GST Calculator

Most of the time you want one of two things from this page. You have a quote of ₹10,000 and need to know what to put on the invoice. Or you have a bill of ₹11,800 from Amazon, a hotel or a DMart receipt, and need the taxable value for your books or your expense claim. The second one is where the arithmetic goes wrong for a lot of people, and the calculator exists mainly for that.

Adding GST, and pulling it back out of a total

The inputs:

• Amount. A pre-tax value if you are adding, an inclusive total if you are removing.
• GST rate. 0, 5, 12, 18 or 28 percent. Most services are 18.
• Direction. Add GST to this amount, or remove GST from this total.
• Type of supply. Intra-state (buyer and seller in the same state) or inter-state. This does not change the tax, only how it splits.

Adding is plain multiplication. With the default ₹10,000 at 18% within one state:

GST   = 10,000 × 0.18 = 1,800
total = 10,000 + 1,800 = 11,800

Total with GST	₹11,800.00
Taxable value	₹10,000.00
Total GST	₹1,800.00
CGST @ 9%	₹900.00
SGST @ 9%	₹900.00

Divide, do not subtract

Now go the other way. You have that ₹11,800 total and want the ₹10,000 back. The instinct is to knock 18% off. Try it:

11,800 × (1 - 0.18) = 9,676     wrong

The 18% was charged on the smaller pre-tax figure, not on the total, so taking 18% of the total removes too much. The correct method divides:

value = 11,800 / 1.18 = 10,000
GST   = 11,800 - 10,000 = 1,800

Method	Taxable value	Error
Divide by 1.18	₹10,000	correct
Subtract 18%	₹9,676	₹324 too low

That is a 3.2% error on every line, and it always goes the same way: it understates the taxable value and therefore understates the input tax credit you can claim. I have seen a full year’s purchase register built in Excel with =B2*0.82 in the column, and the reconciliation with GSTR-2B never had a chance. At 28% the gap is worse: subtracting gives ₹720 on a ₹1,000 base where the true value is ₹781.25.

The same trick applies whenever a percentage was added to a base you cannot see: TDS gross-ups, service charges, a dealer’s “on-road” quote. If the tax was applied to X and you only have X plus tax, divide.

CGST plus SGST, or IGST: what goes on the invoice

The total tax on ₹10,000 at 18% is ₹1,800 whichever way the supply goes. The split is what changes.

Same state. The tax divides equally between the Centre and the state. A Pune designer billing a Mumbai client puts CGST at 9% (₹900) and SGST at 9% (₹900) on the invoice.

Different states. One integrated tax at the full rate. The same designer billing a Bengaluru client puts IGST at 18%, ₹1,800, and the calculator shows CGST and SGST as zero.

	Intra-state	Inter-state
CGST	₹900	nil
SGST	₹900	nil
IGST	nil	₹1,800
Total tax	₹1,800	₹1,800

Which one applies depends on the place of supply, not on where you sit. For a service delivered over email to a registered business, the place of supply is normally the recipient’s registered address, which is why the client’s GSTIN has to be captured before the invoice goes out and not after. Get the split wrong and the money still reaches the government, but the recipient cannot take credit properly in GSTR-3B and both sides end up amending. Fixing an IGST-versus-CGST mix-up means paying the right head and claiming a refund of the wrong one, which takes months.

Which slab, and what the calculator leaves out

Rates are fixed by HSN code for goods and SAC code for services. The September 2025 GST Council changes, effective 22 September 2025, moved most items into the 5% and 18% slabs and thinned out 12% and 28% considerably, with a separate 40% rate for a short list of goods such as pan masala, tobacco, aerated drinks and large cars. The 12% and 28% options are still in the dropdown because older invoices and a few items still carry them. Look up your own code on the CBIC portal rather than going by what a competitor charges; misclassification is your liability, not the buyer’s.

Rate	Where you will meet it
0%	Fresh produce, milk, unbranded atta, education, healthcare, individual health and life insurance
5%	Packaged food, most essentials, economy air travel, restaurants without input credit
12%	Largely emptied by the September 2025 changes; kept for old invoices
18%	Most services, electronics, cement, small cars
28%	Largely emptied; a few items remain, check your HSN

Other things this page does not do:

Compensation cess. Where it still applies (tobacco, mainly), it sits on top of the GST and is not included here. Add it separately.

Reverse charge. On goods transport agency services, an advocate’s fee, imports of services and a few other cases, the recipient pays the GST directly instead of the supplier collecting it. The arithmetic is the same, the invoice looks different.

Discounts. GST is charged on the discounted price provided the discount is on the invoice and was agreed before or at the time of supply. A discount you give later, after the goods have shipped, only reduces the taxable value if it was pre-agreed and can be tied to specific invoices through a credit note. The discount calculator handles stacked discounts before you get to the tax line.

Rounding. Round once, at the invoice total, and GST law lets you round the tax to the nearest rupee. Rounding every line and summing can differ by a few rupees from taxing the total, which matters when someone’s ERP reconciles to the paisa.

Registration. Generally required once turnover crosses ₹40 lakh for goods or ₹20 lakh for services, with lower limits of ₹20 lakh and ₹10 lakh in the special category states. Inter-state supply of goods, selling through an e-commerce operator and reverse charge liability make it mandatory from the first rupee. Below the limit you cannot charge GST at all, so a small freelancer who adds 18% to a bill without a GSTIN is doing something they should not.

If what you actually need is the tax to withhold from a vendor’s bill rather than the tax to add to your own, that is TDS, not GST, and the TDS calculator does the deduction and the gross-up. For the income tax side of a freelancer’s year, the income tax calculator works the slabs.