GST and VAT Calculator

Adding tax and removing it are not mirror images

Adding tax is multiplication: a $100 price at 10 per cent becomes $110. Removing tax is division, and dividing by 1.1 is not the same as subtracting 10 per cent. On a $110 total the tax component is $10, but on a $100 tax-inclusive total it is $9.09, not $10. Getting this backwards is the most common invoicing error there is.

Standard rates used here

Canada is the awkward one. The federal GST is 5 per cent, but most provinces add a provincial tax, and some harmonise the two into a single HST. Ontario, for example, charges 13 per cent combined. If you are invoicing into a specific province, enter the combined rate rather than the federal one.

Not everything attracts the standard rate

• Australia: most fresh food, medical services, education, and exports are GST-free or input-taxed; the registration threshold is $75,000 of turnover.
• New Zealand: financial services and residential rent are exempt, and exports are zero-rated at 0 per cent.
• United Kingdom: most food, children's clothing, books and public transport are zero-rated; many goods sit at the 5 per cent reduced rate.
• Canada: basic groceries and prescription drugs are zero-rated, which is different from exempt.
• India: GST is split across slabs of 5, 12, 18 and 28 per cent, with some goods outside GST entirely.

Zero-rated and exempt are not synonyms. Zero-rated supplies let you claim input tax credits on what you bought; exempt supplies generally do not. That distinction changes the answer on a business return even when the customer pays nothing.

Using the result on an invoice

1. Decide first whether the figure you have is tax-inclusive or tax-exclusive, and say so on the document.
2. Calculate the tax on the unrounded subtotal, then round once. Rounding each line and adding them up can leave the total a cent or two out.
3. Show the rate, the tax amount and the total separately: most tax authorities require it, and it stops arguments.
4. Reconcile against your activity statement or return using the tax component, not the total.

Rates and thresholds change with legislation. Confirm the current rate for your jurisdiction before you file.

What each input means

The calculator asks for 4 pieces of information. Every field is validated before the maths runs, so an impossible value is refused rather than quietly producing a wrong answer.

A worked example using the default inputs

With country and tax of Australia — GST 10%, amount of 1000, what are you starting from? of A price before tax, the calculator returns A$1,100.00 A$100.00 of GST.

The same arithmetic runs whatever you type into the form above, so you can reproduce any figure by matching the inputs listed here.

How this is calculated

Adding tax: total = amount × (1 + rate ÷ 100). Removing tax: amount = total ÷ (1 + rate ÷ 100), and the tax is the difference between the two.

Working values are kept at full precision and rounded only for display. Impossible dates, zero denominators, negative amounts where they make no sense, and out-of-range values are rejected before any result is shown.

The methods behind these money tools

Almost everything in this section is one of six calculations. Once you know which one a tool is running, the result stops being a black box: you can check it roughly on paper, and you can see which input is doing the work.

Two conventions matter more than any formula. First, the rate and the period must match: a 6% annual rate over monthly payments is 0.5% per month and 360 periods, not 6% per period. Second, money paid later is worth less than money paid now, which is why an amortising loan charges interest on the opening balance of each period rather than on the average balance.

Nominal and real returns are different things. A 7% return with 3% inflation buys roughly 3.9% more each year, not 7%: real rate ≈ (1 + nominal) ÷ (1 + inflation) − 1.

Two worked examples from this section

These scenarios use the same arithmetic as the tools above, so you can follow the numbers from input to result and then repeat them with your own figures.

A 30-year mortgage of $600,000 at 6.1%

The monthly rate is 6.1% ÷ 12 ≈ 0.5083% and there are 360 payments. Feeding those into the amortising payment formula gives a fixed instalment of about $3,636.

Monthly principal and interest

≈ $3,636

Total repaid over 30 years

≈ $1,308,900

Interest paid in total

≈ $708,900

Share of the total that is interest

≈ 54%

Notice that more than half of everything paid is interest. That is the single most useful fact a mortgage calculator produces, and it is why a small change in rate or term moves the total so much more than it moves the monthly figure.

Adding and removing 10% GST on $250

Adding tax is a multiplication; removing it is a division by the same factor, never a subtraction of the same percentage. Subtracting 10% from a GST-inclusive price gives the wrong answer because the tax was charged on the smaller, exclusive amount.

$250 plus 10% GST

$275.00

GST component

$25.00

$275 with GST removed (275 ÷ 1.1)

$250.00

The common mistake (275 − 10%)

$247.50 — too low

The same split works for any rate: divide by one plus the rate to go backwards. For 15% VAT, divide by 1.15; for 18% GST, divide by 1.18.

Assumptions behind every finance figure

Results are computed in exact arithmetic and only rounded for display, so a payment shown as $2,413.65 comes from the unrounded monthly rate rather than from a rounded one. Where a calculator compounds, it compounds at the frequency you choose; where it amortises, it assumes payments land at the end of each period unless the tool says otherwise.

Tax and contribution rates are the legislated rates for the tax year noted in each tool. They change, sometimes retrospectively, and none of these tools can see your personal circumstances: deductions, offsets, salary sacrifice, HELP or student loan balances, private health cover, investment losses, provincial or state tax, and treaty arrangements are all outside the model.

• Lenders and tax offices round differently from us, and they use daily interest, fees and assessment rules that a formula cannot reproduce. Expect small differences from an official statement.
• Nominal returns are not real returns. A 7% return with 3% inflation buys about 3.9% more each year, not 7%.
• Every figure is a planning estimate. For a decision with money at stake, confirm it with the lender, the tax office or a licensed adviser.

Sources and further reading

• Australian Taxation Office: GST
• GOV.UK: VAT rates
• Australian Taxation Office: tax rates and codes
• Reserve Bank of Australia: interest rates
• Moneysmart (ASIC): budgeting and borrowing