What Makes an Invoice Valid
An invoice is a formal request for payment, and most tax authorities expect the same core fields: the word "Invoice", a unique sequential number, the issue date, full seller and buyer details, itemised descriptions with quantity and unit price, the tax applied, and the total amount due with payment terms. GST-registered Indian businesses must additionally show their GSTIN and the tax breakup (CGST + SGST or IGST) — put the GSTIN in the address box and it will appear on the PDF.
How the Totals Are Calculated
| Step | Formula | Example (₹10,000 · 10% disc · 18% GST) |
|---|
| Line amount | Qty × Rate | 2 × ₹5,000 = ₹10,000 |
| Subtotal | Sum of all lines | ₹10,000 |
| Discount | Subtotal × Disc% | ₹10,000 × 10% = ₹1,000 |
| Tax | (Subtotal − Discount) × Tax% | ₹9,000 × 18% = ₹1,620 |
| Total due | Subtotal − Discount + Tax | ₹10,620 |
Working out the GST component of a price instead? Use the GST Calculator. Billing for professional services and wondering what your client will withhold? Check the TDS Calculator.
Frequently Asked Questions
How do I create an invoice for free?
Fill in your business and client details, add line items with quantity and rate, set tax/discount — the preview updates live. Click "Download PDF" and choose Save as PDF. No signup, no watermark.
What must an invoice include to be valid?
The word "Invoice", a unique number, issue date, seller and buyer details, itemised descriptions with quantities and prices, tax applied, total due and payment terms. GST-registered Indian sellers also need their GSTIN and tax breakup.
How is the invoice total calculated?
Total = (Subtotal − Discount) + Tax on the discounted amount. Example: ₹10,000 subtotal, 10% discount, 18% GST → ₹9,000 + ₹1,620 = ₹10,620.
Is my invoice data uploaded to a server?
No — the invoice and the PDF are generated entirely in your browser. It even works offline; nothing you type is stored or transmitted.
How do I number my invoices?
Use a sequential, never-repeating scheme like INV-001, INV-2026-001 or 2026/09/001. Sequential numbering is legally expected in many places and keeps accounting clean.