The one formula that tells you the tax already built into a price, and the common mistake that gives the wrong answer.
Every time I buy something in a shop in India, the price on the pack says "MRP, inclusive of all taxes." That line is easy to skip. But it means the GST is already inside the number. The shop does not add it on top. So a simple question comes up: how much of that price is actually tax?
I built a small GST calculator, and this was one of the first things I had to get right. Here is the maths, step by step, with plain numbers.
What "GST inclusive" means
A GST-inclusive price already contains the tax. Retail MRPs in India are inclusive of all taxes. So if a product costs ₹590 and falls in the 18% slab, the ₹590 is made of two parts: ₹500 of value and ₹90 of GST.
You cannot see that split on the pack. You have to work it out.
The formula
There is one formula you need:
Taxable value = Inclusive price × 100 ÷ (100 + GST rate)
Once you have the taxable value, the GST is just the difference:
GST = Inclusive price − Taxable value
Let us check it on the ₹590 example at 18%:
- ₹590 × 100 ÷ 118 = ₹500 taxable value
- ₹590 − ₹500 = ₹90 GST
That matches.
A shortcut for 18%
For the 18% slab, "× 100 ÷ 118" is the same as dividing by 1.18. So:
- ₹23,600 ÷ 1.18 = ₹20,000 taxable value
- ₹23,600 − ₹20,000 = ₹3,600 GST
The same idea works for other slabs. For 5%, divide by 1.05. Same pattern for any rate.
Example at 5%
Say a bill shows ₹2,100 and the rate is 5%.
- ₹2,100 × 100 ÷ 105 = ₹2,000 taxable value
- GST = ₹2,100 − ₹2,000 = ₹100
If the sale is within the same state, that ₹100 is split into two equal halves: CGST ₹50 and SGST ₹50.
An example with an odd price
Real prices are rarely round. Take an item with an MRP of ₹1,499 at 18%:
- ₹1,499 ÷ 1.18 = ₹1,270.34 taxable value (rounded)
- GST = ₹1,499 − ₹1,270.34 = ₹228.66
- CGST ₹114.33 + SGST ₹114.33
Now the same ₹1,499 at 5%:
- ₹1,499 ÷ 1.05 = ₹1,427.62 taxable value (rounded)
- GST = ₹71.38
Same price on the pack, but a very different amount of tax inside it. That is why the rate matters so much.
The common mistake
The most common mistake is taking 18% of the inclusive price. It looks right, but it overstates the tax.
On ₹1,180, 18% is ₹212.40. But the real GST inside ₹1,180 is only ₹180, because:
- ₹1,180 ÷ 1.18 = ₹1,000 taxable value
- ₹1,180 − ₹1,000 = ₹180 GST
The 18% applies to the taxable value (₹1,000), not to the final price that already has the tax in it. Taking 18% of ₹1,180 means you are charging tax on the tax.
Can a shop add GST on top of MRP?
No. MRP is the maximum price inclusive of all taxes, so a retailer cannot add GST on top of it. If you see GST added above the MRP on a retail bill, that is worth asking about.
Not sure which rate applies?
Rates for many items changed in September 2025, so it is easy to be unsure which slab a product falls in. One trick: work out the tax inside your amount at every slab side by side. If the bill's GST matches one of them, you know which rate was used. Then confirm the correct rate from the HSN code of the product.
Try it yourself
If you do not want to do the division by hand, I made a free GST inclusive calculator (my own site) that does this split for any amount and shows every slab side by side.
This is just the arithmetic, not tax advice. For questions about what rate applies to your business, a CA is the right person to ask.
Quick recap
- MRP already includes GST.
- Taxable value = Price × 100 ÷ (100 + rate).
- GST = Price − Taxable value.
- Do not take the rate as a percentage of the final price.
- Within a state, GST splits equally into CGST and SGST.
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