How to Calculate GST (Add and Remove)
⏱ 2 min readGST maths comes in two directions: adding GST to a base price (exclusive → inclusive) and removing GST from a total (inclusive → exclusive). The second one is where most mistakes happen. Here are both formulas with worked examples.
Adding GST to a base price
GST amount = base price × rate ÷ 100. Total = base price + GST amount.
Example at 18%: base ₹1,000 → GST = 1,000 × 18 ÷ 100 = ₹180 → total ₹1,180. For an intra-state sale that is ₹90 CGST + ₹90 SGST.
Removing GST from an inclusive price (reverse GST)
Base price = total ÷ (1 + rate ÷ 100). GST amount = total − base price.
Example at 18%: total ₹1,180 → base = 1,180 ÷ 1.18 = ₹1,000 → GST = ₹180.
At 5%: total ₹2,100 → base = 2,100 ÷ 1.05 = ₹2,000 → GST = ₹100.
The classic mistake: subtracting the percentage directly
Taking 18% off ₹1,180 gives ₹212.40 — which is wrong, because the 18% was charged on the smaller base (₹1,000), not on the total. Whenever you start from a GST-inclusive figure, divide by (1 + rate/100); never multiply the total by the rate.
Do it instantly instead
Our GST calculator does both directions with the CGST/SGST split shown automatically — enter the amount, pick the slab, and choose add or remove.
Frequently asked questions
What is the formula to remove GST from a total?
Base = total ÷ (1 + rate/100). For 18% GST on a ₹1,180 total: 1,180 ÷ 1.18 = ₹1,000 base, so the GST portion is ₹180.
Why can't I just subtract 18% from the inclusive price?
Because the 18% was calculated on the base price, which is smaller than the total. Subtracting 18% of the total removes too much — ₹212.40 instead of ₹180 in the ₹1,180 example.
How do I split GST into CGST and SGST?
Divide the GST amount by two for an intra-state sale: ₹180 GST at 18% = ₹90 CGST + ₹90 SGST. Inter-state sales charge the whole amount as IGST instead.