GST Calculator for B2B Invoices: CGST, SGST & IGST Explained (With Examples)
How to calculate GST on a B2B invoice — add GST to a base price or extract it from a GST-inclusive quote, when CGST+SGST applies vs IGST, and the invoicing mistakes that break Input Tax Credit claims.
GST calculation on a B2B invoice comes down to two directions and one split decision:
Add GST (exclusive → inclusive): GST Amount = Base Amount × Rate ÷ 100 Extract GST (inclusive → exclusive): GST Amount = Total × Rate ÷ (100 + Rate) Split: CGST + SGST (same state) or IGST (different states)
Use the free GST Calculator at cisuitepro.com/calculators/gst-calculator to compute both directions instantly, with the correct CGST/SGST/IGST split — no signup required.
Getting this wrong doesn't just produce a wrong number on a quote — it produces a tax invoice that doesn't match what the buyer expects, and in a B2B relationship where the buyer needs to claim Input Tax Credit (ITC), an invoice that's structured incorrectly can hold up their tax filing and your payment.
The GST Calculation Formula
There are exactly two calculation directions, depending on which number you already have:
- GST-exclusive (you have the base price, need the total): GST Amount = Base Amount × Rate ÷ 100, then Total = Base Amount + GST Amount
- GST-inclusive (you have the total, need the base price): GST Amount = Total × Rate ÷ (100 + Rate), then Base Amount = Total − GST Amount
Worked Example: Adding GST to a Quote
A vendor is pricing a fabrication job at a base value of ₹1,00,000, GST rate 18%:
GST Amount = 1,00,000 × 18 ÷ 100 = ₹18,000 Invoice Total = 1,00,000 + 18,000 = ₹1,18,000
This is the calculation to use whenever you're building up a quote from a known cost-plus-margin base price.
Worked Example: Extracting GST from an Inclusive Price
A buyer receives a lump-sum quote of "₹1,18,000, GST included" and needs the taxable value for their purchase order and books:
GST Amount = 1,18,000 × 18 ÷ (100 + 18) = 1,18,000 × 18 ÷ 118 = ₹18,000 Base (taxable) Value = 1,18,000 − 18,000 = ₹1,00,000
A common mistake here is subtracting 18% of the total directly (1,18,000 × 0.18 = ₹21,240) — that overstates the GST amount because it applies the rate to a number that already includes tax. Always divide by (100 + Rate), not multiply by Rate, when extracting from an inclusive figure.
CGST + SGST vs IGST — Which Applies to Your Invoice?
The split depends entirely on where the supplier and the "place of supply" are located — not on the type of goods or service:
| Scenario | Tax Applied | Split on ₹18,000 GST (18% rate) |
|---|---|---|
| Supplier and buyer in the same state | CGST + SGST | ₹9,000 CGST + ₹9,000 SGST |
| Supplier and buyer in different states | IGST | ₹18,000 IGST |
| Import of goods/services into India | IGST | ₹18,000 IGST (plus customs duty where applicable) |
| Export of goods/services (zero-rated) | IGST at 0% (or refund route) | ₹0 — subject to LUT/bond conditions |
GST Rate Slabs for Industrial Goods and Services
Most industrial B2B transactions fall into one of four standard slabs. The exact slab depends on the HSN (goods) or SAC (services) code, not the industry in general — always confirm against the current code rather than assuming by category:
| Slab | Typical Examples |
|---|---|
| 5% | Select capital goods, some transport and job-work services |
| 12% | Certain machinery, business correspondence and some construction services |
| 18% | Most industrial equipment, AMC and maintenance services, IT services, consulting, freight (most modes) |
| 28% | A narrow list of goods, mostly outside typical industrial procurement |
What a Valid B2B Tax Invoice Must Show
A GST tax invoice under Section 31 of the CGST Act needs specific line items, not a single all-in price:
- Taxable (base) value — shown separately, before tax
- GST rate applied, and the tax amount, split into CGST + SGST or shown as IGST
- GSTIN of both supplier and recipient
- HSN/SAC code for each line item
- Invoice number and date, in sequence
- Place of supply, when it differs from the buyer's billing address
Common GST Invoicing Mistakes That Break Input Tax Credit
These are the errors that most often cause a buyer's ITC claim to get flagged or rejected — worth checking before an invoice goes out, not after:
- Quoting only a GST-inclusive lump sum with no taxable value or tax amount shown separately
- Charging CGST+SGST when the place of supply is actually a different state (should have been IGST), or vice versa
- Using the wrong HSN/SAC code, which can put the transaction in the wrong rate slab
- GSTIN typos — even a single-digit error can cause the invoice to not reflect correctly in the buyer's GSTR-2B
- Rounding the tax amount instead of calculating it precisely from the taxable value and rate
Calculate GST Instantly
To calculate GST you need three things: the amount, the rate, and whether you're adding GST to a base price or extracting it from an inclusive total.
Enter those into the free GST Calculator at cisuitepro.com/calculators/gst-calculator — it computes the taxable value, GST amount, and the correct CGST+SGST or IGST split instantly, for both intrastate and interstate supply.
Frequently Asked Questions
How do I calculate GST on a B2B invoice?
To add GST to a base price: GST Amount = Base Amount × Rate ÷ 100, then Total = Base Amount + GST Amount. Example: ₹1,00,000 at 18% = ₹18,000 GST, total ₹1,18,000. To extract GST from an inclusive total: GST Amount = Total × Rate ÷ (100 + Rate).
When do I charge CGST + SGST instead of IGST?
CGST + SGST (split equally) applies when the supplier and the place of supply are in the same state. IGST (full rate, no split) applies when they are in different states, or for imports/exports. The total tax amount is identical either way — only the split between central and state government changes.
How do I extract the base price from a GST-inclusive quote?
GST Amount = Total × Rate ÷ (100 + Rate); Base Amount = Total − GST Amount. Example: ₹1,18,000 inclusive at 18% → GST = 1,18,000 × 18 ÷ 118 = ₹18,000, base value = ₹1,00,000. Do not simply take 18% of the total — that overstates the tax amount.
What GST rate applies to industrial AMC and maintenance services in India?
Most industrial AMC, maintenance, and repair services fall under the 18% GST slab as a supply of services. Always confirm against the specific SAC code for the service, since a small number of specialised services fall outside the standard slabs.
Why does my vendor's invoice need to show GST separately instead of one inclusive price?
A valid B2B tax invoice must show the taxable value, GST rate, and tax amount as distinct line items so the buyer can claim Input Tax Credit (ITC) on the purchase. An invoice with only a single GST-inclusive figure and no breakup does not meet this requirement and can delay or block the buyer's ITC claim.
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