Check IGST vs CGST+SGST on every invoice line in a sales register

The one rule the check rests on

The CBIC's GST FAQ states it plainly: if the place of supply and the location of the supplier are in the same State, the supply is intra-State. In that case CGST and SGST (or UTGST) apply; otherwise the supply is inter-State and IGST applies. Note what the rule compares. It is the supplier's location against the place of supply, not the supplier against the buyer's GSTIN. The two usually agree, which is exactly why billing staff learn the shortcut "same state as the customer means CGST+SGST", and why that shortcut fails on the lines that matter.

State codes are the two-digit numbers at the start of every GSTIN and in the place-of-supply field of an e-invoice. The GST e-invoice portal publishes the full state code master; keep a copy in the workbook as a lookup sheet instead of typing names.

What goes wrong in practice

Four patterns account for most of what this check finds:

None of them is visible from a monthly total. The total tax can look reasonable while individual lines carry the wrong kind of tax, which then surfaces as a mismatch in the buyer's input credit or a query at return time.

The check in pandas

The register below is synthetic, and so are the GSTINs. The supplier is registered in Uttar Pradesh (state code 09).

import pandas as pd

SUPPLIER_GSTIN = "09AAACX1234A1ZV"                  # synthetic; 09 = Uttar Pradesh
STATES = {"06": "Haryana", "07": "Delhi", "09": "Uttar Pradesh", "27": "Maharashtra",
          "29": "Karnataka"}                          # subset of the GST state code list

# Synthetic sales register. place_of_supply is what the invoice records, as a state code.
inv = pd.DataFrame({
    "inv_no": ["NOI/26-27/0101", "NOI/26-27/0102", "NOI/26-27/0103", "NOI/26-27/0104",
               "NOI/26-27/0105", "NOI/26-27/0106", "NOI/26-27/0107"],
    "buyer_gstin": ["09AAGCW3456D1ZE", "07AAFCY5678B1Z3", "27AABCZ9012C1ZI", "06AAGCW3456D1ZE",
                    None, "29AAGPR4821K1ZA", "09AAGCW3456D1ZE"],
    "place_of_supply": ["09", "07", "09", "06", "09", "29", None],
    "rate": [18, 18, 18, 12, 18, 18, 5],
    "taxable": [100000.0, 45000.0, 60000.0, 25000.0, 8000.0, 150000.0, 20000.0],
    "igst": [0.0, 8100.0, 10800.0, 0.0, 0.0, 18000.0, 0.0],
    "cgst": [9000.0, 0.0, 0.0, 1500.0, 720.0, 0.0, 500.0],
    "sgst": [9000.0, 0.0, 0.0, 1500.0, 720.0, 0.0, 500.0],
})

supplier_state = SUPPLIER_GSTIN[:2]
inv["buyer_state"] = inv["buyer_gstin"].str[:2]
inv["expected"] = inv["place_of_supply"].map(
    lambda p: None if pd.isna(p) else ("CGST+SGST" if p == supplier_state else "IGST"))
inv["charged"] = inv.apply(lambda r: "IGST" if r.igst > 0 and r.cgst == r.sgst == 0
                           else "CGST+SGST" if r.igst == 0 and r.cgst > 0 else "mixed/none", axis=1)
tax_due = (inv["taxable"] * inv["rate"] / 100).round(2)
charged_total = inv["igst"] + inv["cgst"] + inv["sgst"]

def findings(r, due, total):
    out = []
    if pd.isna(r.expected):
        out.append("place of supply missing: cannot decide tax type")
    elif r.expected != r.charged:
        out.append(f"charged {r.charged}, place of supply {r.place_of_supply} implies {r.expected}")
    if abs(total - due) > 1:
        out.append(f"tax {total:,.0f} vs {due:,.0f} at {r.rate}%")
    if r.cgst != r.sgst:
        out.append("CGST and SGST differ")
    if isinstance(r.buyer_state, str) and isinstance(r.place_of_supply, str) and r.buyer_state != r.place_of_supply:
        out.append(f"buyer registered in {STATES.get(r.buyer_state, r.buyer_state)}, "
                   f"supply placed in {STATES.get(r.place_of_supply, r.place_of_supply)}: confirm")
    return "; ".join(out)

inv["check"] = [findings(r, d, t) for r, d, t in zip(inv.itertuples(), tax_due, charged_total)]
print(inv.loc[inv["check"] != "", ["inv_no", "place_of_supply", "charged", "check"]]
      .to_string(index=False))
assert (inv["check"] != "").sum() == 4

Seven invoices go in and four come out on the list:

• 0103 charged IGST because the buyer is registered in Maharashtra, but the invoice itself records Uttar Pradesh as the place of supply. Either the tax or the place of supply field is wrong, and only the person who knows where the goods went can say which.
• 0104 is the reverse: a Haryana place of supply billed with CGST and SGST.
• 0106 has the right tax type but ₹18,000 of IGST on ₹1,50,000 at 18%, which should be ₹27,000. The amount looks like a 12% calculation.
• 0107 has no place of supply at all, so the script refuses to guess.

Invoice 0105 has no buyer GSTIN (a sale to an unregistered customer) and passes, because its tax type is still decided by the recorded place of supply.

Three design choices

Expected type comes from the invoice's own place of supply. The script does not try to work out where the supply should be placed. That decision depends on the kind of supply and the contract, and a script that silently overrides it would produce confident wrong answers. It only checks internal consistency: is the tax charged the tax that this place of supply implies?

Buyer-state mismatches are informational. The GST portal's own GSTR-2B FAQ discusses invoices where the supplier and the place of supply are in one State while the recipient is in another, so such invoices exist and are not automatically wrong. The note "confirm" sends the line to a person without calling it an error.

Tolerance of ₹1 on recomputed tax. Invoices are usually rounded per line or per invoice, so an exact equality test would flood the list with paise differences. Keep the tolerance small, state it in the output, and widen it only if you know your billing software rounds at invoice level.

The same test in Excel

If the register has place of supply in column C, taxable value in E, rate in F, IGST in G, CGST in H and SGST in I, and your own state code sits in a named cell Own_State, three helper columns in J, K and L do the job:

Expected   =IF(C2="","MISSING",IF(TEXT(C2,"00")=Own_State,"CGST+SGST","IGST"))
Charged    =IF(AND(G2>0,H2=0,I2=0),"IGST",IF(AND(G2=0,H2>0,I2>0),"CGST+SGST","MIXED"))
Flag       =IF(OR(J2<>K2,H2<>I2,ABS(G2+H2+I2-E2*F2/100)>1),"REVIEW","")

TEXT(C2,"00") keeps state code 09 from turning into 9 when the column was stored as a number, which is the most common reason this check reports every line as inter-State. Filter the Flag column to REVIEW and sort by taxable value so the largest lines are read first.

Before and after the check

Validate the GSTIN format first, so that a truncated or mistyped number does not produce a wrong state code; the steps are in validate GSTIN and PAN formats before they reach a report. After correcting the register, the purchase side of the same problem appears when you reconcile GSTR-2B with the purchase register.

This article covers a data-consistency check, not tax advice. Rates, exemptions and place-of-supply rules for particular transactions should be confirmed with your tax adviser or the notifications on the CBIC site.

Turning a rule like this into a repeatable check with a clear exception list is a core habit in our Data Analyst course, where learners build such controls on realistic registers before they build dashboards on top.