How to Calculate GST: Inclusive and Exclusive Prices

Almost every GST mistake on a small business bill comes from one confusion: is the number you are looking at the price before tax or the price after tax? The two cases need different formulas, and mixing them up gives a tax figure that is wrong by hundreds or thousands of rupees. This guide shows both formulas, works through examples in rupees, and explains where a simple calculator stops being enough.

This article is general information, not tax or legal advice. Rates and rules are set by the GST Council and the government and can change, so confirm anything that affects a filing with the GST portal or a qualified professional.

The two directions

Every GST calculation is one of these:

Exclusive to inclusive (adding tax). You know the base price and want the amount the customer pays. Typical for a freelancer quoting a fee or a shop pricing stock.
Inclusive to exclusive (extracting tax). You know the final price, say a supplier's bill or a price tag, and want to know how much of it is tax and what the base price was.

The words "exclusive" and "inclusive" refer to whether GST is already inside the number. A quote of "Rs. 10,000 plus GST" is exclusive. A price tag of "Rs. 11,800 inclusive of all taxes" is inclusive.

Formula 1: adding GST to a base price

The arithmetic is two lines:

GST = base × rate / 100
Total = base + GST

With a base of Rs. 10,000 at 18%, GST is Rs. 1,800 and the total is Rs. 11,800. For a supply within one state, the GST splits equally into CGST and SGST, Rs. 900 each. A base of Rs. 2,000 at 5% gives GST of Rs. 100 and a total of Rs. 2,100, with Rs. 50 each of CGST and SGST.

The GST calculator does exactly this. You enter an amount and a rate, and it shows the base, the GST, the total, and the CGST and SGST halves. It accepts any rate you type, so it does not need updating when rates change, but it also means it will not tell you whether the rate you typed is the right one.

Formula 2: removing GST from an inclusive price

This is the direction people get wrong, and it is the one our calculator does not do directly. The calculator only adds tax to the number you enter. To go backwards you work out the base first:

Base = total × 100 / (100 + rate)
GST contained = total − base

For a tag price of Rs. 11,800 at 18%: base = 11,800 × 100 / 118 = Rs. 10,000, and the GST inside is Rs. 1,800. For Rs. 1,050 at 5%: base = Rs. 1,000 and GST is Rs. 50. Once you have the base, you can type it into the calculator to see the CGST and SGST split.

Not every inclusive price comes out as round numbers. For Rs. 2,499 at 18%, the base is Rs. 2,117.80 and the GST is Rs. 381.20 (Rs. 190.60 each for CGST and SGST). For Rs. 5,000 at 18%, the base is Rs. 4,237.29 and the GST is Rs. 762.71. Keep two decimal places and round only at the end.

The mistake that costs the most

Many people extract tax by taking the rate off the total directly: 18% of Rs. 11,800 is Rs. 2,124. That is wrong. Rs. 2,124 is 18% of a number that already contains GST, so it overstates the tax by Rs. 324, and the "base" you would then get (Rs. 9,676) is too low. The correct tax inside Rs. 11,800 is Rs. 1,800, because GST is charged on the base, not on the final price. If the figure you are holding includes tax, always divide by (100 + rate) first.

Worked example: a bill with a discount and two rates

Suppose a small business sells a service and a product on one bill, both supplied within the same state.

Service listed at Rs. 12,000 with a Rs. 1,200 trade discount shown on the invoice. Taxable value is Rs. 10,800. At 18%, GST is Rs. 1,944 (Rs. 972 CGST and Rs. 972 SGST), and the line total is Rs. 12,744.
Goods at Rs. 3,000 taxed at 5%. GST is Rs. 150 (Rs. 75 each), and the line total is Rs. 3,150.
Bill total: Rs. 12,744 + Rs. 3,150 = Rs. 15,894. Total GST on the bill is Rs. 2,094.

Two points to take from this. First, the discount comes off before tax is calculated, as long as it is a normal trade discount recorded on the invoice; tax on the undiscounted Rs. 12,000 would be Rs. 2,160, Rs. 216 too much. Second, the two lines are calculated separately. Running one rate over the combined Rs. 13,800 would give the wrong tax. Run the calculator once per rate and add the results.

Which rate do you use?

The calculator cannot answer this, and it is the part that needs care. Since 22 September 2025, following decisions of the 56th GST Council meeting, the main rates on goods and services are 5% and 18%, with a 40% rate for a short list of items such as tobacco and certain luxury goods. The older 12% and 28% slabs were retired, and many items moved to one of the two main rates. Some goods and services are exempt or nil rated, and some have special rates, so the right rate depends on the item.

The rate is fixed by the classification of what you sell (its HSN code for goods or SAC code for services), not by what the customer thinks it should be. Look up your product or service in the current rate schedule on the GST portal or CBIC notifications, or ask your accountant, rather than relying on an old price list or a rate you remember from a few years ago.

Same state or different state

The calculator shows the tax as CGST plus SGST, which applies when the supplier and the place of supply are in the same state. For a sale to another state, the whole amount is IGST at the full rate. The total tax is the same number; only the label and the head it is deposited under change. In the Rs. 10,000 at 18% example, an inter-state invoice shows IGST of Rs. 1,800 and no CGST or SGST. Do not copy the CGST and SGST rows into an inter-state invoice.

Turning the numbers into a bill

Once you know the amounts, the invoice generator can lay them out as a text invoice. It takes a subtotal and a GST rate, computes the tax and total in the same way (subtotal × rate / 100, then added on), and prints a single GST line. Note what that means: it applies one rate to the subtotal you enter. If your bill has lines at different rates, as in the example above, it will not split them for you; you would need to prepare the lines yourself.

If you are pricing for profit rather than only compliance, the profit margin calculator helps check what is left after costs. Use the base price, not the GST-inclusive price, as your revenue, because the GST you collect is owed to the government and is not your income.

A quick decision framework

Is the number you have inclusive or exclusive of GST? If you cannot tell, ask or check the invoice wording.
If exclusive: GST = base × rate / 100, total = base + GST.
If inclusive: base = total × 100 / (100 + rate), then GST = total − base.
Apply any trade discount before computing tax.
Calculate each rate on the bill separately.
Same state: split into CGST and SGST. Different state: show IGST.

Limits: what this does not cover

The calculator uses the rate you type. It does not look up rates, HSN or SAC codes, or tell you whether a supply is exempt.
It does not extract tax from an inclusive price. You do that step by hand with the formula above.
It does not handle cess, reverse charge, input tax credit, rounding rules on invoices, or e-invoicing.
It shows only the CGST and SGST split and has no inter-state IGST mode; you must relabel the tax yourself.
It handles one amount and one rate per calculation, so mixed-rate bills mean separate calculations.

For registration thresholds, returns, input credit and anything that affects what you actually file, rely on the official GST portal or a qualified tax professional. The calculator is for quick, correct arithmetic once you already know the right rate.