GST Calculator: How to Work Out Inclusive vs Exclusive Tax

GST confusion almost always comes down to one question: is the number in front of you before tax or after tax? Get that backwards and you either overcharge a customer or under-quote a price and pay the difference yourself. This guide explains inclusive and exclusive amounts, the exact formulas for both directions, the current GST rates in India, and the mistakes that most often end up on invoices.

Current GST rates in India

From 22 September 2025, the GST Council simplified the main rate structure into two standard slabs:

• 5% for essentials and many everyday goods and services.
• 18% for most other goods and services. This is the most common rate for services.
• 40% for a short list of luxury and "sin" goods.
• 0% for exempt items.

The old 12% and 28% slabs were folded into these. You will still need them when checking or correcting invoices dated before 22 September 2025, which is why calculators keep them as options. A few special rates also remain for specific goods, such as precious metals. Rates are set per category by HSN code (for goods) or SAC code (for services), so check your item's classification rather than assuming.

GST-exclusive vs GST-inclusive

• A GST-exclusive amount is the base price before tax. You calculate GST on top of it. B2B quotes and invoices are usually written this way, with tax shown on a separate line.
• A GST-inclusive amount already has the tax inside it. The price shown is what the customer pays, and the tax portion has to be extracted, not added. MRP and retail prices are inclusive.

The formulas

Adding GST to an exclusive amount

GST = base × rate ÷ 100 and Total = base + GST.

Example at 18%: base ₹1,000 → GST ₹180 → total ₹1,180.

Extracting GST from an inclusive amount

Base = total ÷ (1 + rate ÷ 100) and GST = total − base.

Example at 18%: total ₹1,180 → base = 1,180 ÷ 1.18 = ₹1,000 → GST ₹180.

A shortcut for the tax portion alone is GST = total × rate ÷ (100 + rate). At 18% that is total × 18/118, about 15.25% of the total. At 5% it is total × 5/105, about 4.76%.

The most common mistake

Taking 18% of the inclusive total to find the tax is wrong. On ₹1,180, 18% is ₹212.40, but the actual tax inside that price is ₹180. The tax was calculated on the base, so it has to be extracted by dividing, never by taking a percentage of the total. The error always overstates the tax, and on a large invoice the difference is substantial.

How to calculate GST without doing it by hand

1. Open the GST Calculator and enter your amount.
2. Select the rate: 5%, 18% or 40% for current transactions, 0% for exempt items, or 12% and 28% for invoices dated before 22 September 2025.
3. Choose the direction: exclusive to add GST to a base price, or inclusive to extract the GST already inside a total.
4. Read the breakdown of base, GST and final amount, ready to put on an invoice or check one you received.

CGST, SGST and IGST

These are the same tax split by geography, not extra tax:

• Within one state (seller and buyer in the same state): the rate is split equally between central and state GST. 18% shows as 9% CGST + 9% SGST.
• Between states: the full rate is charged as IGST. 18% shows as 18% IGST.

The customer pays the same total either way; only the allocation between governments differs. A common invoicing mistake is charging CGST + SGST on an inter-state sale, or IGST within the state. The place of supply decides which applies.

Rounding

Calculate GST on each line or on the invoice total consistently, and round to two decimal places (paise). Small differences between line-wise and total-wise rounding are normal, but a customer's accounting system will flag an invoice whose GST doesn't match the rate applied to the taxable value, so make sure the figures on the invoice agree with each other.

Why this matters for invoicing

• If you quote "₹10,000" to a business client, say whether it is "+ GST" or "inclusive of GST". Ambiguity here is the cause of most payment disputes.
• If you are registered and quote an inclusive price by mistake, the tax comes out of your margin: on ₹10,000 at 18%, you keep ₹8,474.58, not ₹10,000.
• If you are not registered, you can't charge GST at all, so don't add it to your invoices.

Frequently asked questions

Do freelancers need to charge GST?

Registration is generally required once annual turnover from services exceeds ₹20 lakh (₹10 lakh in special-category states), and a few categories of business must register whatever their turnover. Below the threshold, registration is optional, though some freelancers register anyway because business clients prefer GST invoices. Check with a tax professional for your specific case.

Is GST charged on the discounted price?

Generally yes. A discount shown on the invoice at the time of sale reduces the taxable value, and GST is calculated on the reduced amount.

How do I check the GST on a restaurant bill or receipt?

Enter the bill total as an inclusive amount at the rate shown on the bill. The calculator extracts the GST so you can compare it with what is printed.

What about other countries' GST?

The inclusive and exclusive formulas work for any GST or VAT rate. Australia's 10%, New Zealand's 15% and the UK's 20% VAT all calculate the same way.