Working out GST on a price comes down to one piece of arithmetic: multiply the base price by the rate, then divide by 100. Everything else in this guide is that same calculation applied to different numbers. If you'd rather skip the manual math, the GST calculator does this instantly, but it helps to know what it's doing under the hood.
What GST Calculation Actually Involves
The gst calculation formula is built on three numbers: the base price of the good or service, the applicable GST rate, and the resulting tax amount. The base price is the value before tax is added, sometimes called the taxable value or transaction value. The GST rate is the percentage set for that category of goods or services. The tax amount is what you get once the rate is applied to the base price.
Once you have those three, the calculation itself doesn't change based on what you're buying or selling. A packet of biscuits and a consulting invoice go through the same arithmetic. What differs between products is only the rate that applies to them, which depends on how the item is classified under GST.
It's worth being clear about one thing early: a "price" quoted to you could be the base price, or it could already include GST. The formula works the same way either direction, but you need to know which one you're starting from before you calculate anything. This guide covers the standard forward calculation, adding GST to a base price. For pulling GST back out of a tax-inclusive price, see the detailed GST amount guide.
The GST Formula: Adding Tax to a Base Price
The formula for calculating GST on a base price is:
GST amount = (Base price × GST rate) ÷ 100
Price including GST = Base price + GST amount
Or combined into one step:
Price including GST = Base price × (1 + GST rate ÷ 100)
Say a service costs ₹1,000 before tax, and the applicable rate is 18%. The GST amount is (1,000 × 18) ÷ 100, which comes to ₹180. Add that to the base price and the final billed amount is ₹1,180.
The rate is the only variable that changes between transactions. If the same ₹1,000 base price fell under the 5% slab instead, the GST amount would be ₹50, for a total of ₹1,050. Under the 40% slab that applies to certain luxury and sin goods, it would be ₹400, for a total of ₹1,400.
This guide sticks to the forward calculation, tax added to a known base price, because that's what most people need when pricing an invoice or checking a bill. It does not cover splitting the tax into CGST and SGST, or working IGST for interstate supply. Those follow their own rules and are covered separately in the GST amount guide.
Worked Examples Across GST Slabs
The table below applies the formula to three different base prices, each under a different rate slab.
| Base Price | GST Rate | GST Amount | Total Price |
|---|---|---|---|
| ₹850 | 5% | ₹42.50 | ₹892.50 |
| ₹2,000 | 18% | ₹360 | ₹2,360 |
| ₹12,00,000 | 40% | ₹4,80,000 | ₹16,80,000 |
For example, on a base price of ₹850 taxed at 5%, the GST amount is (850 × 5) ÷ 100 = ₹42.50. Add that to the base price and the total comes to ₹892.50.
For example, on a base price of ₹2,000 taxed at 18%, the GST amount is (2,000 × 18) ÷ 100 = ₹360. The total price is ₹2,360.
For example, on a base price of ₹12,00,000 taxed at 40%, the GST amount is (12,00,000 × 40) ÷ 100 = ₹4,80,000, bringing the total to ₹16,80,000. The 40% rate generally applies to select luxury and sin goods rather than everyday purchases, so this last example is included to show how the same formula scales, not as a typical transaction.
The arithmetic doesn't get more complicated as the rate goes up. The only thing that changes is which percentage you multiply by.
Quick Reference Table for Common Amounts
For quick lookups without doing the calculation of gst yourself, here's the GST amount and total price for a handful of common base amounts across the main rate slabs.
| Base Price | GST at 5% | GST at 18% | GST at 40% |
|---|---|---|---|
| ₹100 | ₹5 | ₹18 | ₹40 |
| ₹500 | ₹25 | ₹90 | ₹200 |
| ₹1,000 | ₹50 | ₹180 | ₹400 |
| ₹5,000 | ₹250 | ₹900 | ₹2,000 |
| ₹10,000 | ₹500 | ₹1,800 | ₹4,000 |
This table is for quick reference only. It won't tell you which rate applies to a specific item since that depends on classification, and rates for specific goods and services do get revised from time to time. For an exact figure on a specific amount and rate, the GST calculator is faster than reading down a table, and it's kept current when rates change.
FAQs
What is the basic formula to calculate GST on a price?
GST amount equals the base price multiplied by the GST rate, divided by 100. To get the final price, add that amount back to the base price. So a ₹1,000 item taxed at 18% carries GST of ₹180, making the total payable ₹1,180. The formula stays the same regardless of the rate slab or the type of goods or services involved.
Is GST calculated on MRP or the base price?
GST is calculated on the base price, technically the transaction value, before tax. On packaged retail goods, the printed MRP is generally treated as already inclusive of GST, so the tax has already been factored in. If you're working from an MRP and need to find the tax component within it, that's a different calculation than the one covered here; see the linked guide for that method.
Does the GST formula change across different rate slabs?
No. The formula itself, base price multiplied by rate divided by 100, stays identical whether the applicable rate is 5%, 18%, or 40%. Only the rate figure changes depending on how the good or service is classified. There's no separate calculation method for higher or lower slabs, just a different number plugged into the same equation. For the current category-by-category rate list, see the GST rates 2026 guide.
Is the calculation different for services compared to goods?
The arithmetic is identical for goods and services alike. What differs is the process of determining which rate applies, since goods are classified by HSN code and services by SAC code, and the two use separate rate schedules. Once the correct rate is identified, the same formula applies regardless of whether you're taxing a product sale or a service invoice.
Is This Right For You?
This guide covers the standard calculation for a single, correctly classified base price. It's not enough on its own if you're dealing with disputed invoices, mixed supplies with multiple rates, cross-border transactions, input tax credit claims, or amounts large enough that a classification error would be costly. In those situations, the arithmetic is the easy part; getting the classification and compliance right is where a chartered accountant or GST practitioner earns their fee.
Conclusion
The GST calculation formula itself is short: multiply, divide by 100, add back to the base price. The part that actually takes effort is figuring out which rate slab applies to what you're buying or selling, since that depends on classification rules that sit outside this formula. Once you know the rate, the math is the same every time. For the CGST/SGST/IGST split and TDS/TCS mechanics that build on this formula, see the GST calculator guide.
Try the Calculator
For a faster way to run this calculation with any base price and rate, use the GST calculator. It handles the arithmetic instantly and works for both adding GST to a base price and extracting it from a tax-inclusive amount.