How to add or remove 10% GST on a calculator
To add 10% GST, multiply by 1.1. To take it out, divide by 1.1, or by 11 for the GST alone. Worked to the cent, with ATO rounding and GST-free items.
To add 10% GST to a price, multiply it by 1.1, so $59.00 becomes $64.90. To take GST out of a GST-inclusive price, divide by 1.1 for the price before GST ($59.00), or divide by 11 for the GST itself ($5.90).
ASIC’s Moneysmart gives the same multipliers with its GST calculator. The prices in this article are made-up examples, and every sum was run through stepbystepcalculator.com, so the displays quoted are the ones you’ll see there.
How do you add GST to a price?
Multiply the price before GST by 1.1. Section 9-70 of the GST Act sets the GST on a taxable sale at “10% of the value of the taxable supply”, and adding 10% is the same as multiplying by 1.1. Type 59 × 1.1 = and the calculator shows 64.9, which is $64.90.
The calculator’s % key gets there too. After + or −, the % key adds or takes away that percentage of the total so far, so 59 + 10% = also shows 64.9. The How it works page covers the rest of the % key, and splitting a bill step by step uses the same × 1.1 for a tip.
For several prices, total them first. 24.95 + 12.45 × 1.1 = shows 38.645, because the calculator multiplies before it adds and only the $12.45 picks up GST. Press 24.95 + 12.45 = for 37.4, then × 1.1 = for 41.14. 24.95 + 12.45 + 10% = also gives 41.14, since a % after + finishes the running total first.
How do you take GST out of a GST-inclusive price?
Divide the GST-inclusive price by 11 to find the GST, or by 1.1 to find the price before GST. The ATO’s small-business course says you work out the GST on a purchase “by dividing the total price of the purchase by 11” (Calculating GST on purchases). For $64.90, 64.9 ÷ 11 = shows 5.9, a GST of $5.90, and 64.9 ÷ 1.1 = shows 59, a price before GST of $59.00.
The 11 comes from the 10%. Ten parts of price gain one part of GST, making eleven, and the GST is one of the eleven. $59.00 plus $5.90 is $64.90, so the two answers check each other.
An invoice that only says “Total price includes GST” works the same way. The ATO’s Tax invoices page allows that wording only “if the GST amount is exactly one-eleventh of the total price”.
The calculator’s % key also has a shortcut. After × or ÷, % divides the number by 100, so 110% is 1.1 and 64.9 ÷ 110% = shows 59. That’s this calculator’s rule; on another one, check the result against ÷ 1.1.
Six GST sums for a $64.90 price
For an item that costs $59.00 before GST and $64.90 with it, the calculator gives these results:
| Keys | Display | What it gives |
|---|---|---|
59 × 1.1 = | 64.9 | Price with GST: $64.90 |
59 + 10% = | 64.9 | Price with GST, using the % key: $64.90 |
64.9 ÷ 11 = | 5.9 | GST in $64.90: $5.90 |
64.9 ÷ 1.1 = | 59 | Price before GST: $59.00 |
64.9 ÷ 110% = | 59 | Price before GST, using the % key: $59.00 |
64.9 − 10% = | 58.41 | Wrong: 59 cents short of the $59.00 price before GST |
Why taking 10% off a GST-inclusive price is wrong
Taking 10% off a GST-inclusive price leaves too little, because GST is 10% of the price before GST, not of the price after it. The ATO’s course puts it plainly: “don’t make the mistake of dividing the total price by 10. This is a common error.” On $64.90, 64.9 − 10% = shows 58.41 against the real $59.00, 59 cents too low. The GST comes out at $6.49 (64.9 ÷ 10 =) instead of $5.90, 59 cents too high.
The error is always the GST-inclusive price divided by 110, or one tenth of the real GST. A tenth of the price is the price ÷ 10, the real GST is the price ÷ 11, and the gap is the price ÷ 110. For $64.90, 64.9 ÷ 110 = shows 0.59.
On a $1,100 invoice it’s $10. 1100 − 10% = shows 990, when 1100 ÷ 1.1 = shows 1000, and you’d record $110 of GST where the invoice carries $100 (1100 ÷ 11 =).
How is GST rounded on an invoice?
GST is rounded to the nearest cent, with half a cent rounding up. Section 9-90 of the GST Act sets that rule for an invoice with one taxable sale, and the ATO’s Tax invoices page repeats it. The calculator doesn’t round to cents. It shows up to 12 significant digits (why 12), so the cent rounding is yours.
Take one item at $24.95 before GST. 24.95 ÷ 10 = shows 2.495, which ends in half a cent and rounds up to $2.50. 24.95 × 1.1 = shows 27.445, and $24.95 plus $2.50 is $27.45. Going back, 27.45 ÷ 11 = shows 2.49545454545, which rounds to the same $2.50.
An invoice with more than one taxable sale has two allowed methods, and they can land a cent apart. Add a second item at $12.45 before GST: its GST is 1.245 (12.45 ÷ 10 =), and the pair comes to $37.40 before GST (24.95 + 12.45 =). The ATO names the two methods like this:
| Rule | How the GST is worked out | GST on $24.95 and $12.45 |
|---|---|---|
| Total invoice rule | Add the unrounded GST for each sale, then round once | 2.495 + 1.245 = shows 3.74: $3.74 GST, $41.14 total |
| Taxable supply rule | Round each sale’s GST to what the accounting system records (whole cents here), then add | 2.5 + 1.25 = shows 3.75: $3.75 GST, $41.15 total |
Both results are lawful. Section 9-90 lets the supplier choose for the GST it pays and the buyer choose for the credits it claims, and the ATO says, “You and your customers don’t need to use the same rounding rules.” A receipt a cent away from your own sum may still be right.
GST-free items have no GST to take out
A GST-free item has no GST in its price, so dividing it by 11 finds GST that was never charged. The ATO’s GST-free sales list includes most basic food, some education courses, some medical, health and care services, some medicines, and exports. How an item is sold can change that: the ATO’s GST-free food page notes that “bread rolls are GST-free unless they are consumed in a restaurant.”
On a mixed receipt, divide only the taxable part by 11. A tax invoice with both kinds of item must “clearly show which items are taxable”, the ATO’s Tax invoices page says. Say a $30.50 receipt has $8.50 of GST-free groceries and $22.00 of taxable items. 22 ÷ 11 = shows 2, so the GST is $2.00. Dividing the whole $30.50 by 11 shows 2.77272727273, a GST of $2.77 and 77 cents too much.
Check a GST sum by running it backwards
The price before GST times 1.1 should give back the GST-inclusive price exactly. After 64.9 ÷ 1.1 = shows 59, carry on with × 1.1 = and the display returns to 64.9. The 10%-off answer fails the same test: 58.41 × 1.1 = shows 64.251. The calculator keeps each finished sum as a numbered step in your browser, so both attempts stay in the history where you can compare them.
This article covers the arithmetic only and is general information, not tax advice (see the terms of use). The ATO pages in the Sources list set out the rules, and a registered tax agent can advise on your own business.
Sources
- A New Tax System (Goods and Services Tax) Act 1999, section 9-70: The amount of GST on taxable supplies (AustLII, accessed 9 November 2026): GST is 10% of the value of a taxable supply.
- A New Tax System (Goods and Services Tax) Act 1999, section 9-90: Rounding of amounts of GST (AustLII, accessed 9 November 2026): rounding to the nearest cent, the two methods for invoices with several taxable sales, and who chooses between them.
- GST calculator (ASIC Moneysmart, last updated 27 October 2026): multiply by 1.1 to add GST, divide by 11 for the GST component.
- Goods and services tax (GST): Calculating GST on purchases (Australian Taxation Office, Essentials to strengthen your small business, last modified 28 June 2024): divide by 11, and dividing by 10 is “a common error”.
- Tax invoices (Australian Taxation Office, last updated 18 September 2026): “Total price includes GST”, invoices with taxable and non-taxable items, and rounding of GST.
- GST-free sales (Australian Taxation Office, last updated 14 September 2026): the main GST-free products and services.
- GST-free food (Australian Taxation Office, last updated 26 February 2026): examples of GST-free food, and when bread rolls are taxable.