You've raised an invoice. Your client asks: "Can you add 18% GST to this?" And suddenly your brain goes blank — do you multiply by 1.18? Do you divide? Is CGST and SGST included?
You're not alone. This exact confusion costs Indian freelancers and small business owners hours every month — sometimes even money, when they calculate it wrong and under-charge.
This guide gives you the exact formula, clear examples, a ready table, and a free calculator — so you never get this wrong again.
The Exact Formula to Add 18% GST
The formula is simpler than it looks:
GST Amount = Original Amount × 18 ÷ 100
Total Amount (with GST) = Original Amount × 1.18
That's it. No complicated steps. If your original invoice amount is ₹10,000:
- GST at 18% = ₹10,000 × 18 ÷ 100 = ₹1,800
- Total payable = ₹10,000 + ₹1,800 = ₹11,800
Or simply: ₹10,000 × 1.18 = ₹11,800
How CGST and SGST Split Works (For Indian Invoices)
When you sell to a customer in the same state as you (intra-state), the 18% GST is split into two equal halves on your invoice:
| Tax Component | Rate | Amount (on ₹10,000) |
|---|---|---|
| CGST (Central GST) | 9% | ₹900 |
| SGST (State GST) | 9% | ₹900 |
| Total GST | 18% | ₹1,800 |
| Grand Total | ₹11,800 |
When you sell to a customer in a different state (inter-state), the full 18% goes as IGST (Integrated GST) — no splitting required.
Ready-Made GST Amount Table at 18%
Use this table for quick reference. No calculator needed for these common amounts:
| Original Amount | 18% GST | Total Payable |
|---|---|---|
| ₹1,000 | ₹180 | ₹1,180 |
| ₹2,500 | ₹450 | ₹2,950 |
| ₹5,000 | ₹900 | ₹5,900 |
| ₹7,500 | ₹1,350 | ₹8,850 |
| ₹10,000 | ₹1,800 | ₹11,800 |
| ₹15,000 | ₹2,700 | ₹17,700 |
| ₹20,000 | ₹3,600 | ₹23,600 |
| ₹25,000 | ₹4,500 | ₹29,500 |
| ₹50,000 | ₹9,000 | ₹59,000 |
| ₹75,000 | ₹13,500 | ₹88,500 |
| ₹1,00,000 | ₹18,000 | ₹1,18,000 |
| ₹2,00,000 | ₹36,000 | ₹2,36,000 |
What Services Attract 18% GST?
18% is the most common GST rate in India. Here are the services and goods that fall under it:
Services at 18% GST:
- IT services, software development, SaaS products
- Consulting and professional services (management, legal, accounting)
- Advertising and marketing services
- Telecom services
- Financial services (not exempt ones)
- Restaurant services (non-AC restaurants used to be 5% but this varies)
- Hotel stays between ₹2,501 and ₹7,500 per night
Goods at 18% GST:
- Computers, laptops, smartphones
- Capital goods and machinery
- Most branded consumer products
- Hair care products, cosmetics
Not sure what GST rate applies to your specific service? Use our SAC Code Finder to look it up instantly using your service description.
Reverse GST: How to Extract 18% GST from a Total Amount
This is the most common mistake people make. If someone gives you a GST-inclusive price and you want to find the original base price, do NOT subtract 18% directly. That gives the wrong answer.
Wrong way: ₹11,800 − 18% = ₹11,800 − ₹2,124 = ₹9,676 ❌
Correct formula: Original Price = Total ÷ 1.18 GST Amount = Total − Original Price
Correct example:
- Total (inclusive of 18% GST) = ₹11,800
- Original price = ₹11,800 ÷ 1.18 = ₹10,000 ✅
- GST = ₹11,800 − ₹10,000 = ₹1,800 ✅
Calculate reverse GST instantly → Reverse GST Calculator
Real Example: Freelancer Invoice with 18% GST
Meet Priya. She is a freelance UX designer in Pune who bills a Mumbai-based startup ₹35,000 for a website redesign project. Since both are in Maharashtra (same state), she charges CGST + SGST.
Here is exactly how Priya's invoice looks:
INVOICE — Priya Design Studio
GSTIN: 27XXXXX...
Service: Website UX Redesign
Base Amount: ₹35,000.00
CGST @ 9%: ₹3,150.00
SGST @ 9%: ₹3,150.00
─────────────────────────────────
TOTAL: ₹41,300.00
If Priya's client were based in Delhi (different state), she would charge:
IGST @ 18%: ₹6,300.00
TOTAL: ₹41,300.00
The total is the same — only how the tax is split changes.
GST Rates Across All Slabs — Quick Comparison
Your service or product might not be at 18%. Here is the full picture:
| GST Slab | Common Examples |
|---|---|
| 0% (Exempt) | Education, healthcare, agriculture produce |
| 5% | Basic food items, economy hotel stays, transport |
| 12% | Processed food, business class air travel |
| 18% | IT services, most professional services, electronics |
| 28% | Luxury cars, tobacco, aerated drinks |
3 Common Mistakes to Avoid
Subtracting 18% directly to reverse-calculate GST: As shown above, always divide by 1.18 — never subtract 18% from the inclusive price. The difference can be hundreds of rupees on a large invoice.
Forgetting to split into CGST + SGST on the invoice: If you raise an intra-state invoice without splitting the 18% correctly, your client cannot claim Input Tax Credit (ITC). This is a compliance issue that can lead to penalties.
Charging GST when you're not registered: If your annual turnover is below ₹20 Lakhs (₹10 Lakhs for special category states), you are not required to register for GST and should NOT charge it on your invoices. Charging GST without a GSTIN is illegal.
👉 Calculate GST for Any Amount Instantly
Use our free GST Calculator — enter any base amount, pick your slab, and get CGST + SGST + IGST breakdown in one click. No signup needed.
GST on Professional Services: The ₹20 Lakh Threshold
One question comes up constantly among freelancers: "My annual billing is ₹18 Lakhs. Do I need to add GST?"
The answer depends on your total aggregate annual turnover:
- Below ₹20 Lakhs: GST registration is optional (₹10 Lakhs for Manipur, Mizoram, Nagaland, Tripura)
- Above ₹20 Lakhs: Mandatory registration and charging GST
- Online services (digital goods/software): Different rules — check with a CA
Once registered, you must charge GST on every invoice, file returns monthly or quarterly, and deposit the collected GST to the government after claiming Input Tax Credit.
Final Thoughts
Adding 18% GST is just multiplying by 1.18. Reversing it is just dividing by 1.18. The tricky part is knowing when to apply it, how to split CGST vs SGST, and whether you need to be GST registered at all.
Once you know these three things, GST invoicing becomes completely mechanical — something you can do in 60 seconds per invoice.
Quick FAQs
1. How do I calculate 18% GST on ₹10,000?
Multiply ₹10,000 by 0.18 to get the GST amount: ₹1,800. Add it to get the total: ₹11,800. Or simply multiply ₹10,000 by 1.18 = ₹11,800.
2. What is the formula for adding GST to any amount?
GST Amount = Original Amount × (GST Rate ÷ 100). Total with GST = Original Amount × (1 + GST Rate ÷ 100). For 18% GST: Total = Original Amount × 1.18.
3. How do I add 18% GST and split it into CGST and SGST?
Total GST at 18% is split equally. CGST = 9% of base amount. SGST = 9% of base amount. For example, on ₹10,000: CGST = ₹900, SGST = ₹900, Total = ₹11,800.
4. Can I charge 18% GST if I'm not registered?
No. You cannot charge GST on invoices unless you have a valid GSTIN. Doing so is illegal and can attract penalties. Register for GST first if your annual turnover exceeds ₹20 Lakhs.
5. How do I remove 18% GST from an inclusive price?
Divide the inclusive price by 1.18 to get the original base amount. Then subtract to find the GST portion. Example: ₹11,800 ÷ 1.18 = ₹10,000 base. GST = ₹11,800 − ₹10,000 = ₹1,800.
6. Is 18% GST the same for all professional services?
Most IT services, consulting, legal, and management services attract 18% GST. However, some specific services may have different rates. Always verify the SAC code for your specific service to confirm the applicable rate.
7. When should I use IGST instead of CGST + SGST?
Use IGST when the supplier and recipient are in different states (inter-state transaction). Use CGST + SGST when both are in the same state (intra-state transaction). The total tax rate remains the same — only the split changes.