§ Tools / Series 03 · Personal finance · Tax calculator
GST calculator
A deterministic tax arithmetic calculator — not a time-value-of-money tool like the rest of the suite. Converts between GST-exclusive and GST-inclusive amounts, with the correct CGST/SGST/UTGST/IGST splits and editable, versionedrate presets. Currency-agnostic (USD default, EUR / GBP / INR / JPY / and others one click away) and arithmetically generic enough to handle any "add or extract tax from an amount" workflow.
§ The reverse-GST formula
When extracting GST from an inclusive amount, the correct formula is taxable = invoice / (1 + rate). Most free calculators multiply invoice × rate instead — at 18% that overstates GST by ~18% of itself. This tool does the division and shows the trace so you can verify.
2
Calculation modes
5+
Versioned rate presets
4
Supply-type splits
4
Rounding modes
v1
Methodology
§ Start with a preset
Three starting cases · the inclusive reverse-extract is the one most free GST calculators get wrong.
Inputs
Toggle the mode to flip direction · everything recomputes instantly · no input is silently ignored.
GST to be added
$9,000.00
Taxable value of $50,000.00 + GST at 18% = $59,000.00 total on the invoice. Split as CGST + SGST.
Taxable value$50,000.00
GST amount$9,000.00
Invoice (post-GST)$59,000.00
SupplyCGST + SGST
§ Tax split
How the GST amount maps to invoice line items
| Component | Rate | Amount | Applies when |
|---|---|---|---|
| CGST Central GST | 9.00% | $4,500.00 | Intrastate · Union Territory |
| SGST State GST | 9.00% | $4,500.00 | Intrastate only |
| UTGST Union Territory GST | 0.00% | $0.00 | UT without legislature |
| IGST Integrated GST | 0.00% | $0.00 | Interstate only |
| Total GST | 18% | $9,000.00 | All modes |
§ Rate comparison
What if the classification puts this at a different slab?
| Rate | Taxable | GST | Invoice | vs 18% slab | Source |
|---|---|---|---|---|---|
| 0% | $50,000.00 | $0.00 | $50,000.00 | $-9,000.00 | CBIC · 2026-05-27 |
| 5% | $50,000.00 | $2,500.00 | $52,500.00 | $-6,500.00 | CBIC · 2026-05-27 |
| 12% | $50,000.00 | $6,000.00 | $56,000.00 | $-3,000.00 | CBIC · 2026-05-27 |
| 18%selected | $50,000.00 | $9,000.00 | $59,000.00 | — | CBIC · 2026-05-27 |
| 28% | $50,000.00 | $14,000.00 | $64,000.00 | +$5,000.00 | CBIC · 2026-05-27 |
§ Official sources · verify rate & classification yourself
CBIC GST goods and services rateshttps://cbic-gst.gov.in/gst-goods-services-rates.htmlCBIC GST rates FAQhttps://cbic-gst.gov.in/CGST-bill-e.htmlCBIC GST portalhttps://cbic-gst.gov.in/
Presets versioned with source · retrievedAt · effectiveFrom — refreshed when CBIC publishes a notification, not hardcoded in business logic.
§ Formula trace
- input_amount = 50000 × ones = 50000.00 USD
- gst_rate = 18% = 0.1800
- mode = exclusive (add)
- gst_amount = taxable × rate = 50000.00 × 0.1800 = 9000.0000
- invoice_value = taxable + gst = 59000.0000
- supply_type = intrastate
- cgst = sgst = gst / 2 = 4500.0000 each
- display rounding = half_up @ 2 dp
§ Input audit
amountused
amountUnitused
currencyused
gstRatePctused
modeused
roundingModeused
roundingPrecisionused
supplyTypeused
§ Warnings & out-of-scope
- Verify the GST rate and HSN/SAC classification against official CBIC sources. This calculator does not infer rates from product codes.
- Out of scope: cess, reverse charge, input tax credit (ITC), exemptions, zero-rated supplies, composition scheme, customs/import duties.
- Invoice-line rounding rules vary by accounting system. The display rounding here is shown to the chosen precision; internal math runs at full precision.
§ Golden references · locked in tests
Four fixtures pin the engine. The two inclusive cases are the ones most free GST calculators get wrong — the correct formula is invoice / (1 + rate), not invoice × rate.
Exclusive · $5,000 · 5% · intra
GST $250 · Invoice $5,250
CGST 125 / SGST 125
Inclusive · $5,000 · 5% · intra
Taxable $4,761.90 · GST $238.10
CGST 119.05 / SGST 119.05
Inclusive · $100 · 18%
Taxable $84.75 · GST $15.25
Single-line fixture
Exclusive · $1,000 · 18% · inter
GST $180 · Invoice $1,180
IGST 180 (no CGST/SGST)
§ What this calculator does not do
- No HSN/SAC classification. Rate selection is your responsibility — the page links official CBIC sources for verification.
- No cess, reverse charge, ITC, exemptions, zero-rated supplies, composition scheme, customs / import duties.
- No invoice-line rounding rules beyond display rounding. Real accounting systems apply per-line and per-invoice rounding in ways that vary by software.
- No legal or tax advice. Confirm rate, classification, and applicability against an authoritative source before filing.
- No auto-classification from product codes in v1. A future admin-only rate-pack interface could update presets when CBIC notifications land; HSN/SAC lookup would require a licensed, source-audited rate database.
§ Tax calculator · sibling tools
§ Same engine, headlessly
The exclusive/inclusive arithmetic, all four supply-type splits, the rate comparison, the formula trace, and the input audit are all reachable as a stateless REST endpoint and as an MCP tool. Workbook CRUD is authenticated and stored under calculator_id=gst. Unknown inputs are rejected with 422.
POST/api/v1/financial-calculators/gst/calculate
POST/api/v1/financial-calculators/gst/run
GET/api/v1/financial-calculators/gst/schema
GET/api/v1/financial-calculators/gst/defaults
GET/api/v1/user/financial-calculator-workbooks?calculator_id=gst · individual+
MCP tool
calculate_gstmethodology_version = financial-calculators.v1 · canonical = /en/tools/gst-calculator
§ FAQ
Six things worth knowing
Q01What is GST?+
GST — Goods and Services Tax — is the indirect consumption tax levied on most goods and services in India. A supplier collects GST on top of the taxable value, breaks it into CGST/SGST/UTGST/IGST depending on where the buyer is, and remits the amount to the tax authority. This calculator handles the arithmetic — the GST amount, the taxable value, the invoice value, and the split — once you have decided the applicable rate.
Q02GST exclusive vs GST inclusive — what is the difference?+
Exclusive means the amount you typed is the pre-tax value, and the tool adds GST on top (this is what you quote a customer). Inclusive means the amount is the invoice total and the tool extracts the GST that is already embedded in it (this is what you read off a bill). Both modes share the same rate; only the direction of the formula changes.
Q03How is GST calculated from the taxable value?+
gst_amount = taxable_value × gst_rate, invoice_value = taxable_value + gst_amount. At 18% on $5,000, GST is $900 and the invoice is $5,900. The CGST/SGST split is then half of the GST amount each for intrastate supplies.
Q04How do I reverse-calculate GST from the invoice value?+
The correct formula is taxable_value = invoice / (1 + gst_rate); the GST amount is then invoice − taxable. At $5,900 invoice and 18%, the taxable value is $5,000 and the GST embedded was $900. The common-but-wrong shortcut is invoice × rate — that would give $1,062 and overstate the GST by ~18% of itself. The calculator uses the correct division and the formula trace shows both forms so you can sanity-check.
Q05CGST vs SGST vs UTGST vs IGST — when does each apply?+
CGST + SGST: intrastate supply (buyer and seller in the same state), each is half of the GST amount. CGST + UTGST: supply within a union territory without legislature — same arithmetic, different ledger account because UTGST replaces SGST there. IGST: interstate supply — a single IGST line for the full GST amount, no split. The total tax is identical in every case; only the labels on the invoice change.
Q06Does this calculator determine the correct rate?+
No, and deliberately so. The calculator does the arithmetic; rate selection is your responsibility. HSN/SAC classification varies by product, can be disputed, and changes with CBIC notifications — a calculator that silently picks a rate from a product code becomes wrong over time. The presets are versioned with source URL, retrieved date, and effective date so you can verify them; the page also links the three official CBIC sources for classification.