How to calculate GST: add & remove GST

There are really only two GST sums you'll ever need: adding GST to a base price, and removing GST from a price that already includes it. This guide covers both formulas with clear, worked examples at the current GST 2.0 rates.

By Mantu KumarUpdated 25 October 2026How we check our numbers

The two formulas at a glance

Adding GST to a price

Use this when you have a price before tax and want the final amount the customer pays.

1. Take the base price. Say a service costs ₹10,000 before GST.
2. Find the GST amount. At 18%: ₹10,000 × 18 ÷ 100 = ₹1,800.
3. Add it on. ₹10,000 + ₹1,800 = ₹11,800 total.

The same method works at any slab. At 5%, ₹2,000 × 5 ÷ 100 = ₹100 GST, so the total is ₹2,100. At 40%, ₹2,000 × 40 ÷ 100 = ₹800 GST, so the total is ₹2,800.

Removing GST from a total (reverse GST)

Use this when a price already includes GST — like an MRP or a quoted "all-in" figure — and you need to know the base and the tax hidden inside it. You can't just take a flat percentage off the total; you divide.

1. Take the GST-inclusive total. Say ₹11,800, which includes 18% GST.
2. Back out the base. ₹11,800 ÷ (1 + 18/100) = ₹11,800 ÷ 1.18 = ₹10,000.
3. The rest is GST. ₹11,800 − ₹10,000 = ₹1,800.

A quick check: a ₹1,050 price that includes 5% GST has a base of ₹1,050 ÷ 1.05 = ₹1,000 and ₹50 of GST inside it.

Splitting the GST: CGST, SGST or IGST

The total GST is the same whether the sale is within your state or across states — only the split changes. For a sale within the same state, the GST is divided equally into CGST and SGST. For a sale between two states, a single IGST is charged at the full rate.

So on that ₹1,800 of GST: an intra-state sale shows ₹900 CGST + ₹900 SGST, while an inter-state sale shows ₹1,800 IGST. We cover this fully in CGST vs SGST vs IGST explained.

Common mistakes to avoid

• Subtracting a flat % from an inclusive price. Taking 18% off ₹11,800 gives the wrong base — you must divide by 1.18, not multiply by 0.82.
• Using an old slab. Since GST 2.0 (22 September 2025), the 12% and 28% slabs are gone — see GST 2.0 explained.
• Rounding too early. Round only the final figures, not each intermediate step.

How to calculate the GST amount from a total amount

If you only need the tax hidden inside a GST-inclusive total, there is a one-step formula:

GST amount = Total × rate ÷ (100 + rate)

For ₹11,800 at 18%: 11,800 × 18 ÷ 118 = ₹1,800. For ₹2,100 at 5%: 2,100 × 5 ÷ 105 = ₹100. Subtract the GST from the total to get the base.

How to calculate the basic value from the total value

The basic (taxable) value is the total divided by a fixed divisor for each rate. Keep this table handy:

How to remove GST from a total amount: a full example

A supplier in your state quotes ₹25,960 including 18% GST for a laptop repair. To record it correctly:

1. Basic value: 25,960 ÷ 1.18 = ₹22,000.
2. GST inside: 25,960 − 22,000 = ₹3,960.
3. Split: intra-state, so CGST 9% = ₹1,980 and SGST 9% = ₹1,980.
4. Check: 22,000 × 18% = 3,960, which matches.

What do “excluding GST” and “including GST” mean?

A price excluding GST (also written “exclusive of GST” or “+ GST”) is the basic value before tax: the buyer pays this amount plus GST at the applicable rate. A price including GST (“inclusive” or “all-in”) already contains the tax, so nothing is added at billing. A quote of ₹10,000 + GST at 18% costs ₹11,800; a quote of ₹10,000 including GST costs ₹10,000, of which ₹1,525.42 is tax. In India, the MRP printed on packaged goods is always inclusive of all taxes.

Frequently asked questions

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