GST Calculator 2026 — Free Tool with CGST/SGST/IGST Auto-Split
Calculating GST manually is where most Indian invoicing mistakes start. Wrong slab, wrong split, wrong reverse calculation — and your invoice becomes a credit note. This 2026 guide gives you the exact GST calculation formulas, all 5 rate slabs, 5 worked examples, and a free online calculator that auto-detects whether to split into CGST + SGST or charge IGST. Built for Indian freelancers, CAs, agencies, and SMBs.
🧮 Open Free GST Calculator → No signupWhat's in this guide
The GST formula (3 lines)
Total = Base + Tax
Tax = Total − Base
That's it. Every other rule (split, slab, reverse charge) builds on these two formulas.
All GST rate slabs in 2026
| Slab | Examples | Common HSN/SAC |
|---|---|---|
| 0% (Nil-rated) | Fresh fruits, vegetables, milk, basic medicines, education services, healthcare | 0701, 0702, 0401, 9992, 9993 |
| 5% | Packaged food, transport (air, rail, road), small restaurants, books, tea | 9964, 9966, 9988 |
| 12% | Processed food, cell phones, textiles, butter, ghee, frozen meat | 0405, 8517, 5001-5404 |
| 18% | Most services, electronics, SaaS, IT, restaurants (with AC), telecom, banking | 998, 9983, 8471, 8528 |
| 28% | Luxury cars, tobacco, aerated drinks, premium goods, sin items | 8703, 2402, 2202 |
5 worked examples
Example 1 — Bengaluru freelancer, ₹50,000 design service to Mumbai client
Inter-state sale (Karnataka → Maharashtra). Service falls under 18% slab.
IGST = 50000 × 18 / 100 = ₹9,000
Total invoice = ₹59,000
Example 2 — Chennai agency, ₹1,00,000 marketing retainer to Chennai client
Intra-state sale (Tamil Nadu). Service at 18% — split equally.
CGST = 100000 × 9 / 100 = ₹9,000
SGST = 100000 × 9 / 100 = ₹9,000
Total invoice = ₹1,18,000
Example 3 — Cell phone retailer, ₹15,000 phone sold within Maharashtra
Intra-state sale. Cell phones fall under 12% slab.
CGST = 15000 × 6 / 100 = ₹900
SGST = 15000 × 6 / 100 = ₹900
Total invoice = ₹16,800
Example 4 — SaaS subscription, ₹2,499 to a Delhi customer (you're in Bengaluru)
Inter-state. SaaS at 18%.
IGST = 2499 × 18 / 100 = ₹449.82
Total invoice = ₹2,948.82 (round to nearest rupee per your billing policy)
Example 5 — Restaurant bill, ₹2,360 inclusive of 18% GST (reverse calculation)
The bill shows ₹2,360 total, GST already included. You need to know the base.
Base = 2360 / 1.18 = ₹2,000
Tax = 2360 − 2000 = ₹360
Split (intra-state): CGST ₹180 + SGST ₹180
Reverse calculation — when the total includes GST
Common scenarios where you need to reverse-calculate:
- Customer told you a "round" total (₹10,000) and asked you to make GST work backward
- You're auditing an existing invoice where GST is already included
- Quote shown to customer is GST-inclusive, but ledger entry needs the base
Tax = Total − Base
Worked example: ₹11,800 total inclusive of 18%
Tax = 11800 − 10000 = ₹1,800
Auto-split: how to know if it's CGST + SGST or IGST
The rule is mechanical. Compare the seller's GSTIN state code (first 2 digits) with the place of supply state code:
- Match → CGST + SGST (each at half the GST rate)
- Mismatch → IGST (full rate)
- Place of supply in a UT without legislature → CGST + UTGST
- Export or SEZ supply → IGST at 0% (zero-rated)
Need a deeper guide on the split rule and Section 12 of IGST Act? Read CGST vs SGST vs IGST — The 2026 Guide.
5 calculation mistakes to avoid
1. Mixing up the slab
Treating cell phones (12%) as electronics (18%) overcharges your customer by 6%. Always look up the HSN code first.
2. Double-counting GST in a markup
Marking up cost by 30% AND adding 18% GST on top is fine — but make sure the cost field is the base, not the GST-inclusive landed cost.
3. Rounding mid-calculation
Round only the final total. Rounding the CGST or SGST amount separately can create ₹1-2 mismatches that flag in GSTR-2A reconciliation.
4. Forgetting reverse charge
Some supplies (legal, GTA, security) shift the tax liability to the recipient. Don't add GST to your invoice for those — instead add the line "Tax payable on reverse charge basis."
5. Manual splits in Excel
Hand-typing 9% + 9% on a ₹47,318 invoice with 4 line items is exactly how typos happen. One wrong digit and the entire invoice tax type flips. Use a tool that auto-detects the seller and place of supply.
Stop calculating by hand. Try GST Invoice Generator — ₹9,999 lifetime →Frequently asked questions
How do I calculate GST on an invoice?
Tax = Base × Rate/100. For intra-state, split tax equally between CGST and SGST. For inter-state, the full amount is IGST. Example: ₹10,000 × 18% = ₹1,800 GST.
What are the current GST rate slabs in 2026?
5 main slabs: 0% (essentials), 5% (basic packaged goods), 12% (processed food, cell phones), 18% (most services, electronics), 28% (luxury, sin items).
How do I reverse-calculate GST from a total?
Base = Total / (1 + Rate/100). Then Tax = Total − Base. Example: ₹11,800 inclusive of 18% → Base = ₹10,000, Tax = ₹1,800.
Is the GST rate the same for goods and services?
No. Each HSN code (goods) and SAC code (services) has a specific rate. Most services are 18%; goods span 0-28%. Always check the official rate for your HSN/SAC.
Do I charge GST if my turnover is below ₹20 lakh?
Generally no — businesses below ₹20 lakh turnover (₹10 lakh in special-category states) are exempt. But you MUST register if you sell inter-state, on e-commerce, or in notified categories.
Can I use a GST calculator for export invoices?
Yes. Exports are zero-rated — charge 0% IGST and claim refund of input tax credit. Mark the invoice as an export and reference your LUT/bond.
Is the ByteZBridge GST calculator free?
Yes — the calculator at gstinvoice.app/gst-calculator is free. For PDF invoice generation + GSTR-1 ledger + recurring invoices, the ₹9,999 lifetime plan unlocks the full toolkit.