If you sell anything in India, you run into GST math more often than you'd like. A customer asks for a quote and you need to know whether that number includes tax. An invoice lands on your desk and you need to pull the tax back out to check it. This guide walks through the actual calculations behind a gst calculator, by hand or checked against a tool, plus the CGST/SGST/IGST split and where TDS and TCS fit into a GST invoice. Run any of these numbers through CalcMint's GST calculator if you'd rather skip the arithmetic.
Since the GST 2.0 reform took effect on 22 September 2025, India's slabs are 5%, 18%, and 40%, plus a 0% nil category for essentials. The old 12% and 28% slabs got folded into 5% and 18%. All the examples below use these current rates.
How to Add GST to a Base Price
The gst calculator formula for adding tax is straightforward: multiply the base price by the GST rate, then add that back onto the base price.
GST amount = Base price × (GST rate / 100) Total price = Base price + GST amount
Say you're billing a customer ₹1,000 for a product taxed at 5%.
GST amount = 1,000 × 0.05 = ₹50 Total price = 1,000 + 50 = ₹1,050
Now take a service invoice for ₹10,000 at the standard 18% rate.
GST amount = 10,000 × 0.18 = ₹1,800 Total price = 10,000 + 1,800 = ₹11,800
And a luxury item worth ₹50,000 base, taxed at the top 40% slab.
GST amount = 50,000 × 0.40 = ₹20,000 Total price = 50,000 + 20,000 = ₹70,000
The math is identical no matter which rate applies. Only the percentage changes. The mistake people actually make is applying the rate to the wrong number: always apply it to the base price, never to a price that already has tax folded in. That's a different calculation, covered next.
How to Remove GST from an Inclusive Price
Sometimes you're handed a final price, an MRP sticker, a client's "all-in" quote, and need to work backward to find the base price and the tax hiding inside it. This is a gst calculator reverse lookup, and it trips people up because the obvious approach, just subtracting the percentage, gives the wrong number.
Base price = Inclusive price / (1 + GST rate/100) GST amount = Inclusive price − Base price
Take the ₹1,050 total from the 5% example above.
Base price = 1,050 / 1.05 = ₹1,000 GST amount = 1,050 − 1,000 = ₹50
For the ₹11,800 invoice at 18%:
Base price = 11,800 / 1.18 = ₹10,000 GST amount = 11,800 − 10,000 = ₹1,800
And the ₹70,000 luxury item at 40%:
Base price = 70,000 / 1.40 = ₹50,000 GST amount = 70,000 − 50,000 = ₹20,000
These numbers match the forward examples exactly, and that's the check you can run on any reverse calculation. Subtract 18% straight off the top of an inclusive price and you'll land on a different base than dividing by 1.18 gives you. Division is the correct one, because GST gets calculated on the base, not on the inclusive total, so you have to unwind it the same way it was built.
CGST vs SGST vs IGST Explained
Once you know the total GST amount, the next question is how that tax splits between the central and state governments. The cgst sgst igst difference comes down to one thing: whether the sale crosses a state line.
CGST (Central GST) and SGST (State GST) apply together on transactions within the same state. The total rate splits evenly between the two. On an 18% sale, that's 9% CGST and 9% SGST, both sitting on the same invoice.
IGST (Integrated GST) applies instead of CGST plus SGST when the sale crosses a state border, say a business in Maharashtra selling to a customer in Karnataka. The full rate shows up as one IGST line rather than a split.
| Scenario | Rate applied | Tax breakdown (on ₹10,000 at 18%) |
|---|---|---|
| Intra-state sale (same state) | CGST + SGST | ₹900 CGST + ₹900 SGST = ₹1,800 |
| Inter-state sale (different states) | IGST | ₹1,800 IGST |
The total tax collected is identical either way, ₹1,800 on a ₹10,000 sale at 18%. What changes is which government account the money lands in, and which boxes get filled in on your GST return. Charging CGST plus SGST on an inter-state sale is one of the more common invoicing errors, because the total is right but the money went into the wrong bucket.
GST on TDS/TCS Transactions
GST TDS and TCS are separate from the income-tax TDS most business owners already deal with, and mixing the two up is a common source of confusion when reconciling a gst and tds calculator against your books.
GST TDS applies when certain notified persons, mainly government departments and government-controlled entities, make payments to a supplier under a contract worth more than ₹2.5 lakh. They deduct 2% of the payment (1% CGST + 1% SGST for intra-state supplies, or 2% IGST for inter-state) and deposit it against the supplier's GST account. It's not extra tax the supplier pays; it's an advance credit against their GST liability, similar in spirit to income-tax TDS.
GST TCS applies to e-commerce operators, who collect 1% (0.5% CGST + 0.5% SGST, or 1% IGST) on the net value of taxable supplies made through their platform by other sellers, and deposit it on those sellers' behalf.
Here's the math for tds on gst calculation: if a government department pays a contractor ₹5,00,000 for an intra-state supply, TDS deducted comes to 5,00,000 × 2% = ₹10,000 (₹5,000 CGST + ₹5,000 SGST). The contractor still invoices the full amount and pays their full GST liability. The ₹10,000 shows up as a credit they claim when filing, not cash they lose upfront.
Common GST Calculation Mistakes for Small Businesses
Most gst calculation mistakes come down to a handful of repeat offenders, worth checking your invoices against directly.
Applying the rate to the wrong base. Charging GST on the inclusive price instead of the base price inflates the tax and creates a mismatch the customer eventually catches.
Confusing CGST+SGST with IGST. Splitting tax the wrong way based on the customer's location doesn't change the total, but it does mean your GST return doesn't match what you actually collected.
Rounding errors compounding across line items. Rounding GST separately on each line of a multi-item invoice, instead of on the invoice total, can leave you a few rupees off. Small on one invoice, noticeable across a year of them.
Treating GST TDS or TCS as lost revenue. It's a credit against your GST liability, not a deduction from what you're owed. Suppliers who book it as a loss end up double-counting it later.
Using a rate from before the GST 2.0 reform. The 12% and 28% slabs have been gone since September 2025. Any spreadsheet or invoice template still referencing them is charging the wrong tax. Cross-check unfamiliar items against the current 5%/18%/40% structure in CalcMint's GST rates 2026 guide before assuming an old rate still applies.
Frequently Asked Questions
How do I calculate GST backwards from an MRP that already includes tax?
Divide the MRP by (1 + GST rate/100) to get the base price, then subtract that from the MRP to find the GST amount. For an MRP of ₹1,180 at 18%, the base price is 1,180 / 1.18 = ₹1,000, and the GST amount is ₹180. Don't subtract the percentage directly from the MRP; that gives a slightly wrong base price.
What's the difference between CGST+SGST and IGST?
CGST and SGST apply together on sales within the same state, splitting the total GST rate evenly between the central and state governments. IGST applies instead, as one combined rate, when the sale crosses a state border. The total tax amount is the same either way; only the split changes.
Do I need a different calculation for interstate vs intrastate sales?
The GST rate and the final amount charged stay the same. What changes is how you record it: intrastate sales split the tax into CGST and SGST lines, while interstate sales record it as one IGST line. Get this wrong on the invoice and your GST return won't match what you actually collected.
Is GST applicable on the full invoice value or only the margin?
GST is charged on the full sale value of goods or services, not just your profit margin. If you're registered under a special composition or margin scheme, used in specific cases like second-hand goods, different rules can apply. For standard invoicing, the tax is calculated on the total price charged.
Try It Yourself
Working through these formulas by hand helps you understand the mechanics, but it's easy to slip up on a busy invoicing day. CalcMint's GST calculator handles the addition, reverse extraction, and CGST/SGST/IGST split automatically. Plug in a price and a rate, and it does the rest. For the full category-by-category breakdown of which goods and services fall under which slab after the GST 2.0 changes, see the GST rates 2026 guide.