How to Calculate GST in India (Both Directions, Correctly)

GST arithmetic trips people up in one direction far more than the other, and the direction that goes wrong is the one small businesses need daily.

Adding GST: the easy direction

You have a base amount and need the total.

GST amount = base × rate
Total      = base + GST amount

At 18% on ₹1,000: GST is ₹180, total is ₹1,180. Nothing surprising.

Removing GST: where it goes wrong

You have a tax-inclusive total and need to know how much of it was tax. The intuitive move is to subtract 18% from the total. That is wrong, and it is wrong in a way that looks plausible.

₹1,180 minus 18% gives ₹967.60. But we know the base was ₹1,000. The error is ₹32.40, and it grows with the amount.

The reason is that the 18% was calculated on the base, not on the total. Working backwards needs the reverse formula:

Base = inclusive total ÷ (1 + rate)
GST  = inclusive total − base

₹1,180 ÷ 1.18 = ₹1,000. Correct.

The GST Calculator does both directions, so you can pick "remove GST" and enter the inclusive figure rather than remembering which formula applies.

The rates

The main slabs are 5%, 12%, 18% and 28%, and which one applies depends on the goods or service. Essentials sit at the lower end, most services at 18%, and luxury and sin goods at 28%. Some items are exempt or zero-rated entirely.

If you are unsure which slab applies to what you sell, that is a question for your accountant or the official GST rate finder — not something any calculator can decide for you.

CGST, SGST and IGST

This is the part that confuses people reading their first invoice.

For a supply within a single state, GST splits evenly into two halves:

• CGST — the central government's share
• SGST — the state government's share

So 18% appears on the invoice as 9% CGST + 9% SGST. The customer pays 18% either way; the split is about who receives it.

For a supply across state lines, it is instead charged as a single IGST at the full rate — 18%, not split.

The total is identical. What changes is which boxes on the invoice are filled in, and getting that wrong is a common cause of input credit being rejected.

A worked example

You sell a service for ₹50,000 plus GST at 18%, to a customer in your own state:

• GST: ₹50,000 × 0.18 = ₹9,000
• CGST: ₹4,500
• SGST: ₹4,500
• Invoice total: ₹59,000

If the same customer were in another state, the invoice would show IGST ₹9,000 and the same ₹59,000 total.

One caveat

Calculators produce arithmetic, not advice. Which slab applies, whether a supply is inter-state or intra-state, whether reverse charge applies, and what you can claim as input credit are all questions with real consequences. See our disclaimer, and check anything that matters with a qualified professional.