Whether you're a freelancer creating an invoice, a small business owner pricing your services, or a customer trying to understand what you actually paid in tax on your restaurant bill — understanding GST calculation is genuinely useful.
India's GST system is actually quite logical once you understand the core mechanics. This guide covers everything: the formulas, the different tax splits, the common mistakes, and the situations where people get confused.
India's GST Slabs: What Rate Applies to What?
India has four primary GST rates, plus a zero rate:
| Rate | What It Covers |
|---|---|
| 0% | Essential items — most food grains, vegetables, fresh milk, healthcare services, most education |
| 5% | Basic necessities — packaged food, household goods, economy class air travel, restaurant services (without ITC) |
| 12% | Standard goods — processed food, business class air travel, mobile phones |
| 18% | Most services and manufactured goods — IT services, consulting, restaurants with AC, financial services, insurance |
| 28% | Luxury and sin goods — cars, tobacco, aerated beverages, premium appliances |
For professional services (IT development, design, consulting, legal, accounting), the rate is almost always 18%.
Some items have a 3% rate (gold) or a special composition rate, but these are niche cases.
The Two Core Calculations Everyone Needs
Calculation 1: Adding GST to a Base Price (Exclusive GST)
This is what you do when you have a base price and need to find the total amount a customer pays.
Formula:
- GST Amount = Base Price × (GST Rate ÷ 100)
- Total (GST Inclusive) = Base Price + GST Amount
Example 1: A web developer quotes ₹25,000 for a website (18% GST).
- GST = ₹25,000 × 18% = ₹4,500
- Total invoice = ₹25,000 + ₹4,500 = ₹29,500
Example 2: A retailer sells a product at ₹500 base price (12% GST).
- GST = ₹500 × 12% = ₹60
- Customer pays: ₹500 + ₹60 = ₹560
Calculation 2: Extracting GST from a Total Price (Reverse/Inclusive GST)
This is used when you know the total amount paid and want to find the original base price and the tax component.
Formula:
- Base Price = Total Price ÷ (1 + GST Rate ÷ 100)
- GST Amount = Total Price − Base Price
Example 1: Your restaurant bill is ₹826 (18% GST inclusive). What was the food cost?
- Base price = ₹826 ÷ 1.18 = ₹700
- GST paid = ₹826 − ₹700 = ₹126
Example 2: You see a product listed at ₹1,180 (inclusive of 18% GST). What's the actual product price?
- Base price = ₹1,180 ÷ 1.18 = ₹1,000
- GST = ₹1,180 − ₹1,000 = ₹180
CGST, SGST, and IGST: The Three-Way Tax Split
Here's something that confuses a lot of people. GST isn't just one tax — it's actually split into different components depending on whether a sale is within one state or across states.
Intra-State Sale (Buyer and Seller in the Same State)
GST is split equally between the Central Government and the State Government:
- CGST: Central GST (goes to Delhi)
- SGST: State GST (stays with your state)
For 18% GST: 9% CGST + 9% SGST
Example: Freelancer in Pune (Maharashtra) bills a client in Mumbai (also Maharashtra) ₹50,000.
- CGST @ 9% = ₹4,500
- SGST @ 9% = ₹4,500
- Total = ₹59,000
Both CGST and SGST must appear as separate line items on the invoice.
Inter-State Sale (Buyer and Seller in Different States)
When selling across state borders, the full GST goes to the Central Government as a single integrated tax:
- IGST: Integrated GST (full rate, not split)
For 18% GST: 18% IGST (no split)
Example: Freelancer in Pune (Maharashtra) bills a client in Bangalore (Karnataka) ₹50,000.
- IGST @ 18% = ₹9,000
- Total = ₹59,000
The total amount is the same (₹59,000) regardless of intra or inter-state. What changes is only how the government distributes it internally — not what your client pays.
How do you determine intra vs inter-state? Compare your GST registration state with the client's billing address state. If same state = CGST + SGST. Different state = IGST.
The GST on Common Services (What Rate Do You Charge?)
Many freelancers aren't sure which rate applies to their specific service. Here's a quick reference:
| Service | GST Rate |
|---|---|
| Software development / IT consulting | 18% |
| Web design and development | 18% |
| Graphic design / creative services | 18% |
| Content writing and copywriting | 18% |
| Digital marketing / SEO | 18% |
| Legal services | 18% |
| Accounting and bookkeeping | 18% |
| Architecture and engineering | 18% |
| Healthcare / medical services | Generally 0% |
| Educational services (schools, colleges) | Generally 0% |
| Restaurant (AC, with liquor license) | 18% |
| Restaurant (non-AC, without liquor) | 5% |
When in doubt, look up your SAC (Services Accounting Code) on the CBIC portal to confirm the rate.
GST and Export of Services: Zero-Rated (Not the Same as Exempt!)
If you provide services to a client outside India — a US company, a UK startup, etc. — your services are treated as "Export of Services" and are zero-rated under GST.
Zero-rated does NOT mean exempt. The difference is subtle but important:
- Exempt services (like healthcare): You don't charge GST AND you can't claim Input Tax Credit on your inputs.
- Zero-rated exports: You don't charge GST on the invoice, but you CAN claim ITC on your inputs, OR claim a refund of the IGST paid.
For most freelancers with international clients, you should:
- Not charge GST on the invoice to the foreign client
- File a Letter of Undertaking (LUT) with your GST jurisdiction at the start of each year (this allows you to export without paying IGST upfront)
- Claim ITC on any GST you paid on your inputs
This is more complex than domestic invoicing — consult a CA or GST practitioner if your international billings are significant.
Input Tax Credit (ITC): Offsetting What You Pay
If you're GST-registered, you pay GST when you buy services or goods for your business. This GST you paid can be offset against the GST you collect from clients — this is called Input Tax Credit.
Example:
- You collected ₹18,000 in GST from clients this month (18% on ₹1 Lakh of services)
- You paid ₹3,600 in GST on your software subscriptions and equipment this month
- GST you need to deposit to the government: ₹18,000 − ₹3,600 = ₹14,400
The ITC reduces your effective GST burden. However:
- You can only claim ITC if the supplier has filed their GST returns correctly (the credit shows up in GSTR-2B)
- ITC is not available on some items (personal expenses, motor vehicles for general use, etc.)
- ITC cannot be claimed if your invoice doesn't have all mandatory fields
Common GST Calculation Mistakes
Mistake 1: Applying wrong rate Always verify your SAC code. Charging 12% when the correct rate is 18% means you've undercharged — and you're still liable to remit the correct rate (18%) to the government. You'll cover the shortfall from your own pocket.
Mistake 2: Splitting IGST into CGST+SGST for inter-state For inter-state transactions, the entire tax is IGST. Splitting it into CGST+SGST is incorrect and will cause reconciliation issues for your client's ITC claims.
Mistake 3: Forgetting to show GST separately GST must be shown as a separate line item on invoices. You cannot just write "Total ₹59,000 inclusive of GST" without showing the breakdown. Your client needs the explicit GST amount to file their returns.
Mistake 4: Calculating on a GST-inclusive price for the invoice When you quote a client ₹50,000 + GST, the GST is 18% of ₹50,000 = ₹9,000. Not 18% of ₹59,000. The percentage always applies to the base price, not the total.
👉 Let the calculator do the math for you Our GST Calculator handles both forward (adding GST) and reverse (extracting GST) calculations instantly. Just enter the amount and rate — no formula needed.
Quick FAQs
1. I made a mistake in the GST rate on an invoice. What do I do?
Issue a Credit Note or Debit Note to adjust the difference. A Credit Note reduces the previously invoiced amount; a Debit Note increases it. Both are GST-compliant adjustment documents that you declare in your returns.
2. My client is asking me to add GST to my invoice, but I'm not registered. What do I do?
You cannot charge GST if you're not registered. If your client needs a GST invoice, your options are: voluntarily register for GST (if your turnover makes this feasible), or explain to the client that you're not registered and provide a Bill of Supply instead.
3. What's the difference between GST-exclusive and GST-inclusive pricing?
GST-exclusive: ₹50,000 + 18% GST = ₹59,000 total. The quoted price doesn't include tax. GST-inclusive: ₹59,000 total (including GST). The quoted price already has tax built in. Be clear with clients upfront about which you're quoting.
4. Do I charge GST on reimbursements?
Pure reimbursements for actual expenses (where you pass on an exact cost like a train ticket) are generally not subject to GST. However, if there's a markup or the expense is part of your overall service, it may attract GST. Consult a CA for your specific situation.
5. Can I issue a GST invoice in foreign currency?
Yes, for export of services. The invoice can be in USD, GBP, or any other currency. You'll need to record the INR equivalent at the RBI reference rate on the invoice date in your GST returns.
Disclaimer
GST rates and rules are subject to change. Always verify the current applicable rate for your specific product or service category on the CBIC portal or with a GST practitioner.