The quick answer
A tax-exclusive price is the base to which applicable GST is added. A tax-inclusive price already contains GST, so the tax must be extracted rather than added again. Confirm the price basis before entering an item rate.
The formulas below use a hypothetical 18% rate for a valid taxable supply. Actual item rates, discounts, cess and special treatments require their own checks. This arithmetic guide does not determine the legal rate.
Before you begin
- The agreed price basis: inclusive or exclusive.
- The verified applicable rate and correct taxable-value treatment.
- Consistent quantities, units and rounding rules.
See how the pieces connect
Price basis
Decide whether the quoted amount already includes tax.
Base value
Use the exclusive base or extract it from the inclusive total.
Tax
Calculate the tax on that base.
Check
Confirm base plus tax equals the customer amount.
Work through the steps
Confirm what the price means
Ask whether the supplier or customer quote is inclusive or exclusive. Label the item’s price basis clearly. Assuming the wrong basis can overcharge the buyer or reduce the intended margin.
Use the verified rate
Establish the actual rate for the supply from current sources. Use a decimal rate in calculations: a hypothetical 18% is 0.18. Do not copy a sample rate into the item master.
Add tax to an exclusive value
For an exclusive base B and decimal rate r, total is B × (1 + r) and tax is B × r. Calculate on the correct taxable value after applicable adjustments.
Extract tax from an inclusive total
For inclusive total T, base is T ÷ (1 + r) and tax is T − base. Subtracting 18% of the inclusive total is not the same calculation and gives an incorrect base.
Split components where applicable
After determining supply treatment, split the calculated tax into the applicable components. Intrastate versus interstate treatment is a legal supply question, not a rounding preference.
Review line and document rounding
Use sufficient precision for calculations, then the appropriate amount display and rounding. Check multi-line totals and discount treatment. Record the final invoice amount and actual payment separately.
A worked example
The same 18% example from two price bases
Hypothetical rate, no cess or discount. The numbers illustrate arithmetic only.
| Price basis | Taxable value | GST | Total |
|---|---|---|---|
| ₹1,000 exclusive | ₹1,000 | ₹180 | ₹1,180 |
| ₹1,180 inclusive | ₹1,000 | ₹180 | ₹1,180 |
| Incorrect: subtract 18% of ₹1,180 | ₹967.60 | ₹212.40 | ₹1,180 |
Tax is a percentage of the taxable base. Extracting from an inclusive amount requires division by 1 plus the rate, not subtraction of that percentage from the total.
Common mistakes to avoid
- Adding GST again to an already inclusive price.
- Extracting tax by subtracting the rate percentage from the inclusive amount.
- Using the sample’s 18% for every item.
- Rounding repeatedly before combining the lines.
Questions you might have
Can an inclusive item price be stored in billing software?
Use the supported price-basis option and check a simple invoice independently. Confirm the displayed base and tax reconcile with the intended total.
Does a lower base after extraction mean the shop made a loss?
No. Profit needs a separate comparison of sale revenue and relevant cost. GST collected is not automatically business income.
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Sources and further reading
Official rules and portal screens can change. Use the linked authority for the current requirements; the figures in our worked examples are illustrative.