GST (Goods and Services Tax) appears on almost every bill in India, yet the exact maths behind it surprises most people. Once you know whether the GST is on top of the price or already inside it, the calculation is two quick steps every time.
The two GST calculations you will ever need
Every GST problem reduces to one of these two, depending on how the bill is written.
1. Price excludes GST (business quotations, B2B invoices). The tax goes on top: Total = Amount × (1 + rate / 100). A Rs 20,000 invoice at 18% becomes Rs 20,000 × 1.18 = Rs 23,600, with GST of Rs 3,600.
2. Price includes GST (retail bills, MRP prices). The tax is already inside: Base = Amount / (1 + rate / 100). A Rs 23,600 retail bill at 18% has a base of Rs 23,600 / 1.18 = Rs 20,000, so the embedded GST is Rs 3,600.
What are the GST slab rates in India?
India keeps most goods and services on a small set of rates, so you rarely guess the number to use:
- 0% - grains, fruits, vegetables, milk, basic books
- 0.25% - rough diamonds and some rare materials
- 3% - gold, silver and unsorted precious metals
- 5% - staple foods, non-AC restaurants, transport
- 12% - electronics, furniture, processed food
- 18% - most services, mobile phones, AC restaurants (the default service rate)
- 28% - luxury cars, tobacco, aerated drinks, large AC units
CGST + SGST vs IGST: why the total is always the same
Inside a state, GST is split nearly equally into CGST (collected by the Centre) and SGST (collected by the State). Between two states, a single IGST is charged at the full rate. The split only decides which government receives the money - the combined amount you pay is identical for the same slab, which is why the GST calculator works for every invoice.
What is reverse charge GST?
Reverse charge simply means the buyer deposits the GST with the government instead of the seller, which happens for specified supplies such as certain transport and import of services. The amount is still calculated the same way - the difference is only who pays it to the tax office, not how much.
Example walk-through: a restaurant bill
A Rs 1,800 dinner bill at an AC restaurant is GST-inclusive at 18%. Base = 1,800 / 1.18 = Rs 1,525.42, and GST = Rs 274.58. Now book the same meal in a corporate setting where the invoice excludes tax: Rs 1,800 × 1.18 = Rs 2,124. The tax itself (Rs 274.58) is identical in both routes - only the printed base differs.
Skip the arithmetic with the free GST Calculator - pick exclusive or inclusive mode, choose any India slab rate, and the tax plus total appear instantly. For older-style state retail taxes, the Sales Tax Calculator is the matching tool.