GST arithmetic: from base price to tax-inclusive total
Work through forward tax, reverse tax and equal splits with transparent numerical examples.
Open GST SplitterThe first decision in a tax calculation is not which button to press. It is whether the amount you have already includes tax. This walkthrough explains that distinction with simple arithmetic. The rates are illustrative inputs, not a statement of which rate applies to a product, service or transaction.
Start with the meaning of the amount
Call the amount before tax B and the rate as a percentage r. Forward calculation begins with B and adds tax. Reverse calculation begins with a total T that already contains tax and extracts the underlying base. These directions need different formulas because the percentage is applied to the base, not to the final total.
A useful habit is to label the amount before entering it: “before tax” or “including tax.” If an invoice contains several items with different applicable rates, calculate the relevant lines separately. One blended amount and one selected rate do not describe a mixed-rate invoice accurately.
Add tax to a known base
For a base of 1,000 and an illustrative rate of 18%, tax equals 1,000 × 18 ÷ 100, which is 180. The final total is 1,180. In general, tax = B × r ÷ 100 and total = B × (1 + r ÷ 100). At a zero rate, the total stays equal to the base.
Changing the rate changes the calculation, not the meaning of the input. A base of 1,000 entered in the forward direction remains a base even if the rate changes. Recheck the direction whenever you paste a different amount; an inclusive price entered as a base would produce an inflated result.
Recover the base from an inclusive total
For an inclusive total of 1,180 at the same illustrative 18% rate, divide 1,180 by 1.18 to recover the base of 1,000. The included tax is the difference, 180. The general formulas are B = T ÷ (1 + r ÷ 100) and tax = T − B.
Subtracting 18% of 1,180 is a different operation. That would subtract 212.40, leaving 967.60, which is not the original base. The error comes from applying the percentage to the total rather than to the amount on which the tax was originally calculated. Reverse division corrects that mismatch.
Understand what the displayed split means
The tool displays an equal arithmetic division of the tax into CGST and SGST amounts. In the example, 180 divides into 90 and 90. This presentation is useful for checking an equal split, but it does not determine whether that tax treatment applies to a particular supply. It also does not decide registration obligations, classification or eligibility for a credit.
Currency display rounds to two decimal places. With awkward amounts, independently rounded components may differ by a smallest currency unit from another system’s displayed sum. Keep the underlying formula in mind and reconcile against the actual invoice requirements. Use this tool to inspect arithmetic; obtain the applicable rate and treatment from an appropriate authoritative source for your transaction.
Put it into practice
- Read the invoice label and identify whether tax is already included.
- Enter the amount and the rate that applies to your own calculation.
- Choose Add GST for a base or Remove GST for an inclusive total.
- Compare base, tax, split and total against the original amount.
Frequently asked questions
Can the calculator choose a tax rate for me?
No. The selected rate is an input. The tool does not classify goods or services or determine the rules for a transaction.
Why is reverse tax not simple percentage subtraction?
The tax was applied to the base. Dividing the inclusive total by its multiplier recovers that base before calculating the difference.
Is the split a complete invoice calculation?
No. Discounts, mixed rates, rounding conventions and other invoice details may require additional treatment outside this calculator.