How to Calculate GST: Add, Remove and Split CGST/SGST (2026)

To add GST, multiply the price by (1 + rate ÷ 100); to remove GST from a price that already includes it, divide by (1 + rate ÷ 100). At 18%, ₹1,000 becomes ₹1,180, and ₹1,180 inclusive of GST goes back to ₹1,000, not to ₹967.60 as many people calculate. Within one state the tax is split equally into CGST and SGST (9% + 9% at the 18% rate); between states it is charged as a single IGST at the full rate.

The free GST calculator does both directions: pick "add" or "remove", type the rate, and it shows the base price, the GST, the CGST + SGST halves and the IGST figure. This guide shows the working behind those numbers, the rates that apply after the September 2025 changes, and the mistakes that cause wrong invoices.

The two GST formulas

Every GST question is one of two cases. Either you know the price before tax and want the total, or you know the total (an MRP, a quote "inclusive of GST", a bank statement entry) and want to know how much of it is tax.

• Adding GST: GST = base price × rate ÷ 100. Total = base price + GST, or simply base price × (1 + rate ÷ 100).
• Removing GST: base price = total ÷ (1 + rate ÷ 100). GST = total − base price.

The multiplier for adding and the divisor for removing are the same number, so it helps to remember them for the main rates.

The last column is a handy check. Out of any GST-inclusive price at 18%, a little over 15% is tax, never 18%.

Worked example 1: adding 18% to a freelance invoice

You design a website for a client in your own state and agree a fee of ₹45,000 plus GST at 18%.

1. GST = 45,000 × 18 ÷ 100 = ₹8,100.
2. Total = 45,000 + 8,100 = ₹53,100.
3. Split: CGST 9% = ₹4,050 and SGST 9% = ₹4,050.

The calculator in "add" mode with amount 45000 and rate 18 shows exactly this: total ₹53,100.00, GST ₹8,100.00, CGST + SGST ₹4,050.00 + ₹4,050.00, or IGST ₹8,100.00 if the client were in another state.

Worked example 2: taking 18% out of an inclusive price

A price tag says ₹2,499 including 18% GST and you need the pre-tax value for your accounts.

1. Base price = 2,499 ÷ 1.18 = ₹2,117.80.
2. GST = 2,499 − 2,117.80 = ₹381.20.
3. Split: CGST ₹190.60 and SGST ₹190.60.

In "remove" mode, amount 2499 and rate 18, the calculator gives base ₹2,117.80 and GST ₹381.20. As a check, 381.20 is 15.25% of 2,499, which matches the table above.

Why "price minus 18%" gives the wrong answer

The most common GST mistake is to take a GST-inclusive price and knock off the rate. On ₹1,180, 18% is ₹212.40, so you would get a base of ₹967.60. Add 18% back to ₹967.60 and you get ₹1,141.77, not ₹1,180. The figures don't reconcile because the 18% was charged on the base price, not on the total.

Dividing by 1.18 undoes the multiplication exactly: 1,180 ÷ 1.18 = ₹1,000, and 18% of ₹1,000 is the ₹180 of tax. The same logic works for discounts and price rises; the percentage guide covers these reverse percentages in more detail.

CGST, SGST, UTGST and IGST: who gets the tax

The rate you pay doesn't change with the split. What changes is which government receives it and how the invoice must show it.

• Intra-state supply (seller and place of supply in the same state): half is CGST, collected for the Centre, and half is SGST, for the state. An 18% item is 9% CGST + 9% SGST.
• Union territories without a legislature, such as Chandigarh and Ladakh: UTGST takes the place of SGST, still half and half.
• Inter-state supply (seller in Maharashtra, buyer receiving the goods in Karnataka): one IGST at the full rate, 18% in this case.

A seller in Pune billing a customer in Pune for ₹50,000 at 18% charges CGST ₹4,500 + SGST ₹4,500. The same seller billing a customer in Bengaluru charges IGST ₹9,000. The customer pays ₹59,000 either way.

One small catch: invoices round each tax to the paisa. At 40% on a ₹40 inclusive price, the GST is ₹11.43 but each half rounds to ₹5.71, which adds up to ₹11.42. A one-paisa difference between the halves and the total is normal and not an error.

GST rates from 22 September 2025

The 56th GST Council meeting replaced most of the old 5/12/18/28% structure with two main rates, 5% and 18%, plus a special 40% rate for luxury and "sin" goods. Most items that were at 12% moved to 5%, and most at 28% moved to 18%. The changes took effect on 22 September 2025. Cigarettes and most other tobacco products moved later, on 1 February 2026, when the compensation cess ended; they now pay 40% GST plus a separate excise duty, while bidis are at 18%.

The examples below come from the Finance Ministry's official FAQ on the Council's decisions. Always confirm the rate for your exact product using its HSN code (or SAC code for services), because similar-sounding items can sit in different slabs.

A few 5% services, such as hotel rooms up to ₹7,500 and salons, come "without input tax credit": the business can't claim back the GST it paid on its own purchases. That affects the business's costs, not the calculation on your bill.

More worked examples with the calculator

Here is how the new slabs work out on everyday bills. Each line is the calculator's output for the inputs shown.

The two hotel rows show why slabs matter: a room tariff just above ₹7,500 moves from 5% to 18%, so the ₹8,000 room carries ₹1,440 of GST against ₹300 on the ₹6,000 room. The car row is a simplification; on-road prices also include registration, insurance and other charges that are not GST.

GST on your own invoices: practical rules

• Say whether a quote is inclusive or exclusive. On "₹45,000 + GST" at 18% the client pays ₹53,100 and you keep ₹45,000. On "₹45,000 inclusive of GST" the client pays ₹45,000 and you keep ₹38,135.59, because ₹6,864.41 of it is tax. Put it in writing before work starts.
• Only registered businesses can charge GST. If you are not registered, you can't add GST to your bill, and a buyer can't claim credit for it.
• Show the split correctly. Use CGST + SGST when the place of supply is in your state and IGST when it isn't. Charging the wrong type means the buyer may not get credit until it is corrected.
• Price for profit after tax. GST you collect isn't income. Check your margin on the pre-tax price with the profit margin calculator.

The invoice generator lays out a clean bill once you have the figures, and the freelancer invoice guide covers what else to include. Selling abroad? The VAT calculator does the same job for value-added tax.

This is general information to help you check calculations, not tax advice; confirm rates and invoicing rules for your business with a GST practitioner.

Frequently asked questions

How do I calculate 18% GST on ₹1,000?

Multiply by 18 and divide by 100: GST is ₹180 and the total is ₹1,180. Within a state that is ₹90 CGST and ₹90 SGST; between states it is ₹180 IGST.

How do I remove 18% GST from a total amount?

Divide the total by 1.18. For ₹5,900 that gives a base of ₹5,000 and GST of ₹900. Never subtract 18% of the total; that undercounts the base price.

How much GST is included in an MRP?

MRP includes all taxes, so use the remove formula. At 5% the tax inside is 5 ÷ 105 of the MRP (about 4.76%); at 18% it is 18 ÷ 118 (about 15.25%); at 40% it is 40 ÷ 140 (about 28.57%).

Are the 12% and 28% GST slabs still used?

For most goods and services, no. From 22 September 2025 most 12% items moved to 5% and most 28% items moved to 18%, with luxury and sin goods at 40%. A few special rates remain for specific items, so check the HSN code rather than assuming.

Do online orders have CGST + SGST or IGST?

It depends on where the seller is registered and where the goods are delivered. If the seller's state and the delivery state are the same, the invoice shows CGST + SGST; if they differ, it shows IGST. The total tax is the same.

Does the GST calculator handle any rate?

Yes. Type any rate from 0% to 100%, so it works for 5%, 18%, 40% or any special rate. It shows CGST and SGST as equal halves; for UTGST, read the SGST half as UTGST.

Sources: Ministry of Finance, FAQs on the decisions of the 56th GST Council (PIB, 3 September 2025); CGST, SGST, UTGST and IGST Acts, 2017; Gazette notifications ending the GST compensation cess on tobacco from 1 February 2026 (as reported, January 2026).