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You are handed a bill for ₹11,800 and told it includes 18 percent GST. Your accountant, or your own head, says: take 18 percent off. That gives ₹2,124 of tax and a base price of ₹9,676. It feels right. It is wrong, and the mistake repeats on thousands of invoices every day.
The reason is small but stubborn. GST is charged on the price before tax, not on the final amount. So the 18 percent has to be taken from a number you do not know yet, and subtracting it from the total uses the wrong base.
Adding GST is easy
When you start from the price without tax, one multiplication does everything:
Total = Price × (1 + rate ÷ 100)
A ₹10,000 service at 18 percent becomes ₹10,000 × 1.18 = ₹11,800. The tax is the difference, ₹1,800. Nothing surprising here, and nobody gets this one wrong.
Removing GST is where people slip
Now go backwards. You know ₹11,800 and the 18 percent rate. The base price is the number that, multiplied by 1.18, gives 11,800, so you divide by 1.18:
Price before tax = Total ÷ (1 + rate ÷ 100)
11,800 ÷ 1.18 = ₹10,000. The GST inside the bill is 11,800 − 10,000 = ₹1,800. If you only want the tax part directly, use a shortcut that works for any rate:
GST inside = Total × rate ÷ (100 + rate)
That is 11,800 × 18 ÷ 118 = ₹1,800 again. Notice the 118 on the bottom. It is 100 plus the rate, and it is the whole secret.
The wrong method overstates the tax by ₹324 on a single bill. If you claim or pay on that figure, the error compounds across every invoice in the month.
Quick check with two rates
The rates below are examples to show the arithmetic. The rate for your product or service is whatever the current GST schedule says, so confirm it before you invoice.
| You know | Rate | Price before tax | GST | Total |
|---|---|---|---|---|
| ₹10,000 before tax | 5% | ₹10,000 | ₹500 | ₹10,500 |
| ₹10,000 before tax | 18% | ₹10,000 | ₹1,800 | ₹11,800 |
| ₹10,500 final | 5% | ₹10,000 | ₹500 | ₹10,500 |
| ₹11,800 final | 18% | ₹10,000 | ₹1,800 | ₹11,800 |
CGST, SGST and IGST: who gets the tax
The rate you see is the total. Who receives it depends on where the sale happens. Inside one state, the 18 percent is split evenly between the Centre and the state. Between two states, the whole thing goes under a single head.
| Sale | Tax heads | On ₹10,000 at 18% |
|---|---|---|
| Within one state | CGST 9% + SGST 9% | ₹900 + ₹900 |
| Between two states | IGST 18% | ₹1,800 |
The total is the same either way. Only the labels on the invoice change.
This matters because the invoice must show the right heads. A buyer in another state who receives a CGST and SGST invoice, or the other way round, can run into trouble when claiming credit.
Discounts change the base
If you give a discount that is shown on the invoice, tax is generally worked out on the discounted value. A ₹10,000 item with a ₹1,000 discount has a taxable value of ₹9,000, so 18 percent GST is ₹1,620 and the customer pays ₹10,620, not ₹11,800 minus something.
- Work out the discount first, then the tax on what is left.
- Do not add a discount after the tax line, or the invoice will not add up.
- Rounding: software may differ by a paisa or two on long invoices. That is normal, but be consistent.
A quick routine for any invoice
- Find out whether the amount you hold is before or after tax.
- Confirm the rate for that item from the current schedule.
- Multiply by 1 plus the rate to add tax, or divide by the same number to remove it.
- Choose the heads: CGST and SGST inside a state, IGST across states.
- Round once, at the end, and keep the unrounded figures if you will reuse them.
Questions people ask
How do I remove 18% GST from a price?
Divide the final price by 1.18. For ₹11,800 that gives ₹10,000 before tax, and the GST is the difference, ₹1,800. Subtracting 18 percent of 11,800 gives the wrong answer.
What is the formula for the GST amount inside a price?
GST inside = total × rate ÷ (100 + rate). For 18 percent that is total × 18 ÷ 118, which is about 15.25 percent of the final price, not 18 percent.
What is the difference between CGST, SGST and IGST?
CGST and SGST are charged together on a sale within one state, each usually half of the rate. IGST is charged as one tax on a sale between states. The total rate is the same.
Is the GST rate the same for everything?
No. Different goods and services fall under different rates, and the schedule is revised from time to time. Always confirm the current rate for your item before invoicing.
Can I trust a calculator for invoices?
A calculator gives the correct arithmetic for the rate you enter. Whether that rate and treatment apply to your sale is a separate question that depends on current rules.