Adding GST and removing GST are different sums
The most common GST error is treating these as the same operation.
Adding GST to a net amount multiplies: ₹1,000 plus 18% is ₹1,180.
Removing GST from a gross amount divides: ₹1,180 including 18% GST has a base of ₹1,180 ÷ 1.18 = ₹1,000, and the tax is ₹180.
What you must not do is take 18% of ₹1,180, which gives ₹212.40. That is wrong, and it is wrong in a direction that leaves your books out by a few per cent on every inclusive-priced invoice.
The toggle at the top of the calculator sets the direction. Choose Excluding GST when your figure is the base price and you need to add tax; choose Including GST when you have a final price and need to extract the tax inside it.
The formulas
Adding: GST = Amount × Rate ÷ 100, and the total is Amount + GST.
Removing: Base = Amount ÷ (1 + Rate ÷ 100), and GST = Amount − Base.
CGST, SGST and IGST
The total tax is the same whichever applies; only the split differs.
- Intra-state — supplier and buyer in the same state. The tax splits equally into CGST (central) and SGST (state). An 18% rate becomes 9% CGST plus 9% SGST.
- Inter-state — different states. The whole amount is charged as a single IGST line at the full rate.
- Union territories use UTGST in place of SGST.
The calculator shows the CGST and SGST halves because that is the more common case on an invoice. For an inter-state supply, read the same total as one IGST line.
The rate slabs
GST has six rates: 0.25%, 3%, 5%, 12%, 18% and 28%, alongside a nil-rated category and exempt supplies.
Broadly, 0.25% covers rough precious stones and 3% covers gold and silver. Essentials sit at 5%, 12% and 18% carry most goods and services, and 28% applies to luxury and so-called demerit goods, some of which attract an additional cess on top.
Classification is the hard part, not the arithmetic. The rate depends on the HSN code for goods or the SAC code for services, and getting it wrong creates a liability that surfaces at assessment. If you are unsure which slab applies to what you sell, confirm the code before relying on any calculator — this one applies the rate you choose, it does not choose it for you.
How to use this calculator
- Set Amount entered is to Excluding GST or Including GST.
- Enter the total amount.
- Pick the tax slab.
The result shows the net amount, the total GST, the CGST and SGST halves, and the post-GST total.
Input tax credit, briefly
A registered business pays GST on its purchases and collects GST on its sales, and remits only the difference. The tax paid on inputs is credited against the tax collected on outputs, so GST is ultimately borne by the final consumer rather than by each business in the chain.
This is why a registered buyer cares about a supplier's GST compliance: the credit is only available if the supplier has actually reported the invoice. An unregistered buyer has no credit mechanism and simply pays the tax.