7 Common GST Invoice Mistakes and How to Avoid Them

1. Using the wrong GST rate

GST rates depend on the item or service, and the structure changed on 22 September 2025, when most goods and services moved to 5% or 18%, with 40% for select luxury and sin goods. Old price lists, saved templates and billing software can still carry the earlier 12% or 28% rates. How to avoid it: confirm the rate for each item from the official notification or with your accountant, update your product list after any change, and check that the rate matches the date of supply. Read more in our guide to the revised GST slabs.

2. Mixing up CGST + SGST and IGST

For a sale within one state, the GST is split equally into CGST and SGST. For a sale between states, the full GST is charged as IGST. Putting CGST and SGST on an inter-state invoice, or IGST on a local one, is a common slip. Example: 18% on ₹1,000 is ₹180, which is either ₹90 CGST + ₹90 SGST or ₹180 IGST. How to avoid it: decide the tax type from the supplier location and the place of supply before you start, and choose the matching option in your invoice tool.

3. Amount in words that does not match the total

If the figure says ₹11,800.40 and the words say “Eleven Thousand Eight Hundred Rupees only”, the invoice contradicts itself. Many invoice tools write the words from the amount rounded to the nearest rupee, so a total with paise creates a mismatch. How to avoid it: add a round-off of −₹0.40 so that the net amount becomes ₹11,800.00, and make the figure and the words identical. The PraxTools Invoice Generator has a round-off field for this purpose, and Number to Words can check your wording.

4. Wrong or missing GSTIN

A GSTIN has 15 characters. The first two digits are the state code, the next ten are the PAN of the taxpayer, and the last characters include an entity number, a default letter and a check character. A typing mistake makes the invoice unusable for the buyer’s tax records. How to avoid it: copy the GSTIN from the registration certificate or the buyer’s confirmation, check the length and the state code, and, where the portal allows it, verify the number before you invoice.

5. Duplicate or messy invoice numbers

Invoice numbers should be unique and follow a consistent sequence. Reusing a number or skipping around without a system causes confusion in your own records and in audits. How to avoid it: choose a simple scheme, such as a prefix and a running number for each financial year, and write the next number down before you start. Note that the PraxTools Invoice Generator does not increase the invoice number automatically, so you enter it yourself each time.

6. Applying discount and tax in the wrong order

When an invoice shows a discount, GST is generally calculated on the value after the discount. For example, ₹10,000 with a ₹1,000 discount has a taxable value of ₹9,000, so 18% GST is ₹1,620 and the total is ₹10,620. Charging tax on ₹10,000 and then deducting the discount gives a different and incorrect result. How to avoid it: calculate the taxable value first and then the tax on it. If your discount arrangement is unusual, such as a post-sale discount, check the treatment with your accountant.

7. Missing required details and no copies

Depending on the supply, a tax invoice may need details such as the HSN or SAC code, the place of supply, a reverse-charge indication and a signature. The Invoice Generator has no separate fields for place of supply or reverse charge, so add them in the notes if they apply. Another habit worth building is saving a PDF copy of every invoice. The tool does not keep an invoice history, so save the PDF each time you create one. How to avoid it: keep a checklist for your type of business and file every invoice in a folder named by month.

A one-minute checklist before you send

1. Seller and buyer names, addresses and GSTINs are correct.
2. Invoice number and date are unique and in order.
3. Each line has the right HSN or SAC, quantity, rate and GST %.
4. The tax split matches the type of sale.
5. Taxable value, tax and total add up. Test the arithmetic with the GST Calculator.
6. The amount in words matches the total.
7. You have saved a copy.

Frequently asked questions