A reverse GST calculator finds the base price and the tax hidden inside a GST-inclusive amount. Enter the total and the GST rate, and this free tool removes the GST using the formula Base = Amount × 100 ÷ (100 + rate). It then shows the taxable value, the total GST, and splits it into CGST and SGST for intra-state sales or IGST for inter-state sales. For example, ₹1,180 at 18% gives a base of ₹1,000 and ₹180 GST (₹90 CGST + ₹90 SGST).
When a price already includes GST, finding the original amount by hand is easy to get wrong. This free reverse GST calculator does it for you: enter a GST-inclusive total and the rate, and it removes the GST to show the base value and the tax separately, with the correct CGST/SGST or IGST split.
It is built for Indian GST, so it handles 5%, 12%, 18%, 28% and the special 3% and 0.25% rates, plus a custom rate. Useful for invoices, bookkeeping and checking a bill in seconds.
Key takeaways
- Reverse GST formula: Base = Amount × 100 ÷ (100 + rate). The GST is simply Amount minus Base.
- Do not apply the rate on the inclusive total. 18% of ₹1,180 is ₹212.40, which is wrong. The tax was on the base, not the total.
- Intra-state sale: GST splits equally into CGST + SGST. Inter-state sale: it is one IGST line.
- At 18%, a ₹1,180 bill breaks into ₹1,000 base and ₹180 GST. Change the rate and the split updates live.
Why use this reverse gst calculator?
Removes GST correctly
Uses the proper 100 ÷ (100 + rate) formula, not a rate applied on the total.
All Indian slabs
Handles 5%, 12%, 18%, 28%, plus 3%, 0.25% and a custom rate.
CGST/SGST or IGST
Pick intra-state or inter-state and the split updates automatically.
Free and private
No signup, and nothing you type is stored.
How to use it
- Enter the total GST-inclusive amount (the price that already includes tax).
- Choose the GST rate, or type a custom one.
- Pick intra-state (CGST + SGST) or inter-state (IGST).
- Read the base value and the GST instantly.
- Use the split figures directly on your invoice or in your books.
Worked example: remove GST from ₹1,180 at 18%
Say a bill of ₹1,180 already includes 18% GST. To find the base and the tax:
| Step | Calculation | Result |
|---|---|---|
| Base value | 1180 × 100 ÷ 118 | ₹1,000.00 |
| Total GST | 1180 − 1000 | ₹180.00 |
| CGST (intra-state) | 180 ÷ 2 | ₹90.00 |
| SGST (intra-state) | 180 ÷ 2 | ₹90.00 |
| IGST (inter-state) | full 180 | ₹180.00 |
Reverse GST factor for each slab
To remove GST quickly, multiply the inclusive amount by the factor below to get the base value:
| GST rate | Base = inclusive × | GST share of the total |
|---|---|---|
| 5% | 0.9524 | 4.76% |
| 12% | 0.8929 | 10.71% |
| 18% | 0.8475 | 15.25% |
| 28% | 0.7813 | 21.88% |
The mistake most people make
The common error is applying the GST rate on the inclusive price. If you take 18% of ₹1,180 you get ₹212.40, but that is too high. GST was charged on the base of ₹1,000, not on ₹1,180, so the real tax is ₹180. Always divide the inclusive amount by (100 + rate), never multiply the total by the rate.
CGST, SGST and IGST: which one applies?
- Intra-state (buyer and seller in the same state): GST splits equally into CGST (central) and SGST (state). So 18% is 9% CGST + 9% SGST.
- Inter-state (different states, or import/export): the whole GST is charged as a single IGST line.
- The total tax is the same either way, only the split on the invoice changes.
Popular use cases
- Freelancers and businesses raising GST invoices from an all-inclusive price.
- Shopkeepers finding the tax inside an MRP or quoted total.
- Accountants and bookkeepers doing quick base-and-tax checks.
- Buyers verifying the GST printed on a bill or receipt.
- Anyone quoting a final price and needing the taxable value for records.
Frequently asked questions
How does a reverse GST calculation work?
Base value = inclusive amount × 100 ÷ (100 + rate). The GST is the inclusive amount minus the base. For ₹1,180 at 18%, the base is ₹1,000 and the GST is ₹180.
How do I remove 18% GST from an amount?
Multiply the inclusive amount by 100 and divide by 118. For ₹1,180, that is 1180 × 100 ÷ 118 = ₹1,000 base, so the GST is ₹180.
Why can I not just take 18% of the total?
Because GST was charged on the base price, not on the inclusive total. 18% of ₹1,180 is ₹212.40, which overstates the tax. The correct GST is ₹180.
What are the GST rates in India?
The main slabs are 5%, 12%, 18% and 28%, with special rates of 3% (gold) and 0.25% (rough diamonds). This tool supports all of them plus a custom rate.
What is the difference between CGST, SGST and IGST?
For a sale within one state, GST is split equally into CGST (central) and SGST (state). For a sale between states, the same total is charged as one IGST. The tax amount is identical, only the split changes.
Does reverse GST change the total tax?
No. Reverse calculation just separates an inclusive price into base and GST. The tax amount is the same as a forward calculation on that base.
Is this an official GST calculator?
It is a free helper for quick, accurate calculations. For filing and compliance, confirm figures with your accountant or the official GST portal.
Is this reverse GST calculator free?
Yes, completely free with no signup and no limits. Nothing you enter is stored.
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