A GST invoice is not just a bill — it is a legal document. Get the format wrong, and your client cannot claim Input Tax Credit. Get the numbers wrong, and your GSTR-1 filing becomes a nightmare. This guide covers every mandatory field, the rules for CGST/SGST vs IGST, SAC codes for freelancers, invoice numbering requirements, and common mistakes that trip up even experienced professionals.
Mandatory Fields in a GST Invoice
Under Rule 46 of the CGST Rules, 2017, a tax invoice must contain the following fields. Missing any of these can render the invoice non-compliant, blocking your client's ITC claim:
| # | Field | Example |
|---|---|---|
| 1 | Name, address, and GSTIN of the supplier (you) | Priya Sharma, Mumbai — 27AXXPS1234A1ZQ |
| 2 | Invoice number (unique, sequential per FY) | INV-2025-0001 |
| 3 | Date of issue | 15-Apr-2025 |
| 4 | Name, address, and GSTIN of recipient (if registered) | TechCorp Pvt Ltd — 29AABCT1234A1ZQ |
| 5 | HSN/SAC code | 998314 (IT consulting) |
| 6 | Description of services | UI/UX Design — Mobile App |
| 7 | Quantity and unit (if applicable) | 40 hours |
| 8 | Total value of supply | Rs. 2,00,000 |
| 9 | Taxable value (after discounts) | Rs. 2,00,000 |
| 10 | GST rate | 18% |
| 11 | Amount of CGST, SGST, or IGST | IGST: Rs. 36,000 |
| 12 | Place of supply (state code + name) | 29 — Karnataka |
| 13 | Whether tax is payable on reverse charge | No |
| 14 | Signature or digital signature of supplier | Digital signature |
Missing GSTIN = ITC Blocked
CGST, SGST, and IGST Rules
The GST split depends entirely on the place of supply relative to the supplier's location:
| Scenario | Tax Applied | Example |
|---|---|---|
| Intra-state (same state) | 9% CGST + 9% SGST = 18% | You in Mumbai → Client in Pune (both Maharashtra) |
| Inter-state (different state) | 18% IGST | You in Bangalore → Client in Delhi |
| Export (outside India) | 0% (with LUT) or 18% IGST (without LUT) | You in India → Client in USA |
For services, the place of supply is the location of the recipient. So if you are a freelancer registered in Maharashtra (state code 27) and your client is registered in Karnataka (state code 29), it is inter-state — apply 18% IGST, not CGST+SGST.
The state is determined by the first two digits of the GSTIN:
- 27 — Maharashtra
- 29 — Karnataka
- 33 — Tamil Nadu
- 07 — Delhi
- 24 — Gujarat
- 36 — Telangana
- 06 — Haryana
- 09 — Uttar Pradesh
The most common GST invoice mistake: applying CGST+SGST when the client is in a different state. This creates a mismatch in GSTR-1 and blocks your client's ITC. Always check the state code in your client's GSTIN.
SAC Codes for Freelancers
SAC (Services Accounting Code) is the GST equivalent of HSN codes, but for services. Every service invoice must include the correct SAC code. Here are the most common ones for freelancers:
| SAC Code | Description | Who Uses It |
|---|---|---|
| 998311 | Management consulting and advisory | Management consultants, business advisors |
| 998312 | Business consulting services | Business consultants, strategy advisors |
| 998313 | IT design and development | Software developers, web designers |
| 998314 | IT consulting and support | IT consultants, tech advisors |
| 998361 | Literary and artistic creation | Content writers, copywriters, authors |
| 998362 | Audiovisual production | Video editors, animators |
| 998391 | Photographic services | Photographers |
| 998399 | Other professional services n.e.c. | General freelance services not elsewhere classified |
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Invoice Numbering Rules
GST law requires invoice numbers to be:
- Sequential: Invoices must be numbered in a continuous series. No gaps within a series (voided invoices should be noted but the number is consumed).
- Unique: No two invoices can have the same number within a financial year.
- Alphanumeric: You can use letters, numbers, and special characters (/ and -). Example: INV-2025-0001 or HS/25-26/001.
- Maximum 16 characters: The GST portal limits invoice numbers to 16 characters when filing GSTR-1. Keep your numbering scheme within this limit.
- Reset annually: You can (and should) reset the series at the start of each financial year. INV-2025-0001 → INV-2026-0001.
Sample GST Invoice
Here is what a compliant freelancer GST invoice looks like in practice:
| TAX INVOICE | |
|---|---|
| From: Priya Sharma | Invoice #: INV-2025-0012 |
| GSTIN: 27AXXPS1234A1ZQ | Date: 15-May-2025 |
| Mumbai, Maharashtra — 400001 | Place of Supply: 29 — Karnataka |
| To: TechCorp Pvt Ltd | SAC: 998313 |
| GSTIN: 29AABCT5678B1ZQ | Reverse Charge: No |
| Bangalore, Karnataka — 560001 | |
| Description | Hours | Rate | Amount |
|---|---|---|---|
| UI/UX Design — Mobile App Dashboard | 40 | Rs. 3,000/hr | Rs. 1,20,000 |
| Design System Documentation | 8 | Rs. 3,000/hr | Rs. 24,000 |
| Subtotal | Rs. 1,44,000 | ||
| IGST @ 18% (inter-state) | Rs. 25,920 | ||
| Total | Rs. 1,69,920 | ||
Common Mistakes
- Wrong tax split. Applying CGST+SGST when the client is in a different state (should be IGST), or vice versa. Always check the state code in the GSTIN.
- Missing SAC code. Many freelancers skip the SAC code. It is mandatory under Rule 46.
- Invoice number exceeding 16 characters. The GST portal truncates invoice numbers longer than 16 characters in GSTR-1, causing mismatches.
- Not mentioning "Tax Invoice." The document must be titled "Tax Invoice" — not just "Invoice" or "Bill."
- Calculating GST on GST. GST is calculated on the taxable value (base amount), not on the total including GST. If your service is Rs. 1,00,000, GST is 18% of Rs. 1,00,000 = Rs. 18,000. Total = Rs. 1,18,000.
- Not mentioning place of supply. Essential for determining CGST+SGST vs IGST. The state name and code must appear on the invoice.
Non-Compliant Invoices Cannot Be Used for ITC
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Frequently Asked Questions
Yes, but it becomes a B2C invoice. The client cannot claim ITC. For unregistered clients (individuals or businesses below the GST threshold), this is normal. For registered businesses, always ask for their GSTIN — they need it on the invoice to claim ITC, and they expect you to include it.
For services, the invoice must be issued within 30 days from the date of supply (completion of service). For banking and financial institutions, the limit is 45 days. There is no penalty for issuing the invoice within this window, but delayed invoicing can cause issues with quarterly GSTR-1 filing deadlines.
Yes. Export invoices can be in foreign currency (USD, EUR, GBP). The GST portal requires the value in INR for GSTR-1 filing — convert using the RBI reference rate or SBI TT buying rate on the date of invoice. Include both the foreign currency amount and the INR equivalent on the invoice.
Issue a credit note to cancel or correct the original invoice. The credit note should reference the original invoice number and date. Report the credit note in Table 9A of your GSTR-1. You cannot void or delete a GST invoice after it is issued — corrections must go through credit/debit notes.
E-invoicing (IRN generation on the IRP portal) is mandatory only for businesses with aggregate turnover exceeding Rs. 5 crore. Most freelancers are well below this threshold. For freelancers under Rs. 5 crore, a regular tax invoice (generated by a tool like HourSlip or even manually) is sufficient.