Every Indian invoice sheet needs the same handful of GST formulas. Here they are in one place, with the rounding and inter-state logic people usually get wrong. Rates below are examples; always confirm the current rate for your HSN/SAC code.

Add GST to a price (exclusive → inclusive)

Taxable value in B2, rate in C2 (formatted as 18%):

=B2 * C2            ' GST amount
=B2 * (1 + C2)      ' invoice value

Remove GST from an inclusive price

The common mistake is =B2 - B2*18%. That’s wrong: 18% is charged on the base, not on the inclusive figure.

=B2 / (1 + C2)              ' taxable value: ₹1,180 at 18% → ₹1,000
=B2 - B2 / (1 + C2)         ' GST inside the price → ₹180
=B2 * C2 / (1 + C2)         ' same thing in one step

CGST + SGST or IGST?

Intra-state supply: half CGST, half SGST. Inter-state: the full rate as IGST. With your state code in $L$1 and the buyer’s place-of-supply code in D2:

E2 (CGST):  =IF(D2=$L$1, ROUND(B2*C2/2, 2), 0)
F2 (SGST):  =IF(D2=$L$1, ROUND(B2*C2/2, 2), 0)
G2 (IGST):  =IF(D2<>$L$1, ROUND(B2*C2, 2), 0)
H2 (Total): =B2 + E2 + F2 + G2

Rate from a lookup table

Keep an HSN table (code, description, rate) and pull the rate instead of typing it:

=XLOOKUP(A2, HSN[Code], HSN[Rate], "Check HSN")

Rate-wise summary for the return

=SUMIFS(Lines[Taxable], Lines[Rate], 0.18)
=SUMIFS(Lines[IGST], Lines[Rate], 0.18)

Or one pivot table with Rate in rows and Taxable, CGST, SGST and IGST in values.

Rounding

Round each tax line to 2 decimals with ROUND, not just formatting, which hides paise that still add up. Round the invoice total to the rupee only if your billing policy does, and show it as a separate “Round off” line: =ROUND(total,0)-total.

Where people go wrong

Practice

Download the combo practice workbook below. Its Sales sheet of 200 orders (dates, regions, products, customers, amounts) is ready data to try every formula on this page, and the Practice sheet has 50 checked tasks on related combos.

More: all formula combos · Excel function course.