The short answer
Divide the total by 1.18. Whatever comes out is your taxable value, and the difference between that and the total is the GST. For ₹1,180 the taxable value is ₹1,000 and the GST is ₹180, which splits into ₹90 CGST and ₹90 SGST for a same-state supply.
Everything below explains why division rather than subtraction, and what to do with the result once you have it.
Why subtracting 18% gives the wrong answer
The instinct is to take 18% off the total. It feels symmetrical: 18% went on, so 18% should come off. It does not work, and the reason is worth understanding once so you never do it again.
When GST was added, the 18% was calculated on the base amount — the smaller number. When you subtract 18% from the total, you are calculating 18% of the larger number, which is a bigger figure. You remove more than was ever added.
| Step | Amount |
|---|---|
| Base amount | ₹1,000.00 |
| 18% of ₹1,000 added as GST | ₹180.00 |
| Total | ₹1,180.00 |
| 18% of ₹1,180 (what subtraction removes) | ₹212.40 |
| Result of subtracting | ₹967.60 |
The gap is ₹32.40 on a small invoice. On ₹11.8 lakh of billing it is ₹32,400 — enough to cause a genuine mismatch between your books and your returns.
The formula
GST amount = Total − Taxable value
CGST = SGST = GST amount ÷ 2
The general version, which works at any rate, is Total × 100 ÷ (100 + rate). At 5% divide by 1.05, at 12% by 1.12, at 28% by 1.28.
Worked example: ₹1,180
Same-state supply at 18%
- Inclusive total
- ₹1,180.00
- Divide by 1.18
- ₹1,000.00
- GST amount
- ₹180.00
- CGST at 9%
- ₹90.00
- SGST at 9%
- ₹90.00
If the client were in another state, the same ₹180 would appear as a single IGST line instead of two ₹90 lines. The total the customer pays does not change.
The same calculation at other rates
| Rate | Divide by | ₹10,000 inclusive becomes | GST |
|---|---|---|---|
| 5% | 1.05 | ₹9,523.81 | ₹476.19 |
| 12% | 1.12 | ₹8,928.57 | ₹1,071.43 |
| 18% | 1.18 | ₹8,474.58 | ₹1,525.42 |
| 28% | 1.28 | ₹7,812.50 | ₹2,187.50 |
Putting it on the invoice
A tax invoice cannot show a single inclusive figure. Whatever you quoted the customer, the invoice must break out the taxable value, the rate, and the tax under the correct heads.
- Taxable value: ₹1,000.00
- CGST at 9%: ₹90.00
- SGST at 9%: ₹90.00
- Invoice total: ₹1,180.00
This matters for your buyer as much as for you — they can only claim input tax credit against clearly stated tax components.
Three mistakes worth avoiding
- Rounding at the wrong step. Do the division first, then round. Rounding the total before dividing pushes the error into your tax figure.
- Splitting IGST in half. IGST is never divided into CGST and SGST. Check the place of supply before deciding.
- Assuming everything is 18%. Verify the rate against the HSN or SAC code for the actual supply rather than defaulting.
Frequently asked questions
How do I remove 18% GST from a total amount?
Divide the total by 1.18. The result is the taxable value, and the difference between the two is the GST. For ₹1,180 that is ₹1,000 and ₹180.
Why is subtracting 18% wrong?
The 18% was originally applied to the smaller taxable value. Removing 18% from the larger total takes away more than was ever added, so the answer comes out low.
What is the formula for 5%, 12% and 28%?
The same structure with a different divisor — 1.05, 1.12 or 1.28. In general, taxable value = total × 100 ÷ (100 + rate).
Does this work for IGST?
Yes. The division is identical. Only the split changes: inter-state supply carries a single IGST line instead of separate CGST and SGST.
Is a GST-inclusive invoice legal?
You may quote an inclusive price, but the tax invoice itself must show taxable value and tax separately. Quoting inclusive and invoicing itemised are two different things.