Total Invoice = Base + GST = ₹10,000.00 + ₹1,800.00 = ₹11,800.00.
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About the GST Calculator
The Goods and Services Tax is a destination-based indirect tax levied on the supply of goods and services, replacing the layered structure of excise duty, service tax, VAT and a range of state levies that preceded it. Its defining feature is the input tax credit mechanism, which allows a business to offset tax paid on purchases against tax collected on sales, so that tax ultimately falls only on the value added at each stage rather than cascading. For anyone issuing or receiving an invoice, two calculations recur constantly: adding GST to a base price to arrive at the amount payable, and extracting the GST component from a tax-inclusive total to work out the taxable value. The second is where errors are most common, because the tax is a percentage of the base price rather than of the total, so the extraction requires division rather than a simple percentage of the displayed figure.
Mathematical Formula & Logic
Adding GST to a base price and extracting it from an inclusive total.
1. Adding GST (exclusive to inclusive):
GST Amount = Base Price × (Rate ÷ 100)
Total Payable = Base Price + GST Amount
= Base Price × (1 + Rate ÷ 100)
2. Removing GST (inclusive to exclusive) — the reverse charge:
Base Price = Total ÷ (1 + Rate ÷ 100)
GST Amount = Total − Base Price
= Total × Rate ÷ (100 + Rate)
Note that the GST component is NOT the rate applied to the total.
At 18 percent, the tax inside an inclusive price is 18/118 of it,
which is about 15.25 percent of the total, not 18 percent.
3. Splitting the levy by transaction type:
Intra-state supply: CGST = SGST = Total GST ÷ 2
Inter-state supply: IGST = Total GST (single combined levy)
4. Net liability after input tax credit:
Net GST Payable = GST Collected on Sales − GST Paid on Purchases
Step-by-Step Example
Work both directions at an 18 percent rate:
Adding GST to a base price of 10,000
1. GST Amount = 10,000 × (18 ÷ 100) = 1,800
2. Total Payable = 10,000 + 1,800 = 11,800
3. For an intra-state supply this splits into CGST 900 and SGST 900.
4. For an inter-state supply it is a single IGST of 1,800.
Removing GST from an inclusive total of 11,800
5. Base Price = 11,800 ÷ (1 + 0.18) = 11,800 ÷ 1.18 = 10,000
6. GST Amount = 11,800 − 10,000 = 1,800
7. Cross-check with the direct formula: 11,800 × 18 ÷ 118 = 1,800. Correct.
The common mistake
8. Applying 18 percent directly to the inclusive total gives 11,800 × 0.18 = 2,124, which overstates the tax by 324. The tax is 18 percent of the base, not of the total. Expressed as a share of the inclusive price, an 18 percent rate is 15.25 percent.
Input tax credit illustration
9. If this business had earlier paid 1,080 GST on inputs, its net liability for the period is 1,800 − 1,080 = 720, since credit for tax already paid up the chain is set off against tax collected.
Reference Data & Values
rate
gst on_10000
inclusive total
tax share_of_total
divisor to_remove
5%
500
10,500
4.76%
1.05
12%
1,200
11,200
10.71%
1.12
18%
1,800
11,800
15.25%
1.18
28%
2,800
12,800
21.88%
1.28
Frequently Asked Questions
Divide the inclusive total by one plus the rate expressed as a decimal, then subtract to find the tax. For an 18 percent rate, the base price is Total ÷ 1.18, so an inclusive amount of 11,800 has a taxable value of 10,000 and a GST component of 1,800. The equivalent one-step formula is Total × Rate ÷ (100 + Rate). Applying the rate directly to the inclusive figure is the most frequent error in invoice reconciliation and always overstates the tax.
They are the same total tax divided differently depending on where the supply crosses. For a supply within a single state, the levy splits equally into Central GST and State GST, so 18 percent becomes 9 percent CGST plus 9 percent SGST. For a supply between states, a single Integrated GST is charged at the full 18 percent, which the centre later apportions to the destination state. The rate the customer pays is identical either way; only the internal allocation between governments differs, and the correct classification matters for filing.
Input tax credit lets a registered business offset the GST it paid on business purchases against the GST it collected on sales, so that tax applies only to the value the business itself added rather than compounding at every stage. A registered manufacturer who paid 1,080 in GST on raw materials and collected 1,800 on the finished sale remits only the 720 difference. Credit generally requires that the supplier has actually reported the transaction, the invoice is valid, and the goods or services were used for business purposes. Final consumers cannot claim it, which is why the burden ultimately rests with them.
Rates are assigned by category under the GST Council's notified schedules, with the main slabs being 5, 12, 18 and 28 percent, alongside nil-rated and exempt categories and a compensation cess on certain items such as tobacco and luxury vehicles. Essential goods sit at the lower end and non-essential or luxury items at the higher end. Because classifications are revised periodically by the Council, the applicable rate for a specific HSN or SAC code should be confirmed against the current official notification rather than assumed from memory.
Registration becomes mandatory once aggregate annual turnover crosses the prescribed threshold, which differs for goods and services and is lower for certain special category states. Registration is also compulsory regardless of turnover in specific situations, including inter-state supply of goods, e-commerce operators, and businesses liable under reverse charge. Because thresholds and exemptions are amended from time to time, and because voluntary registration is sometimes advantageous for claiming input credit, the current limits should be verified against official guidance before deciding.
Because the rate is applied to the base price, but the share is being measured against the larger inclusive total. On a base of 100 the tax is 18 and the total is 118, so the tax is 18/118 of what the customer pays, which is 15.25 percent. The same arithmetic gives 4.76 percent for a 5 percent rate, 10.71 percent for 12 percent, and 21.88 percent for 28 percent. Keeping this distinction clear prevents systematic over-extraction when working backwards from till receipts.