GST Calculator Guide: CGST vs SGST vs IGST Explained

GST basics: CGST, SGST, and IGST

GST (Goods and Services Tax) sounds like one tax, but on any given invoice it's actually collected as one of two combinations. Which combination applies depends entirely on one thing: whether the sale happens within a state or across state lines. Get that one variable right and the rest of the invoice follows automatically.

• CGST + SGST (intra-state). When the sale is within the same state, the total GST rate splits evenly between Central GST and State GST. An 18% sale becomes 9% CGST and 9% SGST. Both amounts appear as separate line items on the invoice, not combined into one number.
• IGST (inter-state). When the sale crosses state lines, the full rate goes to Integrated GST instead. An 18% sale is 18% IGST, full stop, no split. The center collects it in one piece and apportions the destination state's share to it later, behind the scenes, without the seller or buyer needing to do anything extra.

Why bother splitting it into three names for what is functionally "the tax" from the buyer's point of view? Because GST in India is a dual tax, administered jointly by the central government and the state governments. Every rupee of tax revenue has to be attributed to one or the other, and the CGST/SGST/IGST labels are how that attribution happens at the point of sale, rather than through a separate reconciliation process later.

This isn't a cosmetic difference. Charging IGST on a sale that was actually within the same state, or the reverse, charging CGST+SGST on a sale that crossed state lines, is a compliance error. It shows up in GST returns, complicates input tax credit for the buyer (they may not be able to claim the credit correctly if the wrong tax type was charged), and usually means a correction has to be filed. It's a small mistake to make and a mildly annoying one to fix, which is exactly why it's worth getting right the first time on every invoice rather than catching it during a monthly reconciliation.

A related point worth flagging: neither CGST/SGST nor IGST changes how much tax the buyer ultimately pays. An 18% sale costs the same 18% in GST whether it's split as 9%+9% or charged as a single 18% IGST line. The split only determines how the tax is recorded and which government it's attributed to, not the total burden on the transaction.

How to tell which one applies

The rule is simpler than it looks: compare the supplier's state to the place of supply, which for most goods and services is the buyer's billing or shipping state as it appears on the invoice.

• Same state on both sides → intra-state → CGST + SGST
• Different states → inter-state → IGST

A business registered in Maharashtra selling to a customer also billed in Maharashtra charges CGST+SGST. The same business selling to a customer billed in Karnataka charges IGST instead, even if the goods physically ship from a warehouse in Maharashtra. The state that matters for this rule is the place of supply, not the warehouse or dispatch location.

A few situations trip people up more than the basic rule does:

• Multiple business locations, one buyer. If your business is registered in more than one state (common for larger companies), the relevant "supplier state" is the specific registered location that's actually making the sale, not your company's head office by default.
• Services delivered remotely. For a lot of services, especially digital ones, the place of supply is still the buyer's billing address even though nothing physically crosses a state border. A freelancer in Delhi invoicing a client billed in Punjab charges IGST, even if all the work happened over a video call.
• E-commerce and marketplace sales. Sellers on marketplaces often ship to buyers across many states in a single day. Each order needs its own CGST/SGST-vs-IGST determination based on that specific buyer's billing state, which is exactly the kind of repetitive check that benefits from automation rather than manual lookup per order.

The good news is that once you know the rule, applying it is fast: look at two state fields on the invoice, compare them, and you have your answer. The part that actually takes time is making sure your invoicing tool or process asks for the right state field in the first place, since an invoice built off an incomplete address won't let you apply the rule correctly.

Calculating GST-inclusive vs GST-exclusive amounts

Once you know which GST applies, the math itself is straightforward. Here's a worked example for a product priced at ₹1,000, exclusive of 18% GST:

• GST amount: ₹1,000 × 18% = ₹180
• Total payable: ₹1,000 + ₹180 = ₹1,180
• If intra-state: split the ₹180 into ₹90 CGST + ₹90 SGST
• If inter-state: the full ₹180 is IGST

The reverse calculation comes up just as often: you're handed a sticker price of ₹1,180 that already includes 18% GST, and you need to know how much of that is the base price versus tax.

• Base price: ₹1,180 ÷ 1.18 = ₹1,000
• GST already included: ₹1,180 − ₹1,000 = ₹180

The common mistake here is subtracting the percentage directly from the inclusive price, for example calculating 18% of ₹1,180 and subtracting it. That gives the wrong base price. You have to divide by 1 + rate, not multiply and subtract.

To see why the direct-subtraction shortcut fails, run the numbers: 18% of ₹1,180 is ₹212.40, not ₹180. Subtract that and you'd get a base price of ₹967.60, which is wrong, because the 18% was never calculated on ₹1,180 in the first place. It was calculated on the smaller base price of ₹1,000. Reversing a percentage-on-top calculation always means dividing by (1 + rate), never multiplying the final total by the rate.

This matters most when you're handed a GST-inclusive number and asked to report the tax component separately, which happens constantly in retail pricing, marketplace payouts, and expense reports where a vendor's invoice only shows one final figure. Getting the inclusive-to-exclusive split right the first time avoids a round of back-and-forth with an accountant later.

Common GST rate slabs

India's GST structure groups most goods and services into a handful of standard slabs: 5%, 12%, 18%, and 28%, with a few categories outside this range, such as 0% for certain essentials and higher effective rates (base rate plus cess) on items like luxury cars, aerated drinks, and tobacco.

• 5% generally covers everyday essentials and select mass-consumption goods.
• 12% sits in the middle, covering a mix of processed foods and certain goods and services.
• 18% is the most common slab, applied to the majority of services and a large share of manufactured goods. Most software, SaaS, and professional services fall here.
• 28% is reserved for luxury and "sin" categories, often with an additional cess stacked on top.

In practice, picking the wrong rate for an item is a more common invoicing error than getting the CGST/SGST split wrong. The split is a simple same-state-or-not rule that rarely changes; the rate depends on the specific HSN code (for goods) or SAC code (for services) tied to what you're selling, and those codes and their associated rates do get revised or reclassified from time to time by the GST Council. A business that copies last year's rate onto this year's invoice without checking can end up under-charging or over-charging without realizing it until a return doesn't reconcile.

The practical takeaway: treat the rate as the variable that needs periodic verification, and treat the CGST/SGST-vs-IGST split as the variable that just needs the two states compared correctly on every single invoice. Different failure modes, different fixes.

Doing this without a spreadsheet

None of this math is hard on its own. But doing it by hand for every invoice line, remembering whether to multiply or divide, whether to split the result or not, and which rate applies to which item, is exactly where small errors creep in when you're moving fast. A lot of small businesses end up building a personal spreadsheet with a formula for this, and then spend time re-deriving that formula every time it breaks or someone edits the wrong cell.

A dedicated calculator removes that risk entirely, because the logic only has to be correct once, and every calculation after that just reuses it.

Our own free GST Calculator adds or removes GST from any amount and shows the CGST/SGST/IGST split automatically. Enter the amount, pick the rate (5%, 12%, 18%, 28%, or a custom rate), and choose whether it's intra-state or inter-state. It handles both directions, GST-exclusive to inclusive and inclusive back down to the base price, so you don't have to remember which formula applies to which direction. It's free, works instantly in your browser, and needs no sign-up.

It's worth repeating that a calculator like this is a convenience tool for arithmetic, not a replacement for understanding the underlying rule about intra-state versus inter-state sales. You still need to know which state the buyer is in before you can tell the calculator which split to show. What it removes is the risk of a typo or a wrong formula turning a correct decision into an incorrect invoice.