Data7 min readOct 2, 2026• By Amit Singh

How to Calculate GST: Inclusive vs Exclusive, CGST/SGST vs IGST (With Examples)

Removing 18% from a GST-inclusive price does not give you the price before GST — and that one mistake shows up on invoices everywhere. Here's the correct maths for adding and removing GST, the CGST/SGST vs IGST rule, and the rate slabs after GST 2.0.

GST maths looks trivial until you have to put it on an invoice. The most common mistake we see — in spreadsheets, in billing software set up by hand, even on printed price lists — is backing GST out of an inclusive price by simply subtracting the rate. It gives the wrong number, and the error grows with the rate. Here's the correct way to do every common GST calculation, with worked examples. If you'd rather not do it by hand, our free GST Calculator does all of this instantly.

Adding GST (exclusive price)

When your price doesn't yet include tax, multiply by the rate and add it on. ₹1,000 at 18%: GST = 1,000 × 18 ÷ 100 = ₹180; total = ₹1,180. Simple.

Removing GST (inclusive price) — the right way

When a price already includes GST, the taxable value is total × 100 ÷ (100 + rate). ₹1,180 inclusive of 18%: 1,180 × 100 ÷ 118 = ₹1,000 taxable, so GST is ₹180. The wrong way is 1,180 − 18% = ₹967.60 — that understates the taxable value by ₹32.40 and overstates GST to match. The reason: the 18% was charged on the *pre-tax* amount, not on the total, so you can't take 18% of the total to reverse it.

CGST + SGST, or IGST

It depends on where the supply happens relative to the seller. If the seller and the place of supply are in the same state, the tax is split equally into Central GST and State GST — 18% becomes 9% CGST + 9% SGST. If they're in different states, the whole 18% is charged as Integrated GST. The total tax is identical; what changes is which government it goes to and how it shows on the invoice and in your returns. Charging CGST/SGST on an inter-state sale (or IGST on an intra-state one) is a real error — the tax paid under the wrong head has to be paid again under the right one and the wrong payment claimed back as a refund, so it's worth getting right on the invoice.

Rounding: where the paise go

On an intra-state invoice, compute CGST and SGST and round each to the paisa; GST is their sum. Example: ₹999 inclusive of 5%. Taxable value is about ₹951.43, each half is ₹23.79, total GST ₹47.58, and the taxable value shown becomes ₹951.42 so everything adds back to exactly ₹999. On an inter-state invoice there's one IGST figure, rounded once. Many businesses also round the grand total to the nearest rupee with a separate 'Round off' line — that's fine, as long as the line is shown.

The rate slabs today

Since the GST rate rationalisation that took effect on 22 September 2025 (often called GST 2.0), most goods and services fall into two main slabs — 5% and 18% — with a 40% slab for luxury and 'sin' goods. A few special rates continue, such as 3% on gold and silver and 0.25% on rough diamonds, and some items are exempt. The rate always follows the HSN (goods) or SAC (services) code, so check the current notified rate for your code rather than relying on a generic table — including tables in older blog posts.

Doing this at scale

Calculating GST for one invoice is easy. Doing it correctly across hundreds of invoices, mixed rates, inter- and intra-state customers and credit notes is where spreadsheets break. That's the point to move to a proper billing system or an automated pipeline that applies the rules the same way every time. Our free GST Invoice Generator handles the split per line item, and at HowAutomate we build the automated version — invoices raised straight from your orders, with the GST maths done the same way every time.

Frequently Asked Questions

How do I remove GST from a GST-inclusive price?

Divide the inclusive price by (100 + GST rate) and multiply by 100 to get the taxable value. For ₹1,180 including 18% GST: 1,180 × 100 ÷ 118 = ₹1,000, so the GST is ₹180. Subtracting 18% from ₹1,180 gives ₹967.60, which is wrong.

When do I charge CGST and SGST instead of IGST?

Charge CGST + SGST (half the rate each) when the seller and the place of supply are in the same state. Charge IGST (the full rate) when they are in different states. The total tax amount is the same.

What are the GST rates after GST 2.0?

From 22 September 2025, most goods and services fall into two main slabs, 5% and 18%, with a 40% slab for luxury and sin goods. Special rates such as 3% for gold and silver continue. Always check the notified rate for your specific HSN or SAC code.

How should GST be rounded on an invoice?

Round CGST and SGST each to the paisa and show GST as their sum, so the figures add up exactly. For IGST, round the single amount once. You may also round the invoice total to the nearest rupee if you show a separate round-off line.

Is there a free GST calculator?

Yes — HowAutomate's GST Calculator at tools.howautomate.com/gst-calculator adds or removes GST and shows the CGST/SGST or IGST split. It runs in your browser and needs no signup.

Amit Singh

Founder, HowAutomate — Data Engineering, AI Automation & Cloud Infrastructure

Amit has 6+ years of experience building data pipelines, AI agents, and automation systems for businesses across India and globally. He founded HowAutomate to make enterprise-grade automation accessible to growing businesses.