GST calculation, both directions: adding tax to a price and pulling it out of one

Rates and exemptions depend on the classification, date and circumstances of the supply. The 2025 GST Council reforms changed many rates, with some changes phased in later. Special rates and exceptions also exist. The preset rates in our calculator are arithmetic examples, not a complete tax schedule.

Consult the current CBIC notifications and an adviser for the correct HSN or SAC, treatment, value and place of supply. A rate that applies to one item or service may not apply to another with a similar name.

This is the straightforward case. You quote a price of ₹10,000 for a service, the applicable rate is 18%, and the invoice shows the tax separately.

Take the base amount, multiply by the rate as a fraction, and add it: ₹10,000 × 0.18 = ₹1,800 of GST, invoice total ₹11,800. If the rate were 5%, the tax is ₹500 and the total is ₹10,500. Nothing here requires the calculator — what people actually want is a record of the arithmetic on the invoice, and a check that the split below is right.

GST is a single tax with two collection points. On a supply within one state, the same rate is split equally between the centre and the state, shown as CGST and SGST. On a supply between states, the whole amount is collected as IGST — integrated GST — and the centre distributes the state share afterwards.

So an 18% intra-state supply of ₹10,000 shows CGST 9% of ₹900 and SGST 9% of ₹900. The identical supply sent to a buyer in another state shows a single line: IGST 18% of ₹1,800. The buyer’s total is the same; only the destination of the money differs.

Place-of-supply and input-credit rules depend on the details of the transaction and can have exceptions. Consult the applicable legislation or a tax professional before issuing an invoice.

Here the tax is already inside the number, and this is where invoices and quotations go wrong, because subtracting the rate is the wrong operation. A total of ₹11,800 at 18% does not contain ₹11,800 − 18% = ₹9,676 of base and ₹2,124 of tax. It contains ₹10,000 of base and ₹1,800 of tax.

The correct division is by 1 + rate. To generalise: base = total ÷ (1 + r), and GST = total − base. For an 18% inclusive price, divide by 1.18. For 5%, divide by 1.05. For 40%, divide by 1.40.

The error matters in both directions. If you plan on the ₹9,676 base, you have underestimated your revenue by ₹324 and will find your margin thinner than the sheet said. If you quote inclusively and check your own invoice against the wrong formula, you may believe you are charging less tax than the portal says you owe.

GST payable is normally rounded to the nearest rupee under section 170 of the CGST Act, and most accounting software rounds the invoice total as well. When you hand-calculate, round once at the end and not at each line — rounding CGST and SGST separately and then adding can put the total a rupee away from the header figure, which is exactly the kind of discrepancy that gets a purchase order returned.

For invoices with many line items, calculate tax per line at full precision, sum the lines, then round the invoice total once. If your buyer’s system computes it per line and rounds, expect a rupee-level difference and reconcile it explicitly rather than silently.

A short map of the cases people write in about, with the caveat that classification is a legal question and this is not tax advice.

• Registered business selling to an unregistered consumer within the state: charge at the applicable rate, split CGST/SGST on the invoice.
• Registered business selling to a buyer in another state: IGST at the applicable rate, unless the supply is exempt.
• Composition scheme dealer: a flat rate on turnover (typically 1% for traders and manufacturers, 5% for restaurants), no tax collected separately on the invoice, and no input credit.
• Export of services: generally zero-rated, but the paperwork — LUT or refund route — is not optional.
• Freelancer billing a foreign client: often an export of services with zero rating, conditional on place-of-supply and payment rules.
• Residential and commercial rent can have different treatment and exceptions. Check the current conditions and seek professional advice.

Our GST calculator takes a base or an inclusive amount, a rate you select, and whether the supply is intra-state or inter-state, and shows the CGST/SGST or IGST split with the reverse calculation alongside. It runs entirely on your device, which matters more than it sounds for financial figures: the numbers you type never leave the tab, and there is no server log of what anyone quoted whom.