GST sounds complex but the arithmetic is simpler than almost any other Indian tax. The tricky part is knowing which slab applies, how to split CGST vs IGST, when the composition scheme is worth it, and the handful of edge cases (discounts, reverse charge, exports) that trip up small businesses on their first audit.
The slab map (FY 2026-27)
The GST Council has settled on seven rates over the years. In practice, almost everything falls in 5 / 12 / 18 / 28%:
- 0% — essential goods (fresh milk, fresh vegetables, unpacked food grains, education services, most healthcare services).
- 0.25% — rough diamonds, semi-precious stones. Niche trade slab.
- 3% — gold, silver, processed diamonds, bullion. Separately notified.
- 5% — packaged food, transport of goods by rail or road, economy air travel, coal, LPG for domestic use, life-saving drugs.
- 12% — restaurants (outside air-conditioned chains), business-class rail, mobile phones, packaged juices, butter, ghee.
- 18% — the default slab. Applies to everything not otherwise notified: professional services, software, telecom, hotel rooms ₹1,000-₹7,500/night, restaurants in AC chains, most consumer durables.
- 28% — luxury / sin category: automobiles, AC, refrigerators, cigarettes, aerated drinks, gambling, cinema ≥ ₹100 ticket. Some attract an additional cess (compensation cess on cars, tobacco) taking effective rate to 40-50%.
When in doubt, the official CBIC rate finder (cbic-gst.gov.in) is authoritative. Don’t rely on blog posts — including this one — for a specific HSN code: GST Council notifications can reclassify items quarterly.
Forward GST — price inclusive
This is the math a seller does before raising an invoice:
Invoice total = Base price × (1 + GST rate / 100)
Example: a consulting invoice at ₹1,00,000 base, 18% GST.
- GST amount = 1,00,000 × 0.18 = ₹18,000
- Invoice total = 1,00,000 + 18,000 = ₹1,18,000
Reverse GST — extracting GST from an invoice total
This is the math a buyer does when they have a gross invoice but need the base price for their ledger / ITC claim:
Base price = Invoice total / (1 + GST rate / 100) GST amount = Invoice total − Base price
Example: a ₹1,18,000 invoice with 18% GST included.
- Base = 1,18,000 / 1.18 = ₹1,00,000
- GST = 1,18,000 − 1,00,000 = ₹18,000
A common mistake: applying 18% to the gross amount to “reverse calculate”. That would give ₹1,18,000 × 0.18 = ₹21,240 — wrong by ₹3,240. The correct divisor is (1 + rate/100), not multiply by rate.
Both directions are two clicks in our GST Calculator, with automatic CGST/SGST/IGST split (below) and cess support.
CGST + SGST vs IGST — the state split
- Intra-state supply (seller and buyer in the same state): GST is split 50:50 between Central GST (CGST) and State GST (SGST). An 18% invoice has 9% CGST + 9% SGST.
- Inter-state supply (seller and buyer in different states, or either is an export / SEZ): single IGST at the full rate. An 18% invoice has 18% IGST, nil CGST/SGST.
- Union Territories without a legislature (Ladakh, A&N, D&N Haveli) levy UTGST instead of SGST — same split math, different line item.
The place of supply rule determines which state is the “state of consumption” for the SGST share. For services delivered digitally (consulting, SaaS), the buyer’s registered office state counts. For goods, the state of delivery. Get this wrong on your GSTR-1 and you’ll spend months in reconciliation with GSTN.
Composition scheme — the small-business shortcut
Businesses with annual turnover up to ₹1.5 crore (goods) or ₹50 lakh (services / mixed) can opt into a composition scheme. You pay a flat percentage of turnover instead of the standard 5/12/18/28% on each invoice:
- Traders: 1% of turnover (0.5% CGST + 0.5% SGST)
- Manufacturers: 1% of turnover
- Restaurants (without alcohol): 5% of turnover
- Services: 6% of turnover
The catch: composition taxpayers CANNOT claim input tax credit (ITC) and CANNOT charge GST to their customers. Useful when your supplier base is largely unregistered (so you don’t have much ITC to claim anyway) and your customers are B2C (so they don’t care about claiming ITC from you).
When to register for GST
- Mandatory: aggregate turnover above ₹40 lakh (goods, most states) or ₹20 lakh (services or northeast states).
- Mandatory regardless of turnover: inter-state supplies, e-commerce operators, casual taxable persons, non-resident taxable persons, reverse-charge cases, input service distributors.
- Voluntary: below-threshold businesses can register to claim ITC on B2B purchases — often worth it if you sell to registered buyers who demand tax invoices.
Reverse charge mechanism (RCM)
Under RCM, the buyer pays GST directly to the government, not through the seller. Applies to a specific list of services — notably:
- Legal services from advocates (if provided to a business entity).
- Services from a non-registered supplier (selected cases).
- Goods transport agency (GTA) services, unless the GTA has opted for forward charge at 12%.
- Rent of commercial property from an unregistered owner to a registered tenant.
- Import of services.
RCM-paid tax becomes ITC for the buyer immediately in the same month, so it’s cash-flow neutral for registered buyers.
Common invoicing mistakes
- Charging GST on post-supply discounts. Discounts recorded on the invoice itself reduce taxable value. Discounts given later via credit note reduce value only if they were part of the original agreement AND the buyer can reverse the corresponding ITC.
- Missing the place-of-supply rule. IGST vs CGST+SGST is determined by place of supply, not the seller’s state. Cross-checking the GSTIN state prefix in your buyer’s GSTIN against your registered state is the 10-second manual check.
- Claiming ITC on ineligible items. Motor vehicles (unless for further supply / transport business), food and beverages for employees, memberships to clubs, health insurance for staff under non-mandatory policies — all blocked under Section 17(5).
- Not reporting reverse charge. RCM payments must appear in GSTR-3B table 3.1(d) AND be self-invoiced. Skipping the self-invoice leaves audit exposure.
- Treating shipping / freight as tax-free. Post-2018, transportation is taxable unless specifically exempt. If you’re a registered supplier and bill delivery as a line-item, it attracts GST at the rate of the underlying good.
Freelancers & consultants — the typical flow
Most Indian freelancers earning below ₹20L aggregate turnover don’t register. Above that, 18% service GST applies on every invoice — which many CAs recommend registering for earlier (₹10-15L) so that input costs (laptops, SaaS, coworking) become creditable. An illustrative ₹10 lakh annual freelance income:
- Unregistered: charge ₹10,00,000, keep ₹10,00,000 (before income tax).
- Registered: charge ₹10,00,000 + 18% = ₹11,80,000. Pay ₹1,80,000 as GST. Claim ₹25,000-₹50,000 ITC on business purchases → net GST out-of-pocket ~₹1,50,000. Client pays you ₹11.8L but claims their ₹1.8L ITC, so cost to them is still ₹10L.
Net effect on the freelancer: GST is cash-flow tax (collected and paid over), reduced by ITC. Income tax is separate and applies to the ₹10L gross. See our Income Tax Calculator and tax regime comparison for the income-side math.
Filing calendar
- GSTR-1 (outward supplies): 11th of next month (monthly) or 13th of next quarter (QRMP scheme under ₹5cr turnover).
- GSTR-3B (summary return + tax payment): 20th of next month. QRMP taxpayers: 22nd / 24th of next quarter depending on state group.
- GSTR-9 (annual return): 31 December of next financial year.
- CMP-08 (composition quarterly return): 18th of next quarter.
Late filing attracts ₹50/day (₹20 nil return) per Act — both CGST and SGST. Miss a few months and you also lose ITC from the affected period. Expensive.
Run your own invoice math
Plug base price (forward) or invoice total (reverse) into our GST Calculator. It handles every slab including 0.25% / 3% niche rates, splits CGST/SGST vs IGST based on your supply type, and supports composition-scheme calculation + compensation cess.
Sources
- CBIC (cbic-gst.gov.in) — HSN code rate finder, notifications, exemption lists.
- GST Council meeting minutes — rate changes and rationalisations.
- CGST Act 2017 & IGST Act 2017 — primary statute (amended through Finance Act 2024).
- Rule 42 / 43 CGST Rules — common credit reversal for exempted supplies.