How to Calculate GST: Formula for Adding GST to a Price
To add GST to a price, multiply the base price by the GST rate and divide by 100 to get the GST amount, then add that to the base price for the final inclusive price. For a sale within the same state, split the GST amount equally into CGST and SGST; for a sale between two states, the full amount is charged as IGST instead.
To add GST to a price, multiply the base price by the GST rate and divide by 100 to get the GST amount, then add that amount to the base price to arrive at the final, tax-inclusive price. For a sale within the same state, this GST amount is split equally between CGST and SGST; for a sale between two different states, the full amount is charged as IGST instead.
The forward GST formula
GST amount = Base price x (rate / 100). Inclusive price = Base price + GST amount. For example, at an 18% rate, GST amount = Base price x 0.18, and the inclusive price is the base price plus that amount.
Let the GST calculator do this for you: enter the base price and rate, and it shows the GST amount, the final price, and the CGST/SGST or IGST split.
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Worked example at 18%, within the same state
| Item | Amount |
|---|---|
| Base price | Rs 10,000 |
| GST rate | 18% |
| GST amount | 10,000 x 0.18 = Rs 1,800 |
| CGST (9%) | Rs 900 |
| SGST (9%) | Rs 900 |
| Final inclusive price | 10,000 + 1,800 = Rs 11,800 |
The CGST and SGST split is simply half the GST amount each, since both go to the central and state government respectively for a sale that stays within one state. If this were an interstate sale instead, the full Rs 1,800 would be charged as IGST in one line rather than split into two.
Step by step: calculating GST on a base price
- Note the base price of the goods or service.
- Confirm the correct GST rate for that item. Under GST 2.0, most items fall into 0%, 5%, 18% or 40%.
- Multiply the base price by the rate, then divide by 100, to get the GST amount.
- Add the GST amount to the base price to get the final inclusive price.
- If the sale is within the same state, split the GST amount equally into CGST and SGST. If it crosses state lines, show the full amount as IGST instead.
Why the CGST/SGST vs IGST split matters
India's GST system splits domestic transactions into two categories depending on where the buyer and seller are located. A sale within the same state is taxed under CGST (Central GST) and SGST (State GST), each getting half the total GST amount, so that both the central and the relevant state government receive a share. A sale between two different states is instead taxed entirely under IGST (Integrated GST), which the central government collects and then settles with the states involved. Getting this split right on an invoice is not optional bookkeeping detail; it directly affects which government account the tax is credited to and matters for input tax credit claims on the buyer's side.
If you are unsure whether a transaction is intra-state or interstate, compare the state of the supplier with the place of supply. For goods that are moved, the place of supply is generally where the delivery ends, not the buyer's head office, so a Delhi seller shipping to a Mumbai site charges IGST. For Union Territories without a legislature, UTGST replaces SGST. Confirm unusual cases with a GST practitioner.
Going the other direction: removing GST from a price
If you already have a final, tax-inclusive price and need to work out how much of it is GST, that is the reverse of this calculation: see how to calculate GST from an inclusive price for that formula and worked examples at the current slabs.
Using this for invoicing and quotes
When preparing a quote or an invoice, it is good practice to show the base price, the GST rate applied, the GST amount (split into CGST/SGST or shown as IGST), and the final inclusive price as separate line items, rather than only showing the final total. This transparency makes it easier for the buyer to verify the calculation themselves and matters for GST compliance on both sides of the transaction.
Understanding the current slab structure
Under GST 2.0, effective from 22 September 2025, most goods and services fall into one of four slabs: 0%, 5%, 18% or 40%. The older 12% and 28% slabs have largely been phased out, with items that used to sit in those bands moved into a neighbouring slab. See the current GST slab list for a fuller breakdown of what moved where, and always confirm the exact rate for your specific item before applying this formula, since using an outdated rate is one of the most common billing mistakes right now.
A quick reference for everyday calculations
For quick mental maths, remember that a 5% rate adds one-twentieth of the base price, an 18% rate adds a little under a fifth, and a 40% rate adds two-fifths. These rough checks are useful for spotting an obviously wrong invoice at a glance, though you should still use the exact formula, or the GST calculator, for any figure you plan to record or rely on.
Common mistakes
- Applying the wrong GST rate for an item, especially since GST 2.0 moved several items between slabs.
- Charging CGST and SGST on an interstate sale instead of IGST, or the other way round.
- Forgetting to check whether an item falls under the 0% slab, where no GST amount should be added at all.
- Rounding the GST amount before splitting it into CGST and SGST, which can leave the two halves not adding back up to the rounded total.
Tools for this
Frequently asked questions
What is the formula to calculate GST on a base price?
GST amount = Base price x rate / 100. Add that to the base price to get the final, tax-inclusive price.
When do I use CGST + SGST instead of IGST?
CGST and SGST apply when the sale is within the same state, splitting the GST amount equally between the two. IGST applies for a sale between two different states, charging the full GST amount in one line.
How do I remove GST from a price that already includes it?
That is the reverse calculation: divide the inclusive price by (1 + rate/100) to get the base price, then subtract to find the GST amount. See the guide on calculating GST from an inclusive price for the full method.
What are the current GST rates I should use in this formula?
Under GST 2.0, in force since 22 September 2025, the slabs are 0%, 5%, 18% and 40%. Use the correct rate for your specific item or service.
Does the formula change for services compared to goods?
No, the same formula applies to both. What matters is using the correct GST rate for that specific good or service.
Sources
- PIB: Recommendations of the 56th GST Council meeting (3 Sep 2025) (checked 2026-09)
- CBIC GST: goods and services rate schedules (checked 2026-09)
Rules and limits change. Always check the latest official notification or portal before you submit a form.