13 Mandatory Fields in a GST Invoice

I once watched a business owner in Surat lose ₹3.2 lakh in Input Tax Credit because his accountant forgot to add the place of supply on 47 invoices. Forty-seven invoices. The GST officer rejected every single one during assessment. No place of supply, no way to determine CGST/SGST vs IGST — and therefore, no ITC for the buyer.

That's the thing about GST invoices. They look simple. You fill in your name, the buyer's name, the amount, the tax — done, right? Not quite. Under Rule 46 of the CGST Rules, 2017, every tax invoice must contain exactly 13 mandatory fields. Miss even one, and the invoice becomes non-compliant. Your buyer can't claim ITC, your returns may show mismatches, and during an audit, you're in trouble.

I've been helping small business owners get their invoicing right for years now, and I've seen every possible mistake. So let me walk you through each of the 13 fields — not just what they are, but what goes wrong with each one, why it matters, and how to get it right every time.

The 13 Mandatory Fields — Deep Dive

1. Supplier's Name, Address & GSTIN

This is the very first thing that must appear on your invoice. The registered name of the supplier (that's you, the seller), your complete address as registered on your GST certificate, and your 15-digit GSTIN.

Sounds straightforward, but here's where people mess up. Your name on the invoice must match your name on the GST registration certificate exactly. If your certificate says "Sharma Electronics and Trading" but your invoice says "Sharma Electronics" — that's a mismatch. During GSTR-2A/2B reconciliation, the buyer's system may flag this, and their ITC claim could get stuck.

The GSTIN itself is 15 characters long:

• First 2 digits: State code (e.g., 27 for Maharashtra, 29 for Karnataka)
• Next 10 digits: Your PAN number
• 13th digit: Entity number (for multiple registrations in the same state)
• 14th digit: "Z" by default
• 15th digit: Check digit

Common mistake: Typos in the GSTIN. I've seen invoices where the supplier typed "27AABCS1234F1Z5" instead of "27AABCS1234F1Z6" — one digit off. The buyer entered this wrong GSTIN in their return, the system couldn't match it, and ITC worth ₹78,000 was held up for two quarters until the correction was made through an amendment.

2. Invoice Number

Every GST invoice needs a unique, consecutive serial number. The rules are specific here:

• It can contain alphabets, numerals, and special characters (hyphens, slashes)
• Maximum 16 characters
• Must be sequential — no gaps, no duplicates
• Must be unique within a financial year
• You can have multiple series (e.g., one per branch or product line), but each series must be sequential

Good examples: INV-2526-0001, BC/26/001, MUM-001-2526

Bad examples: Random numbers like "7842", or using the date as the invoice number, or repeating last year's sequence without resetting.

Why this matters: During a GST audit, the officer will look at your invoice series for continuity. If they see INV-001, INV-002, INV-005 — they'll ask about INV-003 and INV-004. Missing numbers raise red flags about suppressed sales. And if you have duplicate invoice numbers? That's a major compliance issue that suggests either incompetence or intentional misreporting.

A garment retailer in Tirupur was using a billing register where the shop assistant would manually write invoice numbers. Over 8 months, they had 14 duplicate numbers and 23 gaps in the sequence. When the GST officer reviewed their records, it triggered a detailed scrutiny that took 4 months to resolve.

3. Date of Invoice

The date on which the invoice is issued. This seems like the easiest field to get right, but there are timing rules you need to know:

• For goods: The invoice must be issued before or at the time of removal of goods (if goods are being moved) or before or at the time of delivery (if goods are not being moved).
• For services: The invoice must be issued within 30 days from the date of supply of service. For banking/financial services, it's 45 days.

Common mistake: Back-dating invoices. Some businesses deliver goods on April 5 but issue the invoice on April 12 with a date of April 3 to fit it into the previous month's return. This is illegal. If caught during an audit (and auditors do check delivery challans against invoice dates), you can face penalties under Section 122 of the CGST Act.

Another mistake: Not issuing the invoice at the right time for services. You completed a consulting project on March 15. You sent the invoice on April 20 — that's 36 days later. You've violated the 30-day rule. The time of supply shifts to the date of completion (March 15), meaning GST should have been paid in March. Late invoicing means wrong return filing, which means interest and potential penalties.

4. Recipient's Name, Address & GSTIN

If the buyer is GST-registered, their GSTIN is mandatory on the invoice. If they're unregistered (B2C customer), then their name and address are sufficient for invoices above ₹50,000. For B2C invoices below ₹50,000, even the buyer's details aren't strictly required — though it's good practice to include them.

The critical mistake here: Not asking the buyer for their GSTIN. I know, it sounds basic. But walk into any wholesale market in Chandni Chowk or Chickpet and watch how many transactions happen without the buyer providing their GSTIN. The seller issues a bill with "Cash" as the buyer name. The buyer then complains months later that they can't claim ITC.

Also important: if the buyer gives you a GSTIN, verify it. Go to the GST portal and search the GSTIN. Check that the name, state, and registration status are correct. Issuing an invoice to a cancelled or non-existent GSTIN creates problems during filing.

Buyer Type	What's Required on Invoice
GST-registered buyer	Name, address, and GSTIN (mandatory)
Unregistered buyer (invoice > ₹50,000)	Name, address, and state with code
Unregistered buyer (invoice ≤ ₹50,000)	Not mandatory, but recommended

5. HSN Code / SAC Code

HSN stands for Harmonized System of Nomenclature — it's an international classification system for goods. SAC stands for Services Accounting Code. Every product or service under GST has a specific code, and you must mention it on the invoice.

The number of digits you need depends on your turnover:

Annual Turnover	HSN Digits Required
Up to ₹5 Crore	4 digits
Above ₹5 Crore	6 digits

Why this field is critical: The HSN/SAC code determines the GST rate applicable to your product or service. A wrong code can mean you're charging the wrong tax rate — and that's a compliance disaster.

Real example: A stationery supplier in Ahmedabad was selling notebooks under HSN 4820 (which attracts 12% GST) but was using code 4901 (printed books, which attract 0% GST). He wasn't charging GST on notebooks for almost a year. When the department caught it, the liability was ₹1.4 lakh in unpaid tax plus ₹18,000 in interest.

How to find the right code: Search on the CBIC website or the GST portal itself. Type your product name and browse through the results. If you sell "cotton fabric," the HSN is 5208. If you provide "IT consulting services," the SAC is 998313. When in doubt, ask your CA — getting the code wrong is much more expensive than a one-time consultation fee.

6. Description of Goods or Services

A clear, specific description of what you're selling. Not "Item 1" or "Services rendered" — that's not going to fly in an audit.

Good descriptions:

• "Cotton Round-Neck T-Shirt, Size L, Navy Blue" (for a garment seller)
• "Website Development — React Frontend + Node.js Backend, 12 pages" (for a developer)
• "Basmati Rice, 25 kg bag, Grade A" (for a food grain trader)
• "Monthly Accounting Services for March 2026" (for a CA firm)

Bad descriptions:

• "Goods" — completely vague
• "As per PO" — the invoice must be self-contained
• "Misc items" — raises red flags during audit
• "Service charges" — what service? Be specific

The description should be detailed enough that someone reading the invoice can understand exactly what was supplied without needing additional documents. During audits, vague descriptions are the first thing officers question — because they can't match the supply with the HSN code and GST rate applied.

7. Quantity & Unit

The quantity of goods supplied along with the unit of measurement. This field uses UQC (Unique Quantity Codes) — standardized codes like PCS (pieces), KGS (kilograms), LTR (litres), MTR (metres), NOS (numbers), etc.

For services, this might be hours, months, projects, or sessions — depending on how you bill.

Common mistake: Not mentioning the unit at all. An invoice that says "Widget — 50 — ₹25,000" doesn't tell anyone whether those are 50 pieces at ₹500 each or 50 kg at ₹500/kg. This ambiguity creates problems during returns and reconciliation, and can lead to disputes with the buyer.

Another issue: inconsistent units. If you sell fabric and sometimes use metres, sometimes yards, and sometimes "pieces" (for cut fabric) — your records become a mess. Standardize your units and stick with them throughout the financial year.

8. Total Value of Supply

This is the total value of the goods or services before taxes. It includes the price of goods/services and any other charges that are part of the supply — like packing charges, freight (if included in the supply), installation charges, etc.

However, discounts given at the time of supply are deducted from this value.

Example:

• Product price: ₹10,000
• Packing charges: ₹500
• Discount: -₹1,000
• Total value of supply: ₹9,500

This ₹9,500 is the base on which GST is calculated. Getting this number wrong means wrong tax calculation, which cascades into wrong returns, wrong ITC for the buyer — the whole chain falls apart.

9. Taxable Value

The taxable value is the value on which GST is actually computed. In most straightforward cases, it equals the total value of supply. But there are situations where they differ:

• When additional discounts apply after the initial billing
• When some items in the invoice are exempt from GST
• When part of the supply is under a different tax treatment

Example for a mixed invoice: You sell 3 items — Product A (₹5,000, taxable at 18%), Product B (₹3,000, taxable at 12%), and Product C (₹2,000, exempt). Your total value of supply is ₹10,000, but taxable values are computed separately for each rate category. This matters because tax must be calculated correctly for each rate slab.

10. Tax Rate

The applicable GST rate, shown as a percentage. This must be broken down properly:

• Intra-state supply (seller and buyer in same state): Show CGST rate + SGST rate. For example, 9% CGST + 9% SGST = 18% total.
• Inter-state supply (seller and buyer in different states): Show IGST rate. For example, 18% IGST.

The most dangerous mistake here? Applying the wrong type of tax. If your business is in Delhi and your buyer is also in Delhi, but you charge IGST instead of CGST+SGST — both of you have a problem. You've deposited tax in the wrong head, and the buyer's ITC claim will show a mismatch.

I've seen a furniture manufacturer in Jodhpur who supplied goods to a Jodhpur-based hotel but charged IGST because his accountant confused the buyer's corporate headquarters (in Mumbai) with the place of supply (Jodhpur). The tax was paid under the wrong head, and correcting it required filing refund claims and amendments — months of paperwork.

11. Tax Amount

The actual rupee amount of tax, broken down into CGST, SGST, or IGST components. This must be calculated from the taxable value multiplied by the applicable rate.

Example:

Item	Taxable Value	CGST (9%)	SGST (9%)	Total
Office Chair	₹8,000	₹720	₹720	₹9,440
Office Desk	₹12,000	₹1,080	₹1,080	₹14,160
Total	₹20,000	₹1,800	₹1,800	₹23,600

Common mistakes:

• Rounding errors — GST amounts should be rounded off to the nearest rupee. Not ₹720.45, just ₹720 or ₹721.
• Calculating tax on the inclusive amount instead of the base amount. If the MRP is ₹1,180, the taxable value is ₹1,000 (not ₹1,180). GST is on the pre-tax amount.
• Not showing the breakup. Just writing "GST: ₹3,600" without splitting it into CGST and SGST is non-compliant.

12. Place of Supply

This field determines which state's GST applies to the transaction. The place of supply is shown as the state name along with its code — for example, "Maharashtra (27)" or "Karnataka (29)".

The rules for determining place of supply are different for goods and services:

• Goods (with movement): The place where the movement of goods terminates — i.e., where the goods are delivered.
• Goods (without movement): The location of the goods at the time of delivery.
• Services (general rule): The location of the recipient of services.
• Immovable property-related services: The location of the property.
• Restaurant/catering: Where the service is performed.

This is arguably the most confusing of all 13 fields, and the one that causes the most problems. Get it wrong, and the entire tax calculation is wrong — CGST+SGST vs IGST, which state gets the revenue, whether ITC matches up in the system.

A software company in Bangalore provided services to a client registered in Mumbai, but the services were actually consumed at the client's Bangalore branch. Where's the place of supply? It's Maharashtra (the location of the registered recipient), not Karnataka. So IGST applies, even though both parties are physically in Bangalore. Counter-intuitive, but that's the law.

13. Signature of the Supplier

The invoice must bear the signature — physical or digital — of the supplier or their authorized representative. In the digital age, this includes:

• Handwritten signature (scanned and placed on the invoice)
• Digital Signature Certificate (DSC)
• Digital signature via Aadhaar-based e-sign
• For e-invoices, the IRN and QR code serve as authentication

Many businesses skip this field entirely, especially when generating invoices through software. Technically, an unsigned invoice is not a valid tax invoice. In practice, most GST officers won't reject an invoice solely for a missing signature — but during formal audits or legal proceedings, the absence of a signature can weaken your position.

What Happens If Fields Are Missing?

This is the part that keeps accountants up at night. Missing fields on a GST invoice don't just look unprofessional — they have real financial and legal consequences.

ITC Rejection

The biggest hit comes to the buyer. If your invoice is missing mandatory fields, the buyer's ITC claim on that purchase can be rejected during assessment. Under Section 16(2) of the CGST Act, possession of a valid tax invoice is a prerequisite for claiming ITC. An invoice missing any mandatory field is not considered "valid."

Think about what this means for your business relationships. If you're a supplier who consistently issues incomplete invoices, your B2B customers are going to stop buying from you. Why would they, when every purchase comes with the risk of losing ITC?

Penalties Under the CGST Act

Under Section 122 of the CGST Act, issuing invoices that don't comply with the prescribed format can attract a penalty of ₹10,000 or the tax due — whichever is higher. For deliberate non-compliance or fraud, the penalty can go up to ₹25,000 or 100% of the tax due.

Violation	Penalty
Issuing invoice without GST registration	₹10,000 or tax due, whichever is higher
Issuing invoice with incorrect tax amount	₹10,000 or tax due, whichever is higher
Not issuing invoice for a taxable supply	₹10,000 or tax due, whichever is higher
Collecting GST but not depositing to government	₹10,000 or tax amount, plus interest at 18% p.a.

GSTR Return Mismatches

When you file GSTR-1 (your outward supply return), the data is matched against your buyers' GSTR-2A/2B. Missing or incorrect fields — especially GSTIN, invoice number, and tax amounts — create mismatches. These mismatches trigger notices from the department, require reconciliation, and can delay your buyer's ITC claims.

In a case I personally witnessed, a textile trader in Surat had 67 mismatches in a single quarter because his invoices had wrong buyer GSTINs (typos, mostly). It took his accountant three weeks of full-time work to resolve the mismatches through amendments — at a cost of about ₹35,000 in CA fees alone.

Audit Complications

During GST audits (which are increasingly common for businesses with turnover above ₹2 crore), the auditor reviews a sample of invoices. If the sample reveals non-compliant invoices, the auditor may expand the scope of the audit — reviewing more invoices, more transactions, more periods. What could have been a routine audit turns into an extensive investigation, consuming your time and your accountant's fees.

Field-by-Field Error Frequency — What Goes Wrong Most Often

Based on what I've seen across hundreds of businesses, here's how often each field causes problems:

Field	Error Frequency	Typical Consequence
GSTIN (supplier/buyer)	Very High	ITC mismatch, return rejection
HSN/SAC Code	High	Wrong tax rate, demand notices
Place of Supply	High	Wrong tax head (IGST vs CGST+SGST)
Invoice Number	Medium	Audit red flags, duplicate invoice issues
Tax Rate/Amount	Medium	Over/under payment of tax
Invoice Date	Medium	Time of supply errors, wrong return period
Description	Low	Audit queries, classification disputes
Quantity/Unit	Low	Reconciliation difficulties
Signature	Low	Validity challenges (rare in practice)

Quick Compliance Checklist

Before you finalize any invoice, run through this checklist. Print it out, stick it next to your computer, or save it on your phone. Takes 30 seconds to verify, saves you lakhs in penalties and lost ITC.

• ✓ Supplier name, address, and GSTIN — matches GST certificate exactly
• ✓ Unique invoice number — sequential, no gaps, max 16 characters
• ✓ Invoice date — within the prescribed time limits
• ✓ Buyer GSTIN — verified on GST portal (for B2B invoices)
• ✓ HSN/SAC codes — correct code, correct number of digits for your turnover
• ✓ Item description — specific and clear, not vague
• ✓ Quantity and unit — standardized UQC codes
• ✓ Total value of supply — before taxes, after discounts
• ✓ Taxable value — correctly calculated for each tax rate
• ✓ Tax rate — CGST+SGST or IGST based on place of supply
• ✓ Tax amount — broken into components, rounded correctly
• ✓ Place of supply — state name with code, determined correctly
• ✓ Supplier's signature — physical or digital

How BillCraft Handles All 13 Fields

Look, I understand that tracking 13 fields on every single invoice sounds tedious. And it is, if you're doing it manually. Handwritten invoices or basic Excel sheets are where most errors happen — because there's no validation, no auto-fill, no system to catch mistakes.

That's exactly why tools like BillCraft exist. When you create an invoice in BillCraft:

• Your supplier details (name, address, GSTIN) are saved once and auto-filled on every invoice
• Invoice numbers are generated automatically in sequential order — no gaps, no duplicates
• The date defaults to today, with the option to change
• You can save frequent buyers and auto-fill their GSTIN and address
• HSN/SAC codes are attached to saved items
• CGST/SGST/IGST is calculated automatically based on the place of supply you select
• Tax amounts are computed and rounded correctly
• Your signature is embedded in every PDF

You're not removing the need to understand these 13 fields — you should still know what each one means and why it matters. But you are removing the chance of human error on repetitive data entry. And for a small business doing 50-200 invoices a month, that alone prevents thousands of rupees in potential compliance issues every quarter.

Frequently Asked Questions

Can I add extra fields beyond the 13 mandatory ones?

Absolutely. Many businesses add purchase order numbers, delivery dates, transport details, payment terms, bank account information, and notes. These aren't mandatory under GST, but they're good business practice. The 13 fields are the minimum — not the maximum.

Do I need all 13 fields for invoices below ₹200?

If you're a GST-registered retailer making B2C supplies below ₹200, you can issue a consolidated invoice at the end of the day instead of individual invoices. But the consolidated invoice must still contain all mandatory fields.

What about e-invoicing? Does it cover all 13 fields?

Yes. In fact, the e-invoicing schema is even more detailed. It requires all 13 fields plus additional ones like the IRN (Invoice Reference Number), QR code, and acknowledgement number. If you're generating e-invoices through the IRP (Invoice Registration Portal), the system validates the fields before generating the IRN.

What if I issue an invoice with errors and need to correct it?

You have two options: issue a credit note and then a corrected invoice, or amend the original invoice through your GST return (via GSTR-1 amendment). Both approaches are valid, but the amendment route is simpler for minor corrections. For significant errors (wrong amount, wrong buyer), a credit note + fresh invoice is cleaner.