GST Calculator
Invoicing goes wrong in one of two directions: adding tax to a price that already includes it, or 'removing' tax by subtracting a percentage. This calculator handles both directions cleanly so the base, the tax and the total reconcile on the invoice.
Inputs
Result
11,800
GST 1,800 on base 10,000
How it works
GST = amount × rate ÷ 100
- 1GST = 10,000 × 18% = 1,800
How the gst calculator works
Adding GST to an exclusive price: Tax = Base × rate; Total = Base × (1 + rate).
Removing GST from an inclusive price: Base = Total ÷ (1 + rate); Tax = Total − Base. Division, not subtraction — this is the whole game.
Where a domestic supply splits the levy, the same total is divided into halves (for example CGST and SGST at 9% each on an 18% rate).
Worked example: an 11,800 inclusive invoice at 18%
- Base = 11,800 ÷ 1.18 = 10,000.
- Tax = 11,800 − 10,000 = 1,800, which splits into 900 + 900 for an intra-state supply.
- The wrong method: 18% of 11,800 is 2,124, implying a base of 9,676 — an error of 324 on a single line item.
- Going the other way, adding 18% to a 10,000 base gives 1,800 tax and an 11,800 total, which closes the loop.
Common mistakes to avoid
Subtracting the rate from an inclusive price
Percentages are not reversible by subtraction. Dividing by 1 + rate is the only way to recover the base, and the error grows with the rate.
Applying the rate to a discounted line incorrectly
Tax is charged on the value after trade discount but generally before financial or cash discounts. Getting the order wrong changes the taxable value.
Rounding each line then totalling
Rounding tax per line and again on the invoice creates mismatches with the filed return. Fix one convention and apply it consistently.
Frequently asked questions
How do I remove GST from a total?
Divide the inclusive amount by 1 plus the rate as a decimal. For 18%, divide by 1.18; for 5%, divide by 1.05.
What is the difference between inclusive and exclusive pricing?
An exclusive price is the base before tax; an inclusive price already contains it. Retail prices are usually inclusive, B2B quotes usually exclusive.
How does an 18% rate split into CGST and SGST?
Equally, at 9% each, for supplies within a state. Inter-state supplies carry a single IGST charge at the full rate instead.
Is GST calculated before or after a discount?
After discounts that appear on the face of the invoice. Post-sale discounts are handled separately and usually require a credit note.
Does the same maths apply to VAT and sales tax?
For adding and removing tax, yes — only the rate and the compliance rules differ. The sales tax calculator uses identical arithmetic.
Learn more
GST Explained: Adding vs Removing Tax
Inclusive and exclusive pricing, and the mistake most people make.
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