GST Calculation — Inclusive vs Exclusive Price
Adding GST to a price is easy. Taking GST out of a price that already includes it is where almost everyone goes wrong — including plenty of adults filling in invoices. The mistake is always the same: subtracting the tax percentage from the inclusive amount instead of dividing by it. This is the reverse-percentage problem from school maths, wearing a tax label, and it is worth getting right for both the exam and the shop counter.
All rates in this guide are illustrative. GST rates depend on the goods or service and are revised from time to time, so check the rate notified by the Central Board of Indirect Taxes and Customs for the item you are actually billing.
The two formulas
GST amount = Base price × rate ÷ 100 · Total = Base price × (1 + rate ÷ 100)
Base price = Total × 100 ÷ (100 + rate) · GST amount = Total × rate ÷ (100 + rate)
What the terms mean
- Exclusive price (also called the taxable value or base price) — the price before tax. This is the number GST is charged on.
- Inclusive price — what the customer actually pays. Retail MRP labels are inclusive by law; B2B quotations are usually exclusive, and say so.
- CGST and SGST — for a sale within one state, the total GST is split in half between the central and state governments. An 18% total appears on the invoice as 9% CGST + 9% SGST.
- IGST — for a sale across state borders, the whole rate is charged as a single integrated tax. The customer pays exactly the same total either way.
Worked example 1 — adding GST
A shop quotes a keyboard at ₹2,000 plus GST at 18%.
GST = 2000 × 18 ÷ 100 = ₹360
Total payable = 2000 + 360 = ₹2,360
On the invoice, for a sale within the state:
CGST @ 9% = ₹180
SGST @ 9% = ₹180
Total tax = ₹360, exactly as before. The split is an accounting matter, not extra tax.
Worked example 2 — removing GST (the one people get wrong)
A bill shows ₹2,360 inclusive of 18% GST. What was the base price, and how much tax was charged?
Correct method — divide.
Base = 2360 × 100 ÷ 118 = 236 000 ÷ 118 = ₹2,000
GST = 2360 − 2000 = ₹360
Check by the direct formula: GST = 2360 × 18 ÷ 118 = 42 480 ÷ 118 = ₹360 ✓
The wrong method — subtracting 18%.
2360 × 0.82 = ₹1,935.20, which is ₹64.80 short of the true base price.
Why the two differ: 18% of the base (₹2,000) is ₹360, but 18% of the total (₹2,360) is ₹424.80. The tax was calculated on the smaller number, so you cannot remove it using the larger one. In percentage language, the base is 100 parts and the total is 118 parts — so the tax is 18/118 of the total, which is about 15.25%, not 18%.
Worked example 3 — an awkward number at 5%
A packaged item sells for ₹999 inclusive of 5% GST.
Base = 999 × 100 ÷ 105 = 99 900 ÷ 105 = ₹951.43 (951.4286 rounded)
GST = 999 − 951.43 = ₹47.57
Check: 951.43 × 1.05 = 999.0015 ≈ ₹999 ✓ (the 0.15 paise gap is only the rounding of the base price to two decimals)
The general rule worth memorising
| GST rate | Divide inclusive price by | Tax as % of inclusive price |
|---|---|---|
| 5% | 1.05 | 4.76% |
| 12% | 1.12 | 10.71% |
| 18% | 1.18 | 15.25% |
| 28% | 1.28 | 21.88% |
Read the middle column as the whole method: to strip tax out, divide. To put tax in, multiply. Everything else is arithmetic.
Worked example 4 — discount and GST together
Order matters here, and the law is clear: GST is charged on the price after a discount that is shown on the invoice.
Listed price ₹2,000, 10% discount, then 18% GST:
Discounted price = 2000 × 0.90 = ₹1,800
GST = 1800 × 0.18 = ₹324
Total = ₹2,124
Had the shop charged GST first and then discounted, the customer would pay 2360 × 0.90 = ₹2,124 as well — the two multiplications commute. What is not the same is the tax shown: ₹324 in the first case and ₹360 in the second, and only the first is correct, because tax is due on the actual transaction value.
Worked example 5 — finding the rate from a bill
Sometimes the invoice gives you the tax and the total but not the rate, and you want to check it against the rate that should apply.
A bill shows a total of ₹2,360 of which ₹360 is GST.
Base = 2360 − 360 = ₹2,000
Rate = 360 ÷ 2000 × 100 = 18%
Do not divide by the total. 360 ÷ 2360 × 100 = 15.25%, which is the tax as a share of the inclusive price — a genuine quantity, but not the GST rate. This is the same base-choosing error as example 2, arriving from the other direction.
Rounding on a real invoice
Two habits keep an invoice internally consistent, and both matter if anyone later checks your arithmetic:
- Compute the tax on each line, then add — rather than computing tax on the grand total — if the lines carry different rates. Items are taxed at their own rates, and a single blended calculation cannot reproduce the correct split.
- Round only the printed figures. Carry the unrounded base and tax through the working, exactly as in example 3, where the base was 951.4286 before being shown as ₹951.43. Rounding first and then multiplying is what leaves an invoice a paisa short.
For an intra-state supply the CGST and SGST halves are each rounded in their own right, so an odd tax amount can split into two halves that differ by one paisa. That is normal and is not an error in your arithmetic.
Common mistakes
- Subtracting the rate from an inclusive price. The most expensive error on this page. Divide by (100 + rate) ÷ 100.
- Adding CGST and SGST on top of the full rate. 9% + 9% is the 18%; it is not 18% plus another 18%.
- Charging both IGST and CGST/SGST. A transaction is either intra-state or inter-state, never both.
- Assuming a single rate for everything. Different goods and services carry different rates; look up the rate for the item.
- Rounding at the wrong step. Compute the base to full precision, round the final rupee figures. Rounding the base first can leave the invoice a paisa out of balance.
- Treating MRP as exclusive. Maximum retail price already includes all taxes, so adding GST to an MRP double-counts it.
Where this maths appears
| Context | Use |
|---|---|
| School maths (Class 7–10) | Percentage, reverse percentage, profit and loss, sales tax word problems |
| Commerce and accountancy | Invoice preparation, input tax credit, taxable value |
| Aptitude tests | Successive percentage change; "find the original price" questions |
| Everyday life | Checking a restaurant or repair bill; small-business billing |
Check an invoice in seconds. The Finance and Everyday section of the calculator suite has a GST tool that works both ways — add tax to a base price, or strip it out of an inclusive total — alongside discount, EMI and income tax calculators.
Open the ABC Chemistry Calculator Suite →Reverse percentages trip up far more Class 9–12 students than they should. ABC Chemistry runs Class 11–12 chemistry coaching at its Gurugram centre plus online classes across India — abcchemistry.in.