How to remove GST from an inclusive price (and the shortcut that gives the wrong answer)
Subtracting 18% from a GST-inclusive price does not give you the taxable value. Here is the correct formula, why the shortcut fails, and what it does to your books over a year.
An invoice says ₹1,180 including GST at 18%. What was the price before tax?
A great many people subtract 18% of ₹1,180 — that is ₹212.40 — and write down ₹967.60. That answer is wrong, and it is wrong in the same direction every single time.
Why the shortcut fails
GST is charged on the base price, not on the final price. When you subtract 18% of the inclusive figure, you are taking 18% of a number that already contains the tax. You remove too much.
The correct formula
Taxable value = Inclusive price × 100 ÷ (100 + GST rate)
For our invoice: 1,180 × 100 ÷ 118 = ₹1,000. The GST is ₹180. Check it: 18% of ₹1,000 is ₹180, and ₹1,000 + ₹180 = ₹1,180. It reconciles.
The shortcut gave ₹967.60 — an error of ₹32.40 on a single ₹1,180 invoice. Run a hundred such invoices a month and you have misstated over ₹38,000 a year.
The divisors for every rate
| GST rate | Divide inclusive price by |
|---|---|
| 5% | 1.05 |
| 12% | 1.12 |
| 18% | 1.18 |
| 28% | 1.28 |
Going the other way
Adding GST is the easy direction — multiply. A ₹1,000 base at 18% becomes 1,000 × 1.18 = ₹1,180. It is only the reverse calculation that catches people out.
Why this matters beyond arithmetic
The taxable value is what you report in your returns, what your input tax credit is computed against, and what your real revenue actually is. Get it wrong consistently and three things follow: your reported turnover drifts from your books, your input credit claims do not match your supplier filings, and your margins look better on paper than they are in your bank account.
Splitting CGST and SGST
For a supply within your own state, the ₹180 splits into ₹90 CGST and ₹90 SGST. For an inter-state supply it stays as a single ₹180 IGST. The total is identical; only the reporting differs.
Let the calculator do it
The GST Profit Calculator handles both conventions — just tell it whether the price you entered includes GST — and shows your input credit, output liability and the net amount actually payable.