How to Calculate GST: Add and Remove GST Formula
How to calculate GST: formulas to add GST to a price and remove it from an inclusive total, with worked examples for India, Australia, NZ, Canada and Singapore.
To calculate GST on a price that excludes tax, multiply the price by the GST rate: at 18%, GST on 10,000 is 10,000 × 0.18 = 1,800, so the total is 11,800. To remove GST from a price that already includes it, multiply by rate ÷ (100 + rate): the GST inside 11,800 is 11,800 × 18 ÷ 118 = 1,800. Those two formulas cover almost every GST calculation you will make on an invoice, in any country that uses a goods and services tax.
Below you will find both formulas, ready-made fractions for common rates, worked examples for India, Australia, New Zealand, Canada and Singapore, and the spreadsheet formulas to automate them. This is general information to help you check your numbers, not tax advice for your business.
The two GST formulas
Adding GST (price excludes GST)
- GST amount = net price × rate ÷ 100
- Gross price = net price × (1 + rate ÷ 100)
Removing GST (price includes GST)
- GST amount = gross price × rate ÷ (100 + rate)
- Net price = gross price ÷ (1 + rate ÷ 100)
The most common error is to remove GST by taking the rate as a straight percentage of the inclusive price. At 18%, 18% of 11,800 is 2,124, which overstates the tax by 324. The tax was calculated on the net price, so you must divide by 1.18 to get back to it.
GST fractions and multipliers for common rates
Use this table as a shortcut. The "GST fraction" extracts GST from an inclusive price; the "divide by" figure gives the net price.
| GST rate | Where you will see it | Add GST: multiply net by | GST fraction (from gross) | Net = gross divided by |
|---|---|---|---|---|
| 5% | India, Canada (federal GST) | 1.05 | 5/105 = 1/21 | 1.05 |
| 9% | Singapore | 1.09 | 9/109 | 1.09 |
| 10% | Australia | 1.10 | 10/110 = 1/11 | 1.10 |
| 13% | Canada (Ontario HST) | 1.13 | 13/113 | 1.13 |
| 15% | New Zealand; some Canadian HST provinces | 1.15 | 15/115 = 3/23 | 1.15 |
| 18% | India (standard rate for most goods and services) | 1.18 | 18/118 = 9/59 | 1.18 |
Rates change, so confirm the current rate for your product before you invoice. Our overview of how GST differs from VAT and sales tax explains why the same formulas work for all three.
How to calculate GST in India (CGST, SGST and IGST)
Since the rate rationalisation that took effect on 22 September 2025, most goods and services in India fall into two main GST slabs, 5% and 18%, with a 40% rate for specified luxury and "sin" goods and a few special rates for items such as precious metals. Our explainer on GST 2.0 and the new GST rates covers which items moved where, and the GST Council has published FAQs on the rate changes.
The calculation itself is the same as anywhere else; the split depends on where the supply goes:
- Intra-state supply (supplier and place of supply in the same state): half the rate as CGST, half as SGST (or UTGST in a union territory).
- Inter-state supply: the full rate as IGST.
Worked example: adding 18% GST
| Item | Intra-state (Pune to Mumbai) | Inter-state (Pune to Bengaluru) |
|---|---|---|
| Taxable value | ₹10,000.00 | ₹10,000.00 |
| CGST 9% | ₹900.00 | – |
| SGST 9% | ₹900.00 | – |
| IGST 18% | – | ₹1,800.00 |
| Invoice total | ₹11,800.00 | ₹11,800.00 |
Worked example: removing 18% GST from an MRP-style price
A shop sells an item for ₹5,000 including 18% GST. GST = 5,000 × 18 ÷ 118 = ₹762.71. Taxable value = 5,000 − 762.71 = ₹4,237.29. Intra-state, that is CGST ₹381.36 and SGST ₹381.35 (or ₹381.36 each with a one-paisa round-off).
Worked example: removing 5% GST
A price of ₹2,100 including 5% GST contains 2,100 × 5 ÷ 105 = ₹100 of GST, so the taxable value is ₹2,000.
A GST tax invoice in India also needs your GSTIN, the buyer's GSTIN for B2B supplies, the place of supply and the HSN or SAC code. The full field list is in our GST invoice guide.
How to calculate GST in Australia (10%)
Australia's GST rate is 10%, which makes the arithmetic easy:
- Add GST: $1,200 × 10% = $120 GST; total $1,320.
- Remove GST: divide the inclusive price by 11. $550 ÷ 11 = $50 GST; net price $500.
For invoices with several taxable sales, the Australian Taxation Office describes two acceptable rounding approaches on its tax invoices page: round GST once on the invoice total, or work it out per sale and round the sum, rounding half a cent up.
How to calculate GST in New Zealand (15%)
New Zealand uses 15%, and the GST fraction is 3/23.
- Add GST: $800 × 15% = $120; total $920.
- Remove GST: $230 × 3 ÷ 23 = $30 GST; net $200.
How to calculate GST and HST in Canada
Canada charges 5% federal GST in provinces without harmonisation, and a single harmonised sales tax (HST) in participating provinces. The Canada Revenue Agency publishes the current GST/HST rates by province; at the time of writing Ontario is 13% and Nova Scotia 14% (reduced from 15% on 1 April 2025).
- GST only (5%): $800 × 5% = $40; total $840.
- Ontario HST (13%): $800 × 13% = $104; total $904.
- Remove 13% HST: $1,130 × 13 ÷ 113 = $130; net $1,000.
Provincial sales taxes such as British Columbia's PST are separate taxes and are calculated on their own line. The rate you charge generally follows the place of supply, not where your business is based.
How to calculate GST in Singapore (9%)
- Add GST: S$2,500 × 9% = S$225; total S$2,725.
- Remove GST: S$1,090 × 9 ÷ 109 = S$90; net S$1,000.
GST formulas in Excel and Google Sheets
Put the rate in a single cell (say F1 = 18%) so you can reuse it.
| Task | Formula (amount in A2) |
|---|---|
| GST on a net price | =ROUND(A2*$F$1,2) |
| Gross price from net | =A2*(1+$F$1) |
| GST inside a gross price | =ROUND(A2*$F$1/(1+$F$1),2) |
| Net price from gross | =A2/(1+$F$1) |
| CGST or SGST (half of GST) | =ROUND(A2*$F$1/2,2) |
For a quick one-off calculation you can also use our tax calculator, which works for GST, VAT and sales tax rates. The method is identical to the one in our guide on how to add and remove VAT.
Calculating GST on a full invoice
On a multi-line invoice, calculate GST after discounts and on any taxable extras. A worked example of the full sequence (line totals, subtotal, discount, shipping, tax, deposit) is in our guide to calculating invoice totals. If you sell items at different GST rates, group them by rate and show the GST for each rate separately.
Rounding is where most small differences come from. India allows tax amounts to be rounded to the nearest rupee, and many Indian invoices add a "Round off" line to reach a whole-rupee total. Our article on invoice rounding rules compares the approaches used in India, Australia and the UK.
GST on discounts, freight and deposits
A few everyday situations change the amount GST is calculated on, even though the formula stays the same.
Discounts shown on the invoice
A discount that is agreed up front and shown on the invoice normally reduces the taxable value. Example at 18%: list price ₹20,000, 10% trade discount = ₹2,000, taxable value ₹18,000, GST ₹3,240, total ₹21,240. Calculating GST on the full ₹20,000 would overcharge ₹360 of tax.
Freight and packing charged by the seller
When you deliver the goods yourself and charge for it on the same invoice, that charge is usually taxed at the same rate as the goods. Add it to the taxable value before you calculate GST. Our guide to shipping and handling charges on invoices covers how this differs by country.
Advance payments and deposits
Depending on the country and on whether you sell goods or services, GST may be due when you receive an advance, not only when you issue the final invoice. On the final invoice, deduct the advance and any GST already accounted for so that tax is not charged twice.
Reverse charge
For some supplies the buyer, not the seller, accounts for GST. The invoice then shows the taxable value and a statement that reverse charge applies, without charging GST. Check your local rules before using it.
Quick reference: GST calculations side by side
| Country and rate | Net price | GST added | Gross price | GST inside the gross price |
|---|---|---|---|---|
| India 18% | 10,000.00 | 1,800.00 | 11,800.00 | 11,800 × 18/118 = 1,800.00 |
| India 5% | 2,000.00 | 100.00 | 2,100.00 | 2,100 × 5/105 = 100.00 |
| Australia 10% | 500.00 | 50.00 | 550.00 | 550 ÷ 11 = 50.00 |
| New Zealand 15% | 200.00 | 30.00 | 230.00 | 230 × 3/23 = 30.00 |
| Canada (Ontario) 13% | 1,000.00 | 130.00 | 1,130.00 | 1,130 × 13/113 = 130.00 |
| Singapore 9% | 1,000.00 | 90.00 | 1,090.00 | 1,090 × 9/109 = 90.00 |
Common GST calculation mistakes
- Taking the rate of the gross price. Always use rate ÷ (100 + rate) when removing GST.
- Charging IGST on a local sale (or CGST/SGST on an inter-state one). The place of supply decides which applies, and the wrong type has to be corrected.
- Using an old rate. After rate changes, update price lists and invoice templates on the effective date.
- Charging GST when you are not registered. Unregistered businesses generally cannot charge GST; check the threshold for your country.
- Mixing inclusive and exclusive prices on the same invoice without saying which is which.
Once your numbers are right, you can create a GST invoice in our free invoice generator: enter your rate, and it adds the tax line and total for you. No account is needed, and the invoice downloads as a PDF.
Frequently asked questions
What is the formula to calculate GST?
To add GST to a price, multiply the price by the rate: GST = price x rate / 100, and the GST-inclusive total = price x (1 + rate / 100). To remove GST from an inclusive price, use GST = inclusive price x rate / (100 + rate). For example, at 18 percent, GST on 10,000 is 1,800, and the GST contained in 11,800 is 11,800 x 18 / 118 = 1,800.
How do I remove 10% GST from a price in Australia?
Divide the GST-inclusive price by 11 to find the GST, because 10 / (100 + 10) equals 1/11. For a 550 dollar price, GST is 550 / 11 = 50 dollars and the GST-exclusive price is 500 dollars. A common mistake is to take 10 percent of the inclusive price, which gives 55 dollars and overstates the GST. Dividing by 1.1 gives the exclusive price directly.
How is GST split into CGST and SGST?
In India, a sale within the same state attracts CGST and SGST, each at half of the total rate. On a 10,000 rupee sale at 18 percent, the invoice shows CGST at 9 percent (900) and SGST at 9 percent (900), totalling 1,800. A sale to another state attracts IGST at the full 18 percent instead, so the invoice shows a single IGST line of 1,800.
What is the GST fraction for 15% GST in New Zealand?
The GST fraction for New Zealand's 15 percent rate is 3/23, because 15 / 115 simplifies to 3/23. Multiply a GST-inclusive price by 3 and divide by 23 to find the GST. For example, a 230 dollar inclusive price contains 30 dollars of GST, and the exclusive price is 200 dollars. Dividing the inclusive price by 1.15 gives the same exclusive figure.
How do I calculate GST in Excel?
If the GST-exclusive price is in A2 and the rate is 18 percent, use =ROUND(A2*18%,2) for the GST and =A2+B2 for the total. If A2 holds a GST-inclusive price, use =ROUND(A2*18/118,2) for the GST and =A2-B2 for the exclusive price. Store the rate in its own cell, such as $F$1, so you can change it once for every row.
Should GST be calculated on each line or on the invoice total?
Both approaches are common, and they can differ by a cent or a rupee because of rounding. Australia's tax office, for example, describes a total invoice rule and a taxable supply rule for invoices with several sales. India allows tax amounts to be rounded to the nearest rupee. Pick the method your accounting software uses and apply it consistently so your records reconcile.
Invoice Creator Editorial Team
Guides on invoicing, cash flow and small-business finance from Invoice Creator, operated by Next Software Development Company. Content is reviewed for accuracy against the fields our own invoice generator produces.
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