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 makes the invoice non-compliant. Missing the core ones — tax amount, description, value, either GSTIN, or place of supply on an inter-State invoice — can cost your client their ITC (Rule 36(2)):
| # | 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 design and development) |
| 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 |
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. A tax invoice must show it: at least the 4-digit code on invoices to registered buyers if your turnover last year was up to Rs. 5 crore, and the 6-digit code on every invoice above that (Notification 78/2020-Central Tax, in force since 1 April 2021). These are the codes HourSlip maps each profession to, described in the words of the CBIC scheme of classification itself:
| SAC Code | Description (CBIC scheme) | Who Uses It |
|---|---|---|
| 998314 | Information technology design and development services | Software Development, Web Development |
| 998313 | Information technology consulting and support services | IT Consulting / Support |
| 998391 | Specialty design services including interior design, fashion design, industrial design and other specialty design services | Graphic Design |
| 999632 | Services of authors, composers, sculptors and other artists, except performing artists | Content Writing |
| 998361 | Advertising services | Digital Marketing |
| 998311 | Management consulting and management services including financial, strategic, human resources, marketing, operations and supply chain management | Management Consulting, Financial Consulting, HR Consulting |
| 998512 | Permanent placement services, other than executive search services | Recruitment / Placement |
| 998382 | Advertising and related photography services | Photography (commercial / product) |
| 998383 | Event photography and event videography services | Photography & Video (events / weddings) |
| 999612 | Motion picture, videotape, television and radio programme production services | Video Production / Animation |
| 999631 | Services of performing artists including actors, readers, musicians, singers, dancers, television personalities, independent models and the like | Voice-over / Performing Artist |
| 999293 | Commercial training and coaching services | Education / Tutoring |
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: 998314 |
| 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." If you are GST-registered, head the document "Tax Invoice" so your client can see it carries claimable GST.
- 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.
FAQ
Can I issue a GST invoice without the client's GSTIN?
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.
What is the time limit for issuing a GST invoice for services?
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.
Can I issue invoices in foreign currency under GST?
Yes. Export invoices can be in foreign currency (USD, EUR, GBP). The GST portal requires the value in INR for GSTR-1 filing. For GST, convert at the rate applicable under generally accepted accounting principles on the date of the time of supply (CGST Rule 34(2)); income tax uses its own rule, so the two INR figures can differ. Include both the foreign currency amount and the INR equivalent on the invoice.
What happens if I issue a wrong 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 9B of your GSTR-1 (CDNR if your client is registered, CDNUR if not). You cannot void or delete a GST invoice after it is issued — corrections must go through credit/debit notes.
Is e-invoicing mandatory for freelancers?
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.
How should I number my GST invoices?
Under Rule 46(b), invoice numbers must be consecutive, unique per financial year, up to 16 characters, and may be alphanumeric (e.g. 2026-27/001). The series resets at the start of each financial year. Never skip or reuse a number — gaps invite scrutiny during GST audits.