How to Calculate GST A Simple Step-by-Step Guide
Figuring out how to calculate GST trips up a lot of people the first time — not because the math is hard, but because the rate, the rules, and even whether GST exists at all change depending on where you’re shopping, invoicing, or running a business from. This guide breaks down the exact formula, walks through real examples, and covers how GST works differently in Australia, Canada, and the USA — so whichever country you’re calculating for, you’ll get the right number.
If you’d rather skip the manual math, our free GST Calculator does the work for you in one click.
What Is GST?
GST (Goods and Services Tax): a tax on consumption placed on the price of most goods and services. It’s collected by businesses at the point of sale and passed on to the government. The principal concept is the same in every application, but the rate at which it is charged and the extent of the charge vary significantly from country to country as well as from region to region, in some cases.
How Is GST Calculated? The Basic Formula
This is the part everyone actually needs: two formulas, depending on whether you’re adding GST to a price or removing it from one.
Adding GST to a price (GST-exclusive → GST-inclusive):
GST Amount = Original Price × (GST Rate ÷ 100)
Final Price = Original Price + GST Amount
Removing GST from a price (GST-inclusive → GST-exclusive):
Original Price = Final Price ÷ (1 + GST Rate ÷ 100)
GST Amount = Final Price − Original Price
That second formula is the one most people get stuck on — it’s tempting to just subtract the percentage directly, but that gives the wrong answer. Dividing by (1 + rate) is what correctly “reverses” the tax.
How Do I Calculate GST? A Worked Example
Let’s say you have a product priced at $100 and a GST rate of 10%.
Adding GST:
- GST Amount = $100 × (10 ÷ 100) = $10
- Final Price = $100 + $10 = $110
Removing GST (if $110 already includes GST):
- Original Price = $110 ÷ 1.10 = $100
- GST Amount = $110 − $100 = $10
Simple enough at a 10% rate — but the rate itself is where Australia, Canada, and the USA all diverge. Here’s how each one actually works.
How to Calculate GST in Australia
Australia uses a flat 10% GST rate on most goods and services, with a short list of GST-free items (basic food, some medical and education services, exports).
To calculate GST in Australia:
- Add GST: Price × 1.10
- Remove GST: Price ÷ 1.10
Example: A $250 item excluding GST → $250 × 1.10 = $275 including GST. A $275 GST-inclusive price → $275 ÷ 1.10 = $250 excluding GST, with $25 being the GST component.
Because the Australian rate is a single flat percentage, it’s the easiest of the three to calculate by hand — or with a dedicated GST calculator Australia tool, which is useful when you’re processing invoices regularly and want to avoid rounding errors.
How to Calculate GST/HST in Canada
Canada is more layered. A federal 5% GST applies everywhere, but several provinces combine it with their provincial tax into a single HST (Harmonized Sales Tax), while others charge GST plus a separate PST, RST, or QST.
| Province/Territory | Tax Type | Rate |
|---|---|---|
| Ontario | HST | 13% |
| Nova Scotia | HST | 14% |
| New Brunswick, PEI, Newfoundland & Labrador | HST | 15% |
| British Columbia | GST + PST | 5% + 7% |
| Saskatchewan | GST + PST | 5% + 6% |
| Manitoba | GST + RST | 5% + 7% |
| Quebec | GST + QST | 5% + 9.975% |
| Alberta, Yukon, NWT, Nunavut | GST only | 5% |
To calculate GST or HST in Canada, use the same core formula with the applicable combined rate for that province. For example, in Ontario: a $100 item × 1.13 = $113 including HST. In Alberta, the same item is just $100 × 1.05 = $105, since only the 5% federal GST applies there.
This is why a generic calculator often isn’t enough for Canadian invoicing — the correct rate depends entirely on the province the sale is considered to take place in.
Does the USA Have GST?
No — a lot of people get this wrong while looking for a GST calculator when they go shopping or invoice in the US. There isn’t any GST or VAT system in use in the USA. It does not use sales tax, which is levied at the state (and sometimes city/county) level rather than nationally, nor does it levy sales tax at every stage of production, as is the case with GST.
They range from state to state, from none in some states (such as Oregon, Montana, and Delaware) to at least 8-9% in some states when combined with local rates. For USA sales tax calculations, the formula is the same as above (Price × (1 + rate/100)), except that the combined state and local rate varies by region, and must be looked up instead of a single national rate.
Common Mistakes When Calculating GST
- Subtracting the percentage instead of dividing when removing GST from an inclusive price — this always gives a slightly wrong (too high) base amount.
- Using the wrong rate for the country or, in Canada’s case, the wrong province.
- Applying GST to GST-free or zero-rated items, like basic groceries in most GST/VAT systems.
- Forgetting rounding rules — some tax authorities require rounding to the nearest cent at a specific step in the calculation, not just at the end.
Frequently Asked Questions
How is GST calculated?
GST is calculated by multiplying the price by the GST rate. To add GST: Price × (1 + rate). To remove GST from an inclusive price: Price ÷ (1 + rate).
How do I calculate GST manually without a calculator?
Convert the GST rate to a decimal (e.g., 10% = 0.10), then multiply by the price to add GST, or divide the GST-inclusive price by 1 plus that decimal to remove it.
What is the GST rate in Australia?
Australia has a single flat GST rate of 10% on most goods and services.
What is the GST rate in Canada?
Canada’s federal GST is 5%, but combined GST/HST rates range from 5% to 15% depending on the province, since several provinces harmonize GST with their provincial sales tax.
Does the USA charge GST?
No. The USA uses state and local sales tax instead of GST, with rates and rules that vary by state.
Is there a faster way to calculate GST than doing it by hand?
Yes — our free GST Calculator adds or removes GST instantly, saves your recent calculations, and lets you copy, print, or share the result.
Try It Instantly
Now that you know the formula, skip the manual math entirely with CalcuHive’s free GST Calculator — enter your amount, choose add or remove, pick your rate, and get an accurate result in seconds.
