GST calculator
Add or remove Goods and Services Tax at any rate, with the net, GST and total shown separately and an even CGST/SGST split for Indian invoices.
Single-rate GST. Mixed-rate baskets and state-specific rules are not modelled.
How this calculator works
GST is a multi-stage consumption tax that works like VAT: each business in the supply chain charges GST on its sale and claims credit for the GST it paid on its purchases, so the tax ultimately lands on the final consumer once. It is used in India, Australia, Canada, New Zealand, Singapore and many other countries, with very different rates and rules: 18% is a common Indian slab, Australia's rate is 10%, and Canada's federal rate is 5% before provincial taxes.
In India, most intra-state sales split the GST evenly between central and state governments as CGST and SGST, while inter-state sales attract a single IGST — which is why invoices show the two halves. The half-split output here shows that even division; it is not a separate tax. This calculator uses one rate for the whole amount; real invoices can mix rates when a basket contains items from different slabs.
The formula
If the amount is net: GST = net × rate ÷ 100. If the amount is GST-inclusive, the net is gross ÷ (1 + rate ÷ 100) and the tax is the difference:
gst = net · r
total = net · (1 + r)
net = total / (1 + r)
CGST = SGST = gst / 2For a 18% GST, the tax fraction of a gross price is 18/118 ≈ 15.25%.
Worked example
A 1,000 amount before GST at 18% attracts $180.00 of GST, for a total of $1,180.00. On an Indian invoice that would typically be shown as $90.00 CGST and $90.00 SGST.
Common mistakes
- Dividing the gross by 1 + rate as a decimal wrong way. At 18%, divide by 1.18 — not by 0.18 and not by 1.018.
- Applying GST to tax-exempt items. Many countries exempt or zero-rate basic food, healthcare and education. Use the rate that applies to each line item.
- Mixing CGST/SGST and IGST. Intra-state sales split into CGST and SGST; inter-state sales use IGST. Do not charge both on the same invoice.
Frequently asked questions
How is GST calculated?
GST is a percentage of the taxable value: net amount × rate ÷ 100. For a GST-inclusive figure, divide by 1 + (rate ÷ 100) to get the net value, and subtract for the tax.
What is the difference between GST and VAT?
Economically they are the same kind of multi-stage consumption tax. The name differs by country, and the rules for registration, rates and credits differ.
What is CGST and SGST?
In India, intra-state GST is split evenly between the central government (CGST) and the state government (SGST). Inter-state transactions use IGST instead.
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