About gst calculator
Goods and Services Tax is charged on almost every bill in India, but the maths trips people up in one place: whether the price you have already includes GST. This guide explains both directions and shows how the tax splits between the Centre and the state.
Adding GST to a price
When a price is quoted before tax, multiply it by the rate and add the result. GST = amount × rate ÷ 100, and the total is amount + GST. For example, a service billed at ₹10,000 with 18% GST carries ₹1,800 of tax, so the customer pays ₹11,800.
Removing GST from a GST-inclusive price
Shop prices and MRPs usually already include tax. To find the pre-tax value you divide, not subtract: base = total × 100 ÷ (100 + rate). A ₹11,800 bill at 18% has a base of ₹10,000, so the GST inside it is ₹1,800. Subtracting 18% of ₹11,800 (₹2,124) is a common mistake and gives the wrong base.
CGST, SGST and IGST
For a sale inside one state, the tax is split equally: half is CGST (collected by the Centre) and half is SGST (collected by the state). On 18% GST that is 9% + 9%. For a sale between states the whole amount is charged as IGST. The total tax is identical either way; only the head it is booked under changes.
Common mistakes to avoid
- Applying the rate on the GST-inclusive price instead of reverse-calculating.
- Using one rate for a whole invoice when items fall in different slabs.
- Forgetting that rates for specific goods and services are changed by the GST Council, so check the current official schedule before filing a return.
