Business
Invoicing, margin & GST calculators
Reverse GST Calculator — Extract GST from a Total
A reverse GST calculation works backwards from a GST-inclusive total to find the original taxable base and the GST amount hidden inside it. The correct formula is base = gross × 100 / (100 + rate) — NOT gross × (1 − rate), which is the single most common mistake. For an 18% GST-inclusive total of ₹1,180 the base is 1,180 × 100 / 118 = ₹1,000 and the GST is ₹180 (which splits into CGST ₹90 + SGST ₹90 for an intra-state supply, or a single IGST ₹180 inter-state). Enter your inclusive total and rate to extract the GST instantly, then hand off to the invoice generator to issue a matching GST tax invoice. Free, no login.
Reverse formula: base = gross × 100 / (100 + rate) — extract the taxable value from a GST-inclusive price
- GST amount = gross − base (the tax portion hidden inside the total)
- Common error to avoid: base ≠ gross × (1 − rate). At 18%, 1,180 × 0.82 = ₹967.60 is WRONG
- Intra-state supply: the extracted GST splits equally — CGST = GST/2, SGST/UTGST = GST/2
- Inter-state supply: the extracted GST is a single IGST at the full rate
- Worked example — ₹1,180 inclusive @ 18%: base = 1,180 × 100/118 = ₹1,000, GST = ₹180
- Worked example — ₹1,050 inclusive @ 5%: base = 1,050 × 100/105 = ₹1,000, GST = ₹50
- Current GST 2.0 slabs to reverse-extract from: 0% / 5% / 18% / 40% (+ niche 3% and 0.25%)
Frequently asked questions
How do I reverse-calculate GST from a total?
Use base = gross × 100 / (100 + rate). To extract 18% GST from a ₹1,180 inclusive total: base = 1,180 × 100 / 118 = ₹1,000, and the GST is ₹1,180 − ₹1,000 = ₹180. For a 5% inclusive total of ₹1,050: base = 1,050 × 100 / 105 = ₹1,000, GST ₹50. Do NOT multiply the total by (1 − rate) — that under-states the base (1,180 × 0.82 = ₹967.60, which is wrong). The full calculator applies the correct 100/(100+rate) formula and also splits the extracted GST into CGST/SGST or IGST.
Does the extracted GST split into CGST and SGST?
Yes, if the supply is intra-state. Once you have extracted the GST from the total, the split follows the place of supply: an intra-state supply divides it equally into CGST (half) + SGST/UTGST (half), while an inter-state supply charges a single IGST at the full rate. The total tax is identical either way — only the split differs. On ₹180 of extracted 18% GST: intra-state shows CGST ₹90 + SGST ₹90; inter-state shows IGST ₹180.
Which GST rate should I use to reverse a total?
Use the rate that applied to the goods or services on the invoice. After the GST 2.0 reform (effective 22 September 2025), the main slabs are 5%, 18% and 40%, plus 0% (nil-rated) and the niche 3% (gold/jewellery) and 0.25% (rough diamonds). The earlier 12% and 28% slabs were removed. If you are reversing an older invoice issued before the reform, use the rate that was in force at that time (e.g. 12% or 28%). Always confirm the correct rate for the specific HSN/SAC code.