GST amount.

A simple worked example

Imagine an invoice for $220, GST inclusive, for a day of consulting work. To find the GST component:

$220 ÷ 11 = $20

So $20 of that $220 is GST, and $200 is the GST-exclusive revenue for the job. Taking 10% of $220 instead would land on $22, which is the wrong number by $2. On a single invoice that might look small, but across a year of invoicing, consistently calculating it the wrong way around adds up to a real discrepancy between what's been recorded and what's actually owed or claimable.

The same logic runs in both directions. Working out what a GST-exclusive amount becomes once GST is added means multiplying by 1.1. Pulling the GST component back out of a price that already includes it means dividing by 11. It's easy to mix these two operations up, especially when moving between quotes, which are often GST-exclusive, and invoices, which are usually GST-inclusive.

Why this trips people up in practice

The maths itself is simple once it's been seen clearly, but a few everyday situations make it easy to apply incorrectly.

Partial payments and deposits

When a client pays a deposit rather than the full invoice, the GST component of that deposit is still 1/11th of the deposit amount, not 1/11th of the eventual full invoice. A $550 deposit on a larger job carries $50 of GST in it at the point it's received, regardless of what the final invoice total will be. Treating deposits as GST-free until the "real" invoice is issued is a common way people lose track of exactly when GST was actually collected, which matters because GST is generally accounted for in the period the payment or invoice occurs, depending on the reporting method used.

Discounts and adjustments

If a discount is applied to a GST-inclusive price, the 1/11th calculation needs to be run again on the new, discounted total rather than on the original price. A $110 item discounted by $11 becomes a $99 GST-inclusive price, and the GST component of that $99 is $9, not the $10 that applied before the discount. The same applies to credit notes, returns and any adjustment that changes the final amount a customer actually pays. Running the 1/11th calculation on the original, undiscounted figure after a discount has been applied is a quiet source of overstated GST.

Rounding across many transactions

A single transaction dividing cleanly by 11 is the exception rather than the rule. Most real-world prices produce a GST component with cents that need rounding, and the rounding convention used matters more once volume increases. A business issuing a handful of invoices a month might never notice a rounding discrepancy, but one issuing hundreds of small transactions, such as point-of-sale retail, can see rounding differences accumulate into a noticeable gap between invoice-level GST and the total GST actually reported. This is one of the reasons good accounting or invoicing software calculates GST at the line-item level consistently, rather than leaving it to be worked out manually on a running total.

Where this shows up in your records

Mixing up GST-inclusive and GST-exclusive figures tends to show up in a few predictable places. Quotes drafted without GST, then converted to invoices with GST added, are a common point of confusion if the conversion isn't done consistently. Expense receipts are another: a supplier's receipt usually shows a GST-inclusive total, and working out the GST credit available on that purchase means applying the same 1/11th logic to the amount actually paid, not re-adding 10% to it.

Keeping income and expenses recorded at consistent, clearly labelled amounts, whether that's GST-inclusive, GST-exclusive, or the GST component itself, makes it far easier to check the numbers later. A spreadsheet or tracking system that stores the GST-inclusive total and calculates the GST component automatically removes the chance of applying the wrong operation on a given day.

Why it matters for your BAS

A BAS return generally asks for GST-exclusive sales figures along with the GST collected and the GST credits claimed, so the 1/11th calculation sits underneath almost every figure that eventually appears on the form. A small, consistent error in how GST is pulled out of inclusive prices doesn't tend to show up as an obvious mistake. It shows up as numbers that are quietly off, quarter after quarter, in a way that's hard to trace back to its source once a lot of transactions have passed through.

Running a quick sanity check periodically, picking a handful of transactions and manually confirming the GST component with the 1/11th rule, is a reasonable habit regardless of what software or process is being used day to day. It's a simple enough calculation that it's worth being able to do by hand, even if a system handles it automatically the rest of the time.

Keeping it simple day to day

The rule itself doesn't need to be complicated: 1/11th of a GST-inclusive price is the GST component, and multiplying a GST-exclusive price by 1.1 produces the GST-inclusive version. The complexity tends to come from the situations around it, deposits, discounts, rounding and the back-and-forth between quotes and invoices, rather than the calculation itself.

Building a habit of checking which type of figure is being worked with before doing the maths, GST-inclusive or GST-exclusive, removes most of the room for error. For anything that still feels unclear about how a particular transaction should be treated, the ATO's own guidance at ato.gov.au covers specific scenarios in more depth, and a registered tax agent can confirm how it applies to a particular situation.