What is GST?
Goods and Services Tax (GST) is India's unified indirect tax. Introduced on 1 July 2017, it replaced VAT, service tax, excise duty, octroi, and a tangle of state-level taxes with a single nationwide tax structure. Every time you buy something — from a restaurant meal to a software subscription — GST is being charged somewhere in the chain.
There are five common slabs: 0%, 5%, 12%, 18%, 28%, plus cess on luxury and "sin" goods (cars above a threshold, tobacco, soft drinks). This calculator computes GST in either direction — adding it to an exclusive amount, or backing it out of an inclusive total.
How is GST calculated?
Two directions matter — and they're not the same math.
Adding GST (price is exclusive of tax)
GST amount = price × (rate / 100)
Total = price + GST amount
Example: ₹10,000 + 18% GST = ₹1,800 GST + ₹11,800 total.
Removing GST (price is inclusive of tax)
Base price = total / (1 + rate / 100)
GST amount = total − base price
Example: ₹11,800 inclusive of 18% GST → base = ₹11,800 / 1.18 = ₹10,000; GST = ₹1,800.
Common mistake: computing 18% of ₹11,800 directly gives ₹2,124 — which is wrong. You have to divide to back-out an inclusive amount, not multiply.
Worked example: SaaS invoice for a freelancer
You charge an Indian client ₹50,000 + GST. GST rate for IT services is 18%.
| Line item | Amount |
|---|---|
| Service fee | ₹50,000 |
| CGST 9% (centre's share) | ₹4,500 |
| SGST 9% (state's share) | ₹4,500 |
| Total invoice | ₹59,000 |
You receive ₹59,000. ₹9,000 goes to the government (via you remitting it during quarterly returns). Your net income is ₹50,000.
If the client is outside your state, replace CGST + SGST with IGST 18% = ₹9,000 (the centre collects, then shares with the destination state).
GST slabs by common product / service
| Slab | Examples |
|---|---|
| 0% | Fresh milk, fresh vegetables, books, newspapers, education up to high school, healthcare |
| 5% | Tea, coffee, packaged paneer, life-saving drugs, economy flights, train tickets, non-AC restaurants |
| 12% | Butter, ghee, cheese, processed food, mobile phones, branded apparel below ₹1,000 |
| 18% | Most services (IT, consultancy, telecom, banking), soap, AC restaurants, hotels under ₹7,500/night, branded apparel above ₹1,000 |
| 28% | Luxury cars, motorcycles, ACs, refrigerators, soft drinks, tobacco (+ additional cess) |
Slabs change at every GST Council meeting. Always confirm the applicable rate for big-ticket purchases.
CGST, SGST, IGST — which one when?
| Type of sale | Tax composition | Example |
|---|---|---|
| Intra-state (same state buyer and seller) | CGST (half rate) + SGST (half rate) | Mumbai shop sells to Mumbai customer at 18%: 9% CGST + 9% SGST |
| Inter-state (different state) | IGST (full rate) | Bangalore seller invoices Mumbai client at 18%: 18% IGST |
| Import | IGST + Basic Customs Duty | Import a laptop into India: IGST + BCD applies |
| Export | Zero-rated; refundable input GST | Indian software exporter charges 0% GST, claims refund of input GST |
The total tax to the consumer is identical (18% either way). Only the centre-state revenue split changes.
Components and inputs explained
Mode (exclusive vs inclusive)
- Exclusive: the amount you type is the base price; GST will be added on top
- Inclusive: the amount you type includes GST; GST will be backed out
If your supplier's quote says "₹X + GST", it's exclusive. If it says "₹X inclusive" or "₹X (all-inclusive)", it's inclusive.
Amount
The base or total amount as per the mode.
GST rate
Pick the applicable slab (0/5/12/18/28%) or enter a custom rate.
Who needs to register for GST?
| Threshold | Required? |
|---|---|
| Turnover > ₹40 L/year (goods) or > ₹20 L/year (services) | Mandatory |
| Special category states (Manipur, Mizoram, Nagaland, Tripura) | Threshold drops to ₹10 L (services) / ₹20 L (goods) |
| E-commerce sellers (any turnover) | Mandatory |
| Inter-state suppliers (any turnover) | Mandatory |
| Casual taxable persons | Mandatory |
| Voluntary registration | Allowed (to claim input credit) |
Most salaried individuals and small freelancers (< ₹20 L turnover) don't need to register.
Input Tax Credit (ITC)
If you're GST-registered:
- GST you pay on business inputs (laptops, office rent, software subscriptions) → claim as input tax credit
- GST you collect from customers on outputs (your services / sales) → output tax
- You pay the government: Output GST − Input Tax Credit
This prevents double-taxation through the supply chain. Salaried individuals can't claim ITC — the GST on your laptop / phone is a final cost.
Considerations
- Quote inclusive vs exclusive matters in negotiations. A "₹1L deal" inclusive is ₹84,746 base + ₹15,254 GST; exclusive is ₹1L base + ₹18,000 GST. ₹15,254 vs ₹18,000 isn't trivial.
- Place of supply rules are tricky for services. Default: location of recipient. Several exceptions for events, immovable property, transportation.
- TCS under GST (different from income tax TCS) applies for e-commerce operators at 1% of net taxable value.
- Reverse charge mechanism: for some categories (legal services from unregistered lawyers, goods from unregistered suppliers), the buyer pays GST directly to the government.
- Composition scheme (small dealer simplified flat rate of 1-6%) is an alternative for turnover up to ₹1.5 cr — gives up ITC.
Tax implications and filing
- GSTR-1: monthly outward supplies summary (by 11th of next month)
- GSTR-3B: monthly summary return with tax payment (by 20th of next month)
- GSTR-9: annual return (by 31 December of next FY)
- GST audit (GSTR-9C): required for turnover > ₹5 cr
Use accounting software (Zoho Books, ClearTax, TallyPrime) to auto-generate returns; manual filing is error-prone.
Limitations
- The calculator only handles single-rate scenarios. For invoices with mixed-rate line items, compute each line separately.
- Doesn't apply cess on luxury/sin goods (28% slab items often have an additional 1–25% cess).
- Doesn't handle reverse charge or composition scheme.
- Doesn't compute place-of-supply rules — for inter-state vs intra-state classification, refer to GST guidelines.
- Doesn't pull historical rates — slab changes after each GST Council meeting.
Related calculators
- Discount Calculator — for sale-price + GST scenarios
- Sales Tax — US-equivalent
- VAT Calculator — UK/EU-equivalent
- Income Tax — direct tax counterpart
- Brokerage / F&O — STT + GST on stock trades
- Stamp Duty — property registration
Final note. GST is the most-frequently-applied number in Indian commerce — every restaurant bill, every invoice, every online purchase. Most people get it slightly wrong on inclusive amounts (multiplying instead of dividing). The two-second discipline that helps: always know whether the amount you're working with is base or total before doing the math. This calculator just removes the guesswork.
