Tools
Reverse GST calculator: find the price before GST
Enter what the customer pays, including GST. The calculator works back to the taxable value and shows how much of the price is tax, split by place of supply.
In short
This free reverse GST calculator from SimpleERP takes an amount that already includes GST and works out the taxable value and the tax inside it, split into CGST and SGST or IGST. It runs the same tax functions as a SimpleERP invoice.
How to use it
Useful when a customer agrees a round price including tax, or when you need the taxable value behind an MRP.
Enter the GST-inclusive amount
The figure the customer pays, such as ₹11,800 for a rack sold at a tax-inclusive price.
Pick the GST rate
The rate for the goods or service. 12% and 28% stay in the list for checking invoices raised before the September 2025 rate changes.
Choose the seller's and buyer's states
Same state gives CGST and SGST; different states give IGST.
Copy the taxable value
That is the rate to put on the invoice line if you bill exclusive of tax.
Why can't I just take 18% off the total?
Because the 18% was charged on the taxable value, not on the total. Taking 18% off ₹11,800 gives ₹9,676, which is wrong. Dividing by 1.18 gives ₹10,000, which is right.
- Divide, do not subtract: total ÷ (1 + rate ÷ 100).
- The tax share of an inclusive price at 18% is 18 ÷ 118, about 15.25%, not 18%.
- At 5% the tax share is 5 ÷ 105, about 4.76%.
Is MRP the price before or after GST?
After. MRP on packaged goods includes all taxes, so a retailer selling at MRP must work the GST out of it. The taxable value is what goes on the invoice line and into the GST return.
- Selling below MRP is allowed; selling above it is not.
- A discount before the sale reduces the taxable value, and the GST is worked out of the discounted price.
- Rates change, so confirm the rate for your goods with your CA or the GST portal before you rely on it.
Illustration of the keyboard-first invoice editor with a focused line, live tax totals and shortcut hints.
Do this automatically in SimpleERP
In SimpleERP you enter the rate before tax on each line, and the invoice adds GST and splits it by the customer's state. Use this calculator when a customer quotes you an inclusive price, then put the taxable rate on the line.
- CGST and SGST or IGST chosen from the place of supply.
- Totals rounded the same way as this calculator, to the paisa per line.
- The invoice prints the total in words, in rupees and paise.
Illustration of GST tax invoice INV-2026-27-0213 to Ravi Stores with CGST 9% and SGST 9%, round off, a UPI QR code and the amount in words.
The maths
How it is calculated
Work back from the inclusive amount, then split the tax as usual.
- Taxable value = inclusive amount × 100 ÷ (100 + rate).
- Within a state: CGST and SGST are each taxable value × (rate ÷ 2) ÷ 100, rounded to the paisa, so they are always equal.
- Across states: IGST = taxable value × rate ÷ 100.
- Total = taxable value + tax. Because each half is rounded on its own, this can be a paisa off the amount you typed, exactly as on an invoice.
Example
Worked example
Lakshmi Steel Furnitures in Coimbatore agrees ₹11,800 including GST for a rack order at 18%, delivered in Tamil Nadu.
| Line | Amount |
|---|---|
| Taxable value: 11,800 × 100 ÷ 118 | ₹10,000.00 |
| CGST 9% | ₹900.00 |
| SGST 9% | ₹900.00 |
| Total | ₹11,800.00 |
Delivered to Karnataka instead, the same ₹1,800 of tax is IGST at 18%.
Honest limits
What this calculator does not do
- Compensation cess inside an inclusive price is not separated out.
- Composition scheme dealers do not charge GST, so there is nothing to remove from their bills.
- It does not tell you the rate for your goods; look up the HSN or SAC code first.
FAQ
Questions, answered
How do I remove 18% GST from a total?
Divide the total by 1.18. For ₹11,800 that gives ₹10,000 as the taxable value, and the GST is the difference, ₹1,800.
Is GST calculated on MRP?
GST is not added on top of MRP; MRP already includes it. The taxable value is MRP (or the lower selling price) × 100 ÷ (100 + rate).
What is the formula for GST-inclusive price?
Inclusive price = taxable value × (100 + rate) ÷ 100. Reverse it to get the taxable value: inclusive price × 100 ÷ (100 + rate).
Why does my reverse calculation differ by a paisa?
Rounding. An inclusive price does not always divide evenly, so the taxable value is rounded to the paisa and the tax takes the remainder, which keeps the total exact.
Related
Keep reading
GST billing
Tax invoices with CGST, SGST or IGST by place of supply, HSN on every line and a UPI QR on the print.
Read morePlace of supply
Where GST treats a supply as happening, which decides CGST and SGST or IGST.
Read moreGST billing guide
Everything a small business needs for GST billing, in order: registration, invoices, numbering, notes, registers, returns and a month-end checklist.
Read more
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