GST in one formula
To add GST to a base amount: GST = Base Amount × (Rate ÷ 100), and the final price is base amount plus that GST. On a ₹10,000 base amount at the current GST slabs:
| Rate | GST amount | Final price |
|---|---|---|
| 5% | ₹500 | ₹10,500 |
| 18% | ₹1,800 | ₹11,800 |
| 40% | ₹4,000 | ₹14,000 |
The formula itself never changes — only the rate does, depending on what's being taxed.
Before relying on a GST result
| Starting amount | Formula |
|---|---|
| Price before GST | GST = base × rate; total = base + GST |
| Price including GST | Base = inclusive price ÷ (1 + rate); GST = inclusive − base |
| Interstate vs intrastate | Total GST rate can be same, but tax components differ |
The current GST slabs
Following the GST 2.0 reform, the structure simplified to two main rates plus a special category — but the applicable rate for any specific good or service depends on its HSN (goods) or SAC (services) classification, and a range of exemptions, nil-rated supplies, and special treatments exist alongside the headline slabs below:
- 0% — exempt goods and services
- 5% — a standard lower rate
- 18% — the standard rate for most goods and services
- 40% — a special rate applicable to specified goods and services (not a universal slab everything above 18% falls into)
This replaced the earlier four-slab structure (which included 12% and 28% rates) — if you see older content online referencing those rates, it's likely describing the pre-reform structure rather than what currently applies. Finpockett's GST Calculator computes the mathematical GST amount once you know the applicable rate for your specific transaction — it does not determine which rate legally applies to a given product or service, which depends on its official classification.
Working through the calculation step by step
Take a ₹25,000 base amount at the standard 18% rate:
- Base amount: ₹25,000
- GST rate: 18%
- GST amount: ₹25,000 × 0.18 = ₹4,500
- Final price: ₹25,000 + ₹4,500 = ₹29,500
The same method applies at any rate or amount — multiply the base by the rate as a decimal, then add that to the base.
CGST, SGST and IGST — same total, different split
The GST amount calculated above is the total tax, but how it's labelled on an invoice depends on whether the transaction is within a state or across states:
- Intra-state sale (buyer and seller in the same state): the total GST splits evenly into CGST and SGST — at 18%, that's 9% CGST + 9% SGST, still totalling 18%.
- Inter-state sale: the full amount is charged as IGST instead — 18% IGST, not split.
Either way, the total tax paid is identical — only the invoice labelling differs based on where the transaction happens.
Where GST arithmetic goes wrong
If you're ever unsure whether a price you're looking at already has GST built in, inclusive-versus-exclusive worked examples covers how to tell the difference before applying either calculation.
Confusing "adding GST" with "removing GST from an inclusive price." These are different calculations — adding GST to a known base amount (what this article covers) uses simple multiplication, while extracting GST from a price that already includes it requires division, not subtraction. See GST Calculator for the exact method.
Applying GST to the printed MRP instead of the actual transaction price. GST is calculated on the actual selling price after any discount, not the label price — if an MRP item is discounted, GST applies to the discounted amount.
Assuming every product uses the same rate. The applicable rate depends on the specific HSN (goods) or SAC (services) classification — always confirm the correct rate for what you're actually calculating, rather than assuming 18% applies universally.
Use one amount, one rate and the correct direction
Enter any base amount and the applicable rate on the GST Calculator to get the exact GST amount and final price instantly — useful for invoicing, budgeting a purchase, or checking a bill's GST line item against what it should actually be.
Worked examples: inclusive and exclusive prices
If ₹10,000 is quoted before GST, adding 18% produces ₹11,800. If ₹10,000 is quoted including GST, the ₹10,000 is already the final amount and the embedded tax must be extracted rather than added again.
| Framing | Base amount | GST at 18% | Final amount |
|---|---|---|---|
| ₹10,000 exclusive of GST | ₹10,000 | ₹1,800 | ₹11,800 |
| ₹11,800 inclusive of GST | ₹10,000 | ₹1,800 | ₹11,800 |
To remove GST from an inclusive price, divide by 1 + GST rate. For ₹11,800 including 18% GST: ₹11,800 ÷ 1.18 = ₹10,000 base, so GST is ₹1,800. Simply subtracting 18% from ₹11,800 would be wrong because 18% applies to the pre-tax base, not to the tax-inclusive total.
Frequently asked questions
What's the GST formula in one line?
GST amount = Base amount × (Rate ÷ 100). Final price = Base amount + GST amount.
Are there really only three GST rates now?
The main operative rates are 0%, 5% and 18%, with 40% reserved for specified goods and services rather than a universal fourth slab. Beyond these headline rates, various exemptions, nil-rated supplies and classification-specific treatments exist — the applicable rate for any given transaction depends on its official HSN or SAC classification, not just which of these four numbers seems closest.
Does GST apply differently to services versus goods?
The calculation method is identical — the difference is in classification (SAC codes for services, HSN codes for goods) and which specific rate applies to a given service or good, not the formula itself.
Can a business get back the GST it pays on its own purchases?
Yes, generally — registered businesses can typically claim Input Tax Credit (ITC) on GST paid for business purchases, offsetting it against GST collected on sales. This is a separate mechanism from the base calculation shown here.