GST Calculator

Enter the taxable value and the slab, then say whether the supply crosses a state boundary. The amount collected is identical either way; what changes is how it is split, and the split is what the return has to match.

Rules differ by country. This tool performs the calculation you ask for. What you are entitled to charge, and how it must be presented, is set by your contract and your local law — check both before relying on a figure here.

What this tool does

Under India’s GST the slab is only half the answer. A supply within one state is levied as two equal components — a central share and a state share — while a supply between states carries a single integrated levy at the whole slab. The customer pays the same figure in both cases. The money goes to different places, and a return reporting the wrong component has reported to the wrong authority rather than reported the wrong amount.

Which case applies is decided by the place of supply, not by where your office sits or where the document was raised. This tool performs the split and the arithmetic around it; it does not classify your supply or choose your slab, both of which follow from what is being sold and to whom. Countries that levy a single-rate GST — Australia, New Zealand, Canada, Singapore — need no split at all, and the inter-state setting gives them the one-line result they use.

How it works

  1. Enter the taxable value

    The value of the supply before the levy. If the figure you were given already includes it — an "all inclusive" number agreed with a client — switch the basis and the taxable value is derived from it instead of being assumed.

  2. Choose the slab

    Slabs are set nationally and a supply falls into one by classification, not by preference. The usual ones are one tap away; anything else can be typed in directly.

  3. Say where the supply lands

    Within one state, the slab halves: 18% becomes 9% central plus 9% state, each shown as its own line because that is how the invoice must carry them. Across state lines it stays whole, as a single integrated line at 18%.

  4. Odd paise land somewhere deliberate

    Half of an odd number of paise cannot be split evenly. The two halves are allocated rather than rounded separately, so they always add back to the total exactly — a detail that is invisible on one document and a reconciliation problem across a thousand.

Worked example

A design studio in Karnataka bills ₹50,000 to a client in the same state at the 18% slab.

Inputs

Taxable value
₹50,000.00
Slab
18%
Supply
Within the state

Result

Central share, 9%
₹4,500.00
State share, 9%
₹4,500.00
Total levied
₹9,000.00
Invoice total
₹59,000.00

Why it matters: Bill the same ₹50,000 to a client in Maharashtra and the invoice total is still ₹59,000 — but it is one integrated line of ₹9,000 rather than two lines of ₹4,500. The customer notices nothing. The two returns look nothing alike, and only one of them is filed correctly.

Best practices

  • Put each component on its own line with its own rate. A single blended line is not enough for a customer claiming input credit, nor for the return that follows it.
  • Record the place of supply on the document itself. It is what determines the split, and it is the first thing asked about when an input credit is queried.
  • Carry both parties’ registration numbers on business-to-business documents. The customer’s claim depends on the invoice showing them, and a missing number is a rejection rather than a correction.
  • Classify a service once, write the reasoning down, and reuse it. Drifting between slabs across invoices for the same work turns a small classification question into a year of amendments.
  • Check the slab against the current schedule rather than memory. Rates are revised on their own timetable and "it was right last year" is not an answer to an assessment.

Common mistakes

Charging the two-component split on an inter-state supply

The amount collected is correct and the filing is not. Fixing it is not a matter of reissuing the document — tax has been reported to an authority that was not owed it, and unwinding that touches both sides of the transaction and both returns.

Deciding the split from your own address

It follows the place of supply, which is a defined thing and not a synonym for where you are based. A Bengaluru supplier billing a Bengaluru-registered client for delivery in Chennai has not automatically made an intra-state supply, and the letterhead is no help in working out which it was.

Treating an all-inclusive quote as the taxable value

If a client agreed ₹59,000 inclusive at 18%, the taxable value is ₹50,000 and the levy is ₹9,000. Entering ₹59,000 as the taxable value bills ₹69,620 and overstates the tax by ₹1,620 — and because the document is internally consistent, nothing flags it.

Halving an odd figure by hand

Splitting ₹9,000.01 in two by eye produces two numbers that do not add back to it. On one invoice that is a rounding curiosity. Across a quarter of invoices it is a set of component totals that refuse to agree with the tax actually charged.

Frequently asked questions

What is the difference between CGST, SGST and IGST?

CGST and SGST are the central and state halves of a supply made within a single state — an 18% slab becomes 9% and 9%. IGST is the single integrated levy on a supply that crosses a state boundary, charged at the full 18%. The customer pays the same total under either; the difference is which government is owed it and which boxes the return fills.

How do I calculate GST on an inclusive price?

Divide by 1 plus the slab. ₹1,180 at 18% contains ₹180 of tax on a taxable value of ₹1,000. Multiplying the inclusive figure by 18% instead gives ₹212.40 — an overstatement of ₹32.40 that repeats on every document handled the same way.

Which slab applies to my service?

Classification decides it, and classification depends on what the service actually is rather than what it is called on the invoice. Check the current schedule for your category, and where two readings are arguable, get the position confirmed before the first invoice rather than after the hundredth.

Does the registration number have to be on the invoice?

For business-to-business supplies the recipient’s claim to input credit generally depends on the document carrying both registration numbers along with the place of supply. Requirements for what else must appear are set out in the rules, and they are the difference between a document that supports a claim and one that does not.

Can I use this for GST outside India?

Yes, with the inter-state setting. Australia, New Zealand, Canada and Singapore levy a single-rate GST with no central and state division, so the one-line result is the whole answer. Only India needs the split, which is why it is a choice here rather than an assumption.

Is the tax worked out before or after a discount?

On the value after any discount shown on the document and agreed at or before the time of supply. A reduction given later — a settlement gesture, a goodwill credit — is handled through a credit note rather than by quietly recalculating the tax on the original document.

Turn the answer into an invoice

The result above transfers straight into the invoice generator — dates, amounts and terms already filled in. Free, no account needed.

Open the invoice generator

Sources & further reading

Related

Glossary

Invoice types

Published · General information, not legal, tax or financial advice.