What the calculator does in each mode
The calculator has two modes because GST comes up in two different real-world situations. Add GST mode takes a base price and calculates what the final, tax-inclusive price will be — useful when you're quoting a price to a customer or estimating a purchase cost before tax. Remove GST mode does the reverse: given a final price that already includes GST, it works out what portion of that total was actually tax, and what the original base price was before tax was added.
Both modes use the same underlying rate, but the math isn't simply the same formula run backward — which is the part that trips people up most often.
Why you can't just multiply an inclusive price by the GST rate
A common mistake is taking a ₹11,800 GST-inclusive price at 18% and assuming the tax portion is 18% of ₹11,800, or ₹2,124. That's wrong, because 18% was applied to the original base price, not to the final inclusive price — and those are two different numbers.
The correct approach divides the inclusive price by (1 + tax rate) to first recover the base price: ₹11,800 ÷ 1.18 = ₹10,000. The actual GST portion is then ₹11,800 − ₹10,000 = ₹1,800, not ₹2,124. This is exactly the calculation the calculator's Remove GST mode does automatically, and it's the step most manual calculations get wrong.
How CGST and SGST split on the invoice
For a transaction within the same state, the total GST rate splits equally between Central GST (CGST) and State GST (SGST) — an 18% rate becomes 9% CGST and 9% SGST, shown as two separate lines on the invoice rather than one combined GST line. This is why the calculator's output breaks the total tax into these two components.
For a transaction between two different states, the full rate is instead charged as a single Integrated GST (IGST) line, with no CGST/SGST split. The calculator assumes an intra-state transaction by default, so if you're dealing with an inter-state invoice, the total tax amount is still correct, but it should appear as one IGST line rather than split CGST/SGST lines.
Using the calculator for pricing decisions
If you're setting a price for a product or service and need the customer to pay a clean, round final number — say exactly ₹1,000 — work backward using Remove GST mode on that ₹1,000 target to find the base price you should actually quote before tax, rather than adding tax to a round base price and ending up with an odd final number.
This is also useful for checking whether a vendor's quoted price is calculating GST correctly — run their stated base price through Add GST mode at the applicable rate, and compare the result to what they've actually billed.
Common mistakes when calculating GST manually
Applying the tax rate to the wrong base. Always apply the percentage to the pre-tax base price, not the final inclusive price, when adding tax — and use the divide-by-(1+rate) method when working backward from an inclusive price.
Assuming every product falls under the same rate. Different goods and services fall under different GST slabs, and using the wrong slab rate throws off every downstream number — check the applicable rate for the specific item before calculating.
Mixing up CGST/SGST and IGST. An intra-state invoice needs a CGST+SGST split; an inter-state invoice needs a single IGST line — using the wrong structure is a common invoicing error even when the total tax amount is correct.