Every invoice, quote and purchase bill in India runs into the same question: is this amount including GST or excluding GST? Get that one thing wrong and every number after it — your margin, your customer's bill, your GST return — is wrong too. Freelancers undercharge, shopkeepers overpay tax, and accountants spend hours untangling bills where someone simply subtracted 18% from a total and called the result "the GST part". This guide explains the two correct methods once and for all: how to add GST to a base price, and how to remove GST from a tax-inclusive price, with the exact formulas and worked examples. If you just need the answer fast, our free GST Calculator does both in one click — but read on if you want to understand what it is doing.
First, Know Which Price You Are Holding
An exclusive price is the price before tax. This is what you usually work with when you are preparing a quotation or setting your selling price: you decide your price first, then GST is added on top for the customer. An inclusive price is the final amount after tax is already inside it. MRP printed on packaged goods is always inclusive, and so are most prices you see on e-commerce sites and in retail bills.
The rule is simple: add GST when your amount is exclusive; remove GST when your amount is inclusive. Almost every GST mistake in small-business billing is a case of adding tax to a price that already had tax in it, or removing tax with the wrong formula. The two sections below show the correct method for each case.
The GST Slabs in India
GST is charged at four main rates depending on what you are selling, plus a zero/exempt category. Before any calculation, confirm which slab your product or service falls in — using the wrong rate makes even a perfect formula useless.
- 0% (exempt / nil-rated): Fresh fruits and vegetables, milk, eggs, unbranded flour and rice, books, healthcare and education services.
- 5%: Essential and mass-consumption items — packaged food, edible oils, sugar, tea, coffee, domestic LPG, economy air travel, restaurants without air-conditioning, life-saving drugs.
- 12%: Butter, cheese, ghee, fruit juices, Ayurvedic medicines, mobile phones, business-class air tickets and similar mid-range goods.
- 18%: The standard slab for most services (telecom, IT, banking, professional services) and many manufactured goods like electronics, soap, shampoo and appliances.
- 28%: Luxury and sin goods — cars, high-end motorcycles, aerated drinks, tobacco products, casinos and online gaming, often with an additional compensation cess.
When in doubt, check the HSN code (for goods) or SAC code (for services) — every code maps to one slab. Most day-to-day business calculations in India use 18%, which is why the examples below use it.
Skip the manual maths
Use our free GST Calculator to add or remove GST at 5%, 12%, 18% or 28% with the CGST/SGST split shown instantly.
Open the GST Calculator →How to Add GST to a Price (Exclusive → Inclusive)
When your price does not yet include tax, adding GST is straightforward multiplication:
GST amount = Base price × GST rate ÷ 100
Total price = Base price + GST amount
Worked example: ₹1,000 + 18% GST
- Base price: ₹1,000
- GST amount: ₹1,000 × 18 ÷ 100 = ₹180
- Total payable: ₹1,000 + ₹180 = ₹1,180
That is all there is to it — the customer pays ₹1,180, of which ₹180 is the tax component. If the sale is within your own state, that ₹180 is shown on the invoice as ₹90 CGST + ₹90 SGST (explained below). If your base price and rate are different, the same two steps apply: multiply by the rate, then add. This is the method to use for quotations, proforma invoices and price lists where you quote "plus GST".
How to Remove GST From a Price (Inclusive → Exclusive)
This is where most people go wrong. If a price of ₹1,180 already includes 18% GST, the GST inside it is not 18% of ₹1,180 (which would be ₹212.40). Why? Because the 18% was originally charged on the smaller base price, not on the final total. To work backwards, use this formula:
GST amount = Inclusive price × Rate ÷ (100 + Rate)
Base price = Inclusive price − GST amount
Worked example: extracting GST from ₹1,180
- Inclusive price: ₹1,180, GST rate: 18%
- GST amount: ₹1,180 × 18 ÷ 118 = ₹180
- Base price: ₹1,180 − ₹180 = ₹1,000
Notice we land exactly back at ₹1,000 — the same base we started with in the add-GST example. That round-trip check is the easiest way to verify any GST calculation: add tax one way, remove it the other way, and you should return to your starting number. You will use this remove-GST method constantly in real life: finding the taxable value from an MRP, splitting a retail bill into base + tax for your accounts, or checking whether a supplier's "GST included" quote is fair. Our GST calculator has a dedicated "Remove GST" mode for exactly this.
CGST + SGST vs IGST: How the Tax Is Split
The total GST amount never changes — but who it is paid to depends on where the buyer and seller are located, and your invoice must show the split correctly.
- Sale within the same state (intra-state): GST is split into two equal halves — CGST (Central GST, to the Union government) and SGST (State GST, to your state government). At 18%, the invoice shows CGST 9% + SGST 9%. In Union Territories, SGST is replaced by UTGST.
- Sale to another state (inter-state): The full amount is charged as IGST (Integrated GST) — a single 18% line instead of the 9% + 9% split. Imports into India also attract IGST.
What decides the type is the place of supply — broadly, where the goods are delivered or the service is received. A Delhi shop selling to a Delhi customer charges CGST + SGST; the same shop shipping to Mumbai charges IGST. Charging the wrong type is a common error when businesses start selling across state lines through marketplaces, and it creates mismatches in returns even though the total tax paid is identical.
5 GST Calculation Mistakes Businesses Keep Making
- Subtracting the percentage from the total. Taking 18% off ₹1,180 gives ₹967.60 as the "base" — wrong. The correct base is ₹1,000, found with the Rate ÷ (100 + Rate) formula above.
- Using the wrong slab. Similar-looking products can sit in different slabs (for example, different forms of the same food item). One wrong HSN code and every invoice you issue is wrong.
- Splitting CGST/SGST on an inter-state sale. Cross-state invoices must show IGST as a single amount, not two halves.
- Rounding each line separately. Round the GST on the invoice total, not per item line, or your totals will drift by a few paise against your customer's books and their ITC claim can mismatch.
- Treating "GST included" quotes carelessly. When a vendor quotes ₹50,000 "including GST", your actual cost for margin purposes is the base value — extract it with the remove-GST formula before comparing quotes.
Input Tax Credit (ITC), in One Paragraph
ITC is the mechanism that stops GST from piling up at every stage of a supply chain: when your business buys goods or services, you pay GST on those purchases, and when you sell, you collect GST from your customers — you then deposit only the difference with the government, claiming credit for the tax you already paid on inputs. So if you collected ₹27,000 in GST on sales and paid ₹18,000 in GST on purchases, you pay ₹9,000, not ₹27,000. This is why keeping proper tax invoices for every business purchase matters: no invoice, no credit.
Do the Maths in Seconds, Not Minutes
Formulas are worth knowing — they help you spot a wrong invoice instantly — but for daily billing, a calculator is faster and safer. ToolKit India's free GST Calculator handles both directions (add and remove), all four slabs, and shows the CGST/SGST split automatically, right in your browser with no sign-up. While you are here, you might also find our EMI Calculator useful for loan planning, and our QR Code Generator for creating UPI payment codes for your shop or invoices. And if you are planning a loan alongside your business purchases, our companion guide on how to calculate home loan EMI explains that formula step by step too.