GST Calculator

Four slabs, one tax, and a split that confuses everyone

A ₹1,000 item at the standard 18% rate carries ₹180 of GST, making the invoice ₹1,180. If the seller and buyer are in the same state, that ₹180 does not appear as one line — it splits into ₹90 CGST and ₹90 SGST. Cross a state border and the identical ₹180 becomes a single ₹180 IGST line instead. The tax is the same; only the division between the Centre and the states changes.

Goods and Services Tax replaced central excise duty, service tax, state VAT, octroi and a long list of other levies on 1 July 2017. Its central design idea is input tax credit: every business in a supply chain charges GST on what it sells and claims back the GST it paid on what it bought, so tax is levied only on the value each stage actually adds. Without that credit mechanism, tax would compound on tax at every step — which is precisely what the old system did.

Since the 56th GST Council meeting, effective 22 September 2025, the rate structure has been simplified to four slabs. The earlier 12% and 28% bands were removed, with most items in them moving down to 5% and 18% respectively, and a new 40% band created for luxury and sin goods.

The rate slabs and what they cover

• 0% — nil-rated essentials: milk and dairy staples, lifesaving medicines, educational items, and individual health and life insurance policies.
• 5% — daily essentials: packaged foods, FMCG products, toiletries and most medicines.
• 18% — the standard rate, covering most goods and services: electronics, appliances, cement, most vehicles, and the bulk of professional services.
• 40% — luxury and sin goods: tobacco and pan masala, aerated and sugary drinks, luxury vehicles and online gaming.

A compensation cess continues to apply to tobacco products over and above the 40% rate, which this calculator does not model.

How the two directions are computed

GST calculations run one of two ways, and mixing them up is the most common arithmetic error in Indian billing.

Adding GST to a price that excludes it:

GST = Base × rate ÷ 100, and Total = Base + GST

Removing GST from a price that already includes it — which is *not* simply taking the same percentage off:

Base = Inclusive amount × 100 ÷ (100 + rate), and GST = Inclusive amount − Base

The asymmetry trips people up constantly. On a ₹1,180 inclusive price at 18%, the GST is not 18% of ₹1,180 (₹212.40). It is ₹1,180 × 100 ÷ 118 = ₹1,000 base, so ₹180 of tax. Taking 18% off an inclusive figure overstates the tax every time, because the 18% was calculated on the smaller base, not on the total.

The split then depends on where the supply moves:

• Intra-state (same state): CGST and SGST, each exactly half the GST.
• Inter-state or import: IGST, the full amount as one line.
• Union Territory without a legislature: CGST and UTGST, again half each.

The total never changes. A buyer pays ₹180 either way — the split determines which government receives it.

Worked both ways, at 18%

Adding GST. Base ₹10,000, rate 18%, intra-state.

GST = 10,000 × 18 ÷ 100 = ₹1,800. Invoice total = ₹11,800. Split: CGST ₹900, SGST ₹900.

Removing GST. An all-in quote of ₹11,800, rate 18%.

Base = 11,800 × 100 ÷ 118 = ₹10,000. GST = 11,800 − 10,000 = ₹1,800, again splitting into ₹900 and ₹900.

The two are mirror images, which is the check worth remembering: if you add GST and then remove it, you should land exactly where you started. Had you instead taken 18% off ₹11,800 you would have got ₹2,124 of "tax" and a base of ₹9,676 — an error of ₹324 on a single invoice.

At 5%, inter-state. A base of ₹2,000 carries GST of 2,000 × 5 ÷ 100 = ₹100, invoiced as ₹2,100 with a single ₹100 IGST line, because the supply crosses a state border.

Who needs this calculation

Small businesses and freelancers issuing invoices, who must show the correct split and the correct HSN or SAC code.

Anyone quoting a client. The difference between "₹50,000" and "₹50,000 plus GST" is ₹9,000, and leaving it ambiguous is the origin of a large share of payment disputes.

Buyers checking an inclusive quote, to separate the actual cost of goods from the tax component — particularly when comparing a GST-registered vendor against an unregistered one.

Anyone claiming input tax credit, who needs the tax component isolated correctly to claim it.

Reading your result

The GST amount is the tax on this single supply. The CGST/SGST or IGST lines are how it must appear on a compliant tax invoice — getting the split wrong is a genuine compliance problem, not a presentational one, because the recipient's input tax credit is claimed against those specific heads.

What the calculator does not tell you is your net liability. A registered business does not pay over the GST it collects; it pays the difference between output tax collected and input tax credit available on its purchases. A business that collected ₹1,800 and paid ₹1,200 of GST on inputs remits ₹600.

It also does not cover reverse charge, where the recipient rather than the supplier pays the tax, compensation cess on tobacco and similar goods, or the composition scheme, under which small taxpayers pay a flat percentage of turnover and cannot claim input credit.

Where GST calculations go wrong

Applying the rate to an inclusive amount. As above — the single most frequent error, and it always overstates the tax.

Splitting IGST into halves. Inter-state supplies carry one IGST line. There is no CGST/SGST split.

Using the place of supply incorrectly. The split follows the place of supply, which for services is not always where the provider sits. Getting this wrong means issuing an invoice the recipient cannot claim credit against.

Assuming the old 12% and 28% slabs still apply. They were removed on 22 September 2025. Invoices and price lists built on the earlier structure need updating.

Forgetting registration thresholds. Registration is generally required above ₹40 lakh of turnover for goods and ₹20 lakh for services, with lower limits in special category states, and is mandatory regardless of turnover for inter-state supply and for e-commerce operators.

Treating a GST-inclusive discount casually. A discount given after the invoice is issued only reduces the taxable value if it was agreed beforehand and is linked to the specific invoice — otherwise the GST stands on the original amount.

Ignoring rounding rules. Tax amounts are rounded to the nearest rupee on the invoice total, not line by line, which is why a manually computed figure can differ by a rupee from accounting software.

Assuming a GST-registered vendor is always more expensive. If you are yourself registered, the GST on a purchase is recoverable as input credit, so the real cost to you is the base price alone. An unregistered vendor quoting a lower all-in figure may actually be dearer once the credit you cannot claim is accounted for. Comparing an unregistered quote against a registered one means comparing the unregistered total against the registered *base*, not against the registered total — a distinction that changes the answer surprisingly often.