What you need before you start
Generating a GST invoice is straightforward once the groundwork is in place, so gather three things first. One: your own details — business name, address and GSTIN — which stay the same on every bill. Two: your customer’s details, including their GSTIN if they are registered, and the place of supply (the state where the goods or service are delivered). Three: the items you are selling, each with its HSN or SAC code, GST rate, price and quantity.
If those three are ready, the invoice almost writes itself. The steps below walk through it by hand so you understand every field, and then show how billing software collapses the whole thing into a single action once your items and parties are saved.
Step 1 — Fill the header and a unique invoice number
Start with the header. Put your business name, address and GSTIN at the top, then assign an invoice number. This number must be consecutive and unique for the financial year — up to 16 characters, using only letters, numbers, slash and hyphen. A common pattern is a short prefix plus a running sequence, for example INV/26-27/0042.
Add the date of issue. The rule that trips people up is continuity: numbers must not reset mid-year or repeat, because a broken series is exactly the kind of thing that surfaces at filing. If you generate bills by hand, keep one register so the sequence never forks.
Step 2 — Add the customer and the place of supply
Next, record who you are billing. If the customer is registered, show their name, address and GSTIN. Then set the place of supply — the state of delivery — because this single field decides the tax you charge. For an unregistered customer where the invoice value is above ₹50,000, you still need their name, address and the state of delivery.
Do not skip or guess the place of supply. It is not paperwork; it is the input that drives step four. Reading it off the customer once, correctly, saves a wrong tax split later.
Step 3 — Enter line items with HSN and rate
Now the body of the bill. For each product or service, enter a description, the HSN or SAC code, quantity and unit, the rate, and any discount. Multiply out the taxable value for each line. Every line should carry its HSN — how many digits you must print depends on turnover (4 digits up to ₹5 crore, 6 digits above), so store the fullest code you know against each item.
This is the step that is slowest by hand and instant with software, because a saved item already carries its HSN, price and GST rate. Getting the taxable value right per line is what everything after this depends on.
Step 4 — Apply CGST/SGST or IGST correctly
Now use the place of supply. If your state and the place of supply are the same, the sale is intra-state: split the GST into CGST plus SGST, each half the total rate. If they differ, the sale is inter-state: charge a single IGST at the full rate. So an 18% item sold within Telangana is 9% CGST + 9% SGST; the same item shipped to Maharashtra is 18% IGST.
The total the customer pays is usually the same either way, but the tax must sit under the right heads, or your return will show a mismatch. If you want to sanity-check a split or work out the tax on a price quickly, a free GST calculator like the one at /tools/gst-calculator/ does the arithmetic for you before you commit it to the bill.
Step 5 — Round off, total, and amount in words
Add the taxable values and the tax to get the grand total. Most shops round the total to the nearest rupee and record the small round-off difference so the maths still reconciles. Apply round-off consistently — a stray paisa that does not match is a needless mismatch at filing.
Finally, write the total in words as well as figures, confirm whether reverse charge applies, and sign or digitally sign the invoice. Amount in words is a required courtesy that also prevents tampering. With that, the invoice is complete and compliant.
The one-click way with software
Doing all this by hand for every sale is slow and error-prone, which is why the whole point of billing software is to make a compliant invoice the default output rather than something you assemble. Save each item once with its HSN and GST rate, save each party with their GSTIN and state, and generating a bill becomes: pick the party, add the items, save. The number stays consecutive, the tax heads follow the place of supply, round-off and amount in words are filled in, and a GSTR-1 summary builds itself in the background.
RetailDek works exactly this way — you choose the party and the items and the compliant GST invoice is what comes out, ready to print or share. If you just need a one-off bill without setting up an account, the free invoice generator at /tools/invoice-generator/ produces a clean GST invoice in the browser, and the GST calculator at /tools/gst-calculator/ handles the tax maths. Whether you generate a GST invoice online for a single sale or run every bill through the app, the fields in this guide are the ones that make it correct.