Adding '18%' by hand seems easy until a CA points out a ₹1 mismatch that breaks their reconciliation.

The goal is not to become a calculator; it is to understand what the calculator is doing so you can spot nonsense quickly.

This guide explains how GST should be applied line-by-line and in totals, and how your invoice generator can make mistakes nearly impossible.

Generate a legally compliant, highly professional invoice in under 60 seconds.
No signup required to start. No credit card. Just type, send, done.

Step 1: Decide if your price is before or after GST

B2B prices are usually before GST (exclusive) — you quote ₹1,00,000 + GST extra.
Retail/B2C often uses GST-inclusive pricing — MRP already includes tax.
Your invoice needs to know which mode you are using to reverse-calc correctly.
Pick one strategy per product/channel and stick to it to avoid confusion.

Step 2: Apply the right GST rate and split

For each line, set the GST rate (5%, 12%, 18%, etc.) according to its HSN/SAC.
Let the system handle whether that becomes IGST or CGST+SGST based on state and place of supply.
For GST-exclusive prices: Tax = taxable value × rate; Total = taxable value + tax.
For GST-inclusive prices: Taxable value = total / (1 + rate); Tax = total - taxable value.

Sum taxable values and tax separately; do not round each line randomly.
Use a consistent rounding rule for the final amount (for example, round off to nearest rupee).
Show round-off as a separate minor line so CA can still track unrounded totals if needed.
In Invwow, all of this should happen live as you type, with no manual calculation step.

Step 1: Decide if your price is before or after GST

B2B prices are usually before GST (exclusive) — you quote ₹1,00,000 + GST extra.

Retail/B2C often uses GST-inclusive pricing — MRP already includes tax.

Your invoice needs to know which mode you are using to reverse-calc correctly.

Pick one strategy per product/channel and stick to it to avoid confusion.

Step 2: Apply the right GST rate and split

For each line, set the GST rate (5%, 12%, 18%, etc.) according to its HSN/SAC.

Let the system handle whether that becomes IGST or CGST+SGST based on state and place of supply.

For GST-exclusive prices: Tax = taxable value × rate; Total = taxable value + tax.

For GST-inclusive prices: Taxable value = total / (1 + rate); Tax = total - taxable value.

Step 3: Let the tool do totals and rounding consistently

Sum taxable values and tax separately; do not round each line randomly.

Use a consistent rounding rule for the final amount (for example, round off to nearest rupee).

Show round-off as a separate minor line so CA can still track unrounded totals if needed.

In Invwow, all of this should happen live as you type, with no manual calculation step.