Mixing up these two directions is the single most common invoicing error. For exclusive pricing, tax sits on top of the base: GST = Base × Rate ÷ 100, Total = Base + GST. For inclusive pricing, the tax is already folded into the sticker price, so the calculation runs backward: Base = Total × 100 ÷ (100 + Rate), GST = Total − Base. Applying the rate straight to an inclusive total instead of backing out the base first overstates the tax and understates the base — a mistake that shows up constantly on hand-written invoices.

India taxes different goods and services at different slabs, so the correct rate comes from how the specific item is classified, not from how large or small the transaction is. Broadly:

Slabs are notified and revised periodically by the GST Council, and individual items do move between them — treat this table as orientation, not a classification ruling. What actually fixes the rate is the HSN code (for goods) or SAC code (for services) attached to the supply, not a seller's own sense of what "feels fair." On a new product line, look up the notified HSN classification or ask a GST practitioner rather than copying a similar-looking item's rate.

Holding the base at ₹10,000 and only changing the rate shows how much classification alone moves the tax:

• At 5%: GST ≈ ₹500, total ≈ ₹10,500.
• At 18%: GST ≈ ₹1,800, total ≈ ₹11,800.
• At 28%: GST ≈ ₹2,800, total ≈ ₹12,800, sometimes with a compensation cess layered on for specific luxury or "sin" goods.

The gap between the 5% and 28% totals on an identical base — about ₹2,300 here — is entirely down to which slab the item falls into. Getting the classification right matters more to the final number than the base amount does.

How the same rate plays out on nearby amounts, and how your amount plays out at nearby rates:

The mirror-image question comes up constantly: "I paid ₹11,200 — what was the base and how much was tax?" That's the inclusive calculation from above, run through with your numbers: Base = ₹11,200 × 100 ÷ (100 + 12) ≈ ₹10,000, and GST = ₹11,200 − ₹10,000 ≈ ₹1,200.

This matters most for expense reports, ITC claims, or just checking a vendor charged the rate they say they did, when all you're handed is a final figure. A quick sanity check afterward: multiplying the derived base by the rate should reproduce the same GST shown here, and adding that GST back to the base should reproduce your original total. If it doesn't, the rate you assumed probably isn't the one actually applied on the bill.

A registered business that pays GST on its purchases can typically credit that amount against the GST it collects on its own sales, paying the government only the difference — the mechanism that keeps GST from cascading at every stage of a supply chain the way the older tax system did.

This calculator doesn't evaluate eligibility, and eligibility is where most of the real friction lives: the supplier has to have actually deposited the tax and filed correctly, you need a valid tax invoice, and some categories — specified personal-consumption expenses among them — are blocked from ITC regardless of paperwork. Paying GST is not the same as being guaranteed the credit back.

Most GST-registered businesses reconcile their purchase records against auto-populated data from suppliers' filed returns before claiming ITC, because a mismatch — supplier didn't file, or filed a different figure — can mean the credit gets reversed later with interest. Worth building as a habit even with a small, stable list of suppliers.

Registration is generally mandatory once a business's aggregate turnover crosses the notified threshold in a financial year — thresholds differ for goods versus services and for special-category states — and is compulsory regardless of turnover for inter-state suppliers, e-commerce operators and sellers, and anyone required to pay tax under reverse charge. Voluntary registration below the threshold is also allowed, and can be worth it if your customers need ITC on your invoices.

Small businesses below a specified turnover threshold have a simpler alternative: the composition scheme, which charges a flat, lower rate on turnover instead of the standard slabs, with quarterly instead of monthly filing. In exchange, composition dealers generally can't charge GST separately on invoices, can't claim ITC, and can't make inter-state outward supplies in most cases. This calculator models standard-scheme exclusive/inclusive pricing — check current thresholds and eligibility before assuming composition applies to you.

Once registered under either scheme, returns are due on a fixed schedule whether or not there was any sale that period, and a late filing generally attracts a fee and interest even when the actual tax owed is small.

• Shopkeepers confirming a shelf price against the intended margin before printing a tag.
• Freelancers and consultants above the registration threshold adding tax correctly to a service invoice.
• E-commerce sellers reconciling a marketplace settlement report against the listed price.
• Procurement teams checking a vendor's invoice before approving payment.
• Students practising GST arithmetic for a commerce or accountancy syllabus.

• Applying the rate straight to an inclusive total instead of backing out the base first — overstates both the tax shown and the amount remitted.
• Quoting a price without saying inclusive or exclusive, which reliably causes a dispute at billing time.
• Using the wrong slab because the product or service was misclassified under its HSN/SAC code.
• Assuming ITC is automatic just because GST was paid, when it actually depends on the supplier's filing and your documentation.
• Missing a filing deadline, which attracts a late fee and interest even when the eventual liability is small.
• Charging GST on the pre-discount value when a trade discount is clearly disclosed on the invoice — tax should apply to the discounted price.
• Treating this page's output as filing-ready — it's arithmetic on the amount and rate you gave it, not a substitute for return-preparation software or an accountant's sign-off.

A trade discount shown as a line-item reduction on the invoice itself lowers the taxable value before GST is applied, so tax is charged on the discounted price, not the original list price. A discount given after the invoice is issued — a post-sale credit note — needs its own documentation to reduce liability correctly. Free samples and gifts usually carry no outward GST, but they can restrict the ITC already claimed on the inputs used to make them, a nuance small businesses miss often enough that it's worth flagging here.

Larger businesses above specified turnover thresholds also generate e-invoices through the government's invoice registration portal, and e-way bills for goods moving above a specified value. Neither changes the underlying base/GST/total relationship this calculator works through — they're compliance steps layered on top of it once a business crosses the relevant threshold.

Before GST, a good moving across state lines could be taxed at several separate stages — excise duty, service tax, VAT, central sales tax, octroi among them — often without full credit for tax already paid earlier in the chain. That's "tax cascading": tax charged on top of already-taxed value, and it made goods more expensive to move than the nominal rates alone suggested.

GST folded most of those levies into one structure, split into central and state components for revenue-sharing behind the scenes, with input tax credit designed to flow through the supply chain instead of stacking at each stage. The base/GST/total split this calculator shows reflects that unified structure — one rate, one tax amount, rather than several taxes layered on top of each other.