GST Calculation Formula: How to Add GST and How to Remove It, With Worked Examples

Short answer

• To add GST: GST amount = base price x rate / 100. Total price = base price + GST amount. In one step, total price = base price x (1 + rate / 100).
• To remove GST: base price = total price x 100 / (100 + rate). GST amount = total price - base price. Never take the rate percent of the inclusive price, because that applies the rate to a figure that already contains the tax.
• Tax fraction shortcut: the GST inside an inclusive price is total price x rate / (100 + rate). At 10 percent that fraction is 1/11 of the total. At 15 percent it is 3/23 of the total.
• Worked check: a base price of 10,000 at 18 percent gives GST of 1,800 and a total of 11,800. Working backwards, 11,800 x 100 / 118 returns the same 10,000.
• India split: on a supply within one state, the rate splits equally between CGST and SGST. On a supply between states, the full rate is charged as IGST. The customer pays the same total either way.
• Rates differ by country: India now mainly uses 5 percent and 18 percent, Canada uses 5 percent federal GST with harmonized rates of 13 to 15 percent in some provinces, Australia uses 10 percent, and New Zealand uses 15 percent.

Introduction

Every GST calculation runs in one of two directions. Either you start with a price before tax and add GST to reach the total, or you start with a price that already includes GST and work backwards to find the price before tax. The two directions use different operations, and confusing them is the most common GST arithmetic mistake. Adding uses multiplication. Removing uses division. Applying the adding method to a price that already contains tax overstates the tax, sometimes by a noticeable amount on a large invoice.

This article gives both formulas in plain words and in symbols, shows where each formula comes from so you can rebuild it if you forget it, and then works through examples at several rates, from 5 percent to 18 percent, checking each one in the opposite direction. It also covers the split of Indian GST into CGST, SGST and IGST, the difference between GST inclusive and GST exclusive pricing, the errors that cause invoices to fail a check, and how GST rates and structures differ in India, Canada, Australia and New Zealand. GST stands for Goods and Services Tax, a tax on the supply of goods and services. The formulas are the same in every country that uses GST. Only the rate changes.

This is general information about how the arithmetic works. It is not tax advice for your situation. The rate that applies to a specific supply depends on what is being supplied, how it is classified and where the supply takes place, and rates change over time. Always confirm the rate, the classification and the place of supply from current official guidance, or from a qualified tax professional for your own case, before invoicing or filing. Arithmetic done carefully cannot correct a wrong rate.

What GST Is, and Why the Direction Matters

GST is a consumption tax. A business registered for GST charges it on its sales, claims back the GST it paid on its own business purchases, and hands the difference to the tax authority. The final customer, who cannot claim anything back, ultimately bears the tax. How that burden lands in a price is the subject of tax incidence, and Amberaisle explains that wider idea in Calculating Tax Incidence. For the arithmetic here, what matters is simpler: every price is either stated before GST or stated with GST already inside it, and you must know which one you are holding before you choose a formula.

A supplier quote or business invoice often shows the base price first and adds GST as a separate line. A shop price or consumer receipt usually shows one figure with GST already included, because consumer pricing rules in many countries require the displayed price to be the full amount payable. Neither presentation is wrong. The mistake is to treat one as if it were the other.

Here is the cost of that mistake in one line. An inclusive price of 2,360 at 18 percent contains GST of 360, as the removal examples below prove. Taking 18 percent of 2,360 instead gives 424.80, overstating the tax by 64.80, because it applies the rate to a figure that already contains the tax.

The Two Prices: Exclusive and Inclusive

Three terms cover every calculation in this article.

The base price, also called the GST exclusive price or the taxable value, is the amount before GST is added. It is the figure the rate was applied to when the tax was first charged.

The GST amount is the tax itself. On a tax invoice it appears as its own line, because buyer and seller both need it for their records and, for a registered buyer, for a possible input tax credit claim.

The total price, also called the GST inclusive price, is base price plus GST amount. It is the amount the customer actually pays.

Every formula below moves between these three figures when you know two of them, or one of them plus the rate. If a document gives you only one figure and no rate, no formula can rescue the calculation. Find the rate first.

Adding GST to a Price Before Tax

Use this direction when the price you have excludes tax: a supplier quote, a base price on an invoice, or a price list marked plus GST, ex GST, or GST exclusive.

The formula

GST amount = base price x GST rate / 100

Total price = base price + GST amount

The one step version combines both lines:

Total price = base price x (1 + GST rate / 100)

So at 18 percent you multiply by 1.18, at 5 percent by 1.05, at 10 percent by 1.10, at 15 percent by 1.15 and at 13 percent by 1.13. The multiplier is always 1 plus the rate as a decimal: the whole base price, plus the tax on top of it.

Worked examples: adding GST

Example 1: base price 10,000, rate 18 percent. GST amount = 10,000 x 18 / 100 = 1,800. Total price = 10,000 + 1,800 = 11,800. Check with the one step version: 10,000 x 1.18 = 11,800. The two methods agree.

Example 2: base price 50,000, rate 18 percent. GST amount = 50,000 x 18 / 100 = 9,000. Total price = 50,000 + 9,000 = 59,000.

Example 3: base price 2,500, rate 5 percent. GST amount = 2,500 x 5 / 100 = 125. Total price = 2,500 + 125 = 2,625. Check: 2,500 x 1.05 = 2,625.

Example 4: base price 80,000, rate 5 percent. GST amount = 80,000 x 5 / 100 = 4,000. Total price = 80,000 + 4,000 = 84,000.

Example 5: base price 200, rate 10 percent (the Australian rate). GST amount = 200 x 10 / 100 = 20. Total price = 200 + 20 = 220.

Example 6: base price 400, rate 15 percent (the New Zealand rate). GST amount = 400 x 15 / 100 = 60. Total price = 400 + 60 = 460.

Example 7: base price 250, rate 13 percent (the Ontario HST rate in Canada). GST amount = 250 x 13 / 100 = 32.50. Total price = 250 + 32.50 = 282.50. The same 250 base price in a GST only province at 5 percent would carry tax of 12.50 and a total of 262.50. Same goods, different place of supply, different tax. That is why the rate check comes before the arithmetic.

Example 8: a price with odd cents, base price 999, rate 18 percent. GST amount = 999 x 18 / 100 = 179.82. Total price = 999 + 179.82 = 1,178.82. Work to two decimal places, keep the full figure through the calculation, and round only at the end, in the way your tax authority requires.

Splitting Indian GST: CGST, SGST, IGST and UTGST

In India the total GST amount is recorded in parts, depending on where the supply takes place. The split changes how the tax is recorded and shared between governments. It never changes what the customer pays.

On an intra state supply, where buyer and seller are in the same state, the rate splits equally. Half is CGST, the Central GST, and half is SGST, the State GST. In Example 1 above, the GST of 1,800 at 18 percent splits into 900 CGST (9 percent of 10,000) and 900 SGST (9 percent of 10,000). In Example 2, the 9,000 at 18 percent splits into 4,500 CGST and 4,500 SGST. In Example 3, the 125 at 5 percent splits into 62.50 CGST (2.5 percent of 2,500) and 62.50 SGST.

On an inter state supply, where buyer and seller are in different states, there is no split. The full rate is charged as IGST, Integrated GST. Example 1 charged between states would show IGST of 1,800, and the total would still be 11,800.

In a union territory, the state half is called UTGST instead of SGST. The arithmetic is identical.

One practical warning follows. If an invoice shows CGST and SGST lines, each should be half the rate applied to the same base price, and the two should add up to the GST amount from the main formula. If they do not, the invoice does not reconcile.

Removing GST From a Price That Already Includes It

Use this direction when the price you have already contains tax: a shop price, a receipt total, or an expense you need to split into base price and tax for your records. This is sometimes called a reverse GST calculation. It needs division, not multiplication, and it is the direction people most often get wrong.

Where the formula comes from

Start from the adding formula. Total price = base price x (1 + rate / 100), which is the same as total price = base price x (100 + rate) / 100. To run it backwards, divide the total by (100 + rate), then multiply by 100:

Base price = total price x 100 / (100 + GST rate)

Then the tax is whatever is left:

GST amount = total price - base price

At 18 percent, divide the total by 1.18. At 10 percent, divide by 1.10. At 15 percent, by 1.15. At 5 percent, by 1.05. The divisor is the same multiplier you used to add the tax: removing tax undoes adding it.

The tax fraction shortcut

There is a second way to find the GST inside an inclusive price, and tax authorities themselves use it. Combining the two steps above, the GST amount is:

GST amount = total price x rate / (100 + rate)

The fraction rate / (100 + rate) is called the tax fraction. For the common rates it simplifies to a memorable fraction of the total:

• At 5 percent, the tax fraction is 5/105, which is 1/21 of the total.
• At 10 percent, it is 10/110, which is 1/11 of the total.
• At 13 percent, it is 13/113 of the total.
• At 15 percent, it is 15/115, which is 3/23 of the total.
• At 18 percent, it is 18/118, which is 9/59 of the total.

The Australian Taxation Office states the 10 percent case directly: GST is 1/11th of the amount charged for a sale. New Zealand Inland Revenue uses the 3/23 fraction for its 15 percent rate. Test the 18 percent fraction on Example 1 in reverse: 11,800 x 9 / 59 = 1,800, the same GST amount, written differently.

Worked examples: removing GST

Example 9: total price 11,800, rate 18 percent. Base price = 11,800 x 100 / 118 = 10,000. GST amount = 11,800 - 10,000 = 1,800. This is Example 1 seen from the other side, and the two agree, as they must.

Example 10: total price 2,360, rate 18 percent. Base price = 2,360 x 100 / 118 = 2,000. GST amount = 2,360 - 2,000 = 360. Compare the wrong method from earlier: 18 percent of 2,360 is 424.80, which overstates the tax by 64.80. On a year of purchases entered the wrong way, that error inflates your claimed tax and understates your real costs throughout.

Example 11: total price 1,050, rate 5 percent. Base price = 1,050 x 100 / 105 = 1,000. GST amount = 1,050 - 1,000 = 50. Check with the tax fraction: 1,050 x 1/21 = 50.

Example 12: total price 220, rate 10 percent. Base price = 220 x 100 / 110 = 200. GST amount = 220 - 200 = 20. Check with the Australian shortcut: 220 / 11 = 20.

Example 13: total price 460, rate 15 percent. Base price = 460 x 100 / 115 = 400. GST amount = 460 - 400 = 60. Check with the New Zealand fraction: 460 x 3 / 23 = 60.

Example 14: total price 2,000, rate 15 percent, with cents in the answer. GST amount = 2,000 x 3 / 23 = 260.87, rounded to two decimal places. Base price = 2,000 - 260.87 = 1,739.13. New Zealand Inland Revenue publishes this same case, a 2,000 inclusive price containing 260.87 of GST, as its illustration of the 3/23 fraction. Real world removal calculations often produce cents. Round the GST amount as your authority requires, then subtract it from the total to get the base price, so the two figures always add back to the total exactly.

Example 15: total price 113, rate 13 percent. Base price = 113 x 100 / 113 = 100. GST amount = 113 - 100 = 13.

A reliable habit covers every removal case: add your base price and GST amount together. If they do not reproduce the inclusive total you started with, to the cent, the calculation is wrong.

Finding the Rate From a Base Price and a Total

If you know both the base price and the total, you can check which rate was actually applied. This is useful when an invoice states a total and a base price but the rate line is missing, unclear or suspect.

Rate = (total price - base price) / base price x 100

Example 16: total 11,800, base 10,000. Total minus base = 1,800. Then 1,800 / 10,000 = 0.18. Times 100 gives 18 percent.

If the result is not a rate that exists in that country, something else is wrong: a discount may have been applied after tax, a delivery charge may be inside the total, or one figure may be a typo. Treat an impossible rate as a flag to query the invoice.

GST Inclusive and GST Exclusive Pricing in Practice

The same transaction can be quoted either way. A wholesaler quotes 10,000 plus GST. A retailer takes a shelf price of 11,800 and works backwards to record base price and tax. A customer comparing two quotes, one inclusive and one exclusive, is not comparing like with like until both are in the same form. Convert before you compare.

Discounts need care. In general terms, GST is charged on the value of the supply after a genuine discount that is part of the price, not on the undiscounted list price.

Example 17: a discount applied before tax. List price 10,000, discount 10 percent, rate 18 percent. Discounted base price = 10,000 - 1,000 = 9,000. GST amount = 9,000 x 18 / 100 = 1,620. Total price = 9,000 + 1,620 = 10,620. Charging 18 percent on the full 10,000 and then discounting the total would give a different answer and, in most systems, the wrong one. The treatment of vouchers and cashback offers varies by country and by the terms of the offer, so check current official guidance for anything beyond a straightforward price reduction.

Deposits follow the total, not a separate rule. The GST inside a deposit toward an inclusive total is the same proportion as the GST inside the whole price, found with the tax fraction. The fraction, not the headline rate, is the right tool whenever you split an amount that already contains tax.

Why the GST Amount Matters Separately: Input Tax Credit

A registered business does not hand over all the GST it collects. It claims a credit for the GST it paid on its own business purchases, called an input tax credit in India and Canada and a GST credit in Australia and New Zealand, and remits the difference.

Example 18: a net GST position. A business collects GST of 18,000 on its sales in a period and pays GST of 11,000 on its business purchases in the same period. Net GST payable = 18,000 - 11,000 = 7,000.

That subtraction is why the GST amount has to be separated correctly on every purchase, using the removal formula on inclusive prices. Overstate the GST on your purchases and you overstate your credits by the same amount. A GST credit is also a different mechanism from a deduction against income: it offsets tax collected, dollar for dollar, rather than reducing taxable income. Amberaisle covers the deduction side in What Is a Tax Write Off. The detailed credit rules differ by country and are outside the scope of this arithmetic guide.

How GST Differs by Country

The formulas never change. The rates, the rate structure and the way the tax is displayed and split change a great deal. Rates below were checked against official sources on 3 November 2026, and all of them can change. Treat them as a starting point for the rate check, not as the check itself.

India. India runs a dual GST: CGST plus SGST (or UTGST) within a state or territory, and IGST between states. Following the GST Council rate rationalisation that took effect on 22 September 2025, the structure was simplified from the earlier main rates of 5, 12, 18 and 28 percent to a merit rate of 5 percent and a standard rate of 18 percent for most goods and services, with a nil rate for some essentials and a special demerit rate of 40 percent for a select few goods and services. Some tobacco products were treated on a transitional basis. Because individual items moved between rates in that reform, confirm the rate for a specific item from the current official schedule for its classification, not from memory of the old slabs. The examples in this article at 18 percent and 5 percent use the two main current rates.

Canada. Canada charges a federal GST of 5 percent across the whole country. In participating provinces it is combined with a provincial portion into a single Harmonized Sales Tax, or HST: 13 percent in Ontario, 14 percent in Nova Scotia since 1 April 2025, and 15 percent in New Brunswick, Newfoundland and Labrador and Prince Edward Island. Other provinces charge the 5 percent GST alongside a separate provincial sales tax, and some charge GST only. The Canada Revenue Agency publishes the chart of which rate applies where, and the rate follows the place of supply. The same formulas apply to the combined HST rate: at 13 percent, divide an inclusive total by 1.13, as in Example 15.

Australia. Australia has a single national GST of 10 percent on most goods and services, administered by the Australian Taxation Office, with no state splits and no reduced standard rates, although some supplies are GST free or input taxed. Consumer prices are displayed inclusive, and the office gives the removal shortcut directly: the GST in an inclusive price is 1/11th of the total, as in Example 12.

New Zealand. New Zealand has a single standard GST rate of 15 percent on most goods and services, administered by Inland Revenue, with a small number of zero rated and exempt supplies. Prices are generally advertised inclusive. Inland Revenue expresses the removal step as the tax fraction 3/23, as in Examples 13 and 14.

A note on the United States. The United States has no GST. It uses separate state and local sales taxes instead, usually added at the checkout rather than included in the displayed price, with no input credit chain. Do not apply this article to a United States sales tax figure without checking that system on its own terms.

Common Errors, and How to Catch Them

Taking the rate percent of an inclusive price. The single most damaging error, covered in Examples 10 and 14. If the price already includes GST, divide first. A quick test: your GST amount should always be smaller than the headline rate percent of the inclusive total. If it is not, you have used the wrong method.

Using the wrong rate for the item or the place. A correct formula at a wrong rate is still a wrong invoice. Rates differ by classification in India and by province in Canada, and they change, as the Indian rationalisation of September 2025 and the Nova Scotia HST reduction of April 2025 both show. Date your rate check.

Splitting CGST and SGST unevenly, or splitting IGST. Within a state, the two halves are equal by construction. Between states there is one IGST line, not two half lines. A split that does not add back to the GST amount fails reconciliation.

Rounding at every step. Round once, at the end, in the way your authority requires. Rounding the base price, the tax and the split lines separately can leave totals that no longer add up.

Adding GST to a price that was already inclusive. This double counts the tax, and is common when a retail price is copied into an invoice template that adds tax automatically. The rate check above exposes it: the implied rate comes out far above any real rate.

Comparing an inclusive quote with an exclusive quote. Convert both quotes to the same basis before deciding which is cheaper.

Treating a calculator result as a rate confirmation. A calculator applies the rate you type in. It cannot tell you whether that rate applies to your supply. Classification and place of supply are questions for official guidance, not for arithmetic.

A Five Step Check for Any GST Figure

1. Identify the starting figure. Is it exclusive or inclusive? The words on the document decide this.
2. Confirm the rate for that supply, place and date from current official guidance.
3. Choose the matching direction: multiply to add, divide to remove.
4. Calculate to two decimal places and round once at the end.
5. Reverse your own result. Add GST to the base price you found, or remove GST from the total you produced, and confirm you land back on the figure you started with.

Frequently Asked Questions

What is the GST calculation formula? There are two. To add GST to an exclusive price: GST amount = base price x rate / 100, and total price = base price + GST amount. To remove GST from an inclusive price: base price = total price x 100 / (100 + rate), and GST amount = total price - base price. Every GST calculation is one of these two directions.

What is the fastest way to add GST on a calculator? Multiply the base price by 1 plus the rate as a decimal: by 1.18 at 18 percent, 1.05 at 5 percent, 1.10 at 10 percent, 1.15 at 15 percent. The result is the inclusive total. Subtract the base price if you also need the GST amount on its own.

How do I find the GST inside a price that already includes it? Multiply the inclusive price by the tax fraction, rate / (100 + rate). At 10 percent, divide the total by 11. At 15 percent, multiply by 3 and divide by 23. At 18 percent, multiply by 9 and divide by 59. Do not take the headline rate percent of the inclusive price.

Which formula do I use for a shop price? Shop prices usually already include GST, so use the removal formula to find the base price and the tax inside the price. Supplier invoices that show a base price plus a separate tax line use the addition formula.

Does the CGST and SGST split change what the customer pays? No. The customer pays the same total either way. Within one Indian state the rate splits equally into CGST and SGST. Between states, the full rate is charged as IGST. The split only decides how the tax is recorded and shared.

Why does my GST calculation not match the invoice? Work through the common causes in order: the rate may differ from the one you assumed, the starting price may be inclusive when you treated it as exclusive or the reverse, a discount may have been applied before tax, or rounding may have been done at several steps instead of once. The rate check in this article, (total - base) / base x 100, will often show which explanation fits.

Is GST the same in every country? No. The formulas are the same, but the rates and structures are not. India mainly uses 5 percent and 18 percent with a central and state split, Canada uses 5 percent federal GST with harmonized rates of 13 to 15 percent in some provinces, Australia uses a single 10 percent, and New Zealand uses a single 15 percent. Always confirm the rate for your country, classification and place of supply from current official guidance.

Sources & references

• Press Information Bureau, Government of India, Recommendations of the 56th Meeting of the GST Council, 3 September 2025. Material read via search result excerpts on 3 November 2026: rationalisation of the rate structure into a standard rate of 18 percent and a merit rate of 5 percent, with a special demerit rate of 40 percent for a select few goods and services, with changes in rates of goods and services to take effect from 22 September 2025, and tobacco related products treated on a transitional basis. Publication day recheck required for the current rate of any specific item.
• Canada Revenue Agency, GST/HST guidance and rates material, including the Quick Method guide RC4058 and freight carrier guidance, read via search result excerpts on 3 November 2026. Material read: GST at 5 percent, HST at 13 percent, 14 percent and 15 percent by province, with Nova Scotia at 14 percent since 1 April 2025. Publication day recheck required for the rate for a specific place of supply.
• Australian Taxation Office, How Australian GST works, and GST for small business. Material read via search result excerpts on 3 November 2026: GST is a tax of 10 percent on most goods and services sold in Australia, and GST is 1/11th of the amount charged for sales connected with Australia.
• New Zealand Inland Revenue, Tax Technical guidance on the GST rate and tax fraction, and GST guidance pages. Material read via search result excerpts on 3 November 2026: the GST rate is 15 percent from 1 October 2010, the tax fraction used to calculate GST in an inclusive price is 3/23, and a 2,000 inclusive price contains 260.87 of GST on that basis.