Every small business owner in India runs into the same two questions almost daily. "If I quote ₹10,000 plus GST, what does the customer actually pay?" and "This bill says ₹1,180 including GST, so what was the real price?" Getting either one wrong means a quotation that undercuts your margin, an invoice that doesn't match the customer's books, or an awkward phone call with your accountant.
This guide walks through both calculations step by step, explains how the tax splits into CGST, SGST and IGST, and covers the mistakes people make most often. You can do all of it by hand, or use the free GST Calculator on PKD Ventures to check your numbers in a few seconds.
The two formulas you actually need
There are only two directions of GST calculation.
Adding GST (exclusive price to final price)
- GST amount = Price × Rate ÷ 100
- Final price = Price + GST amount
Example: ₹1,000 at 18% gives GST of ₹180 and a final price of ₹1,180.
Removing GST (inclusive price to base price)
- Base price = Inclusive price ÷ (1 + Rate ÷ 100)
- GST amount = Inclusive price − Base price
Example: ₹1,180 at 18% gives a base price of ₹1,180 ÷ 1.18 = ₹1,000, and GST of ₹180.
That's it. Almost every GST confusion comes from using the first formula when you need the second.
Step by step: adding GST to a quotation
Say you're a web designer quoting ₹25,000 for a website, and your service attracts 18% GST.
- Open the GST Calculator.
- Enter 25000 as the amount.
- Pick 18% as the rate.
- Choose Add GST.
You'll see GST of ₹4,500 and a total of ₹29,500. Results update as you type, so you can try a few different prices without pressing any buttons.
Put both lines on your quotation: the taxable value and the GST. Customers who are registered under GST need to see the tax separately to claim input credit, and a quote that says only "₹29,500" leaves them guessing.
Step by step: removing GST from an MRP or bill
Now flip it. A customer tells you their budget is ₹50,000 "all inclusive", at 18%.
- Enter 50000 as the amount.
- Pick 18%.
- Choose Remove GST.
The base price works out to about ₹42,372.88 and GST to about ₹7,627.12. That base figure is what you can really charge for your work. The calculator rounds results for display, so for a final invoice, decide how your billing software rounds and stay consistent.
This direction is also useful when you buy something with an MRP and want to know how much of what you paid was tax, or when you're checking whether a supplier's bill adds up.
CGST, SGST and IGST in plain words
GST is one tax, but on the invoice it shows up under different names depending on where the buyer is.
- Sale within the same state (intra-state): the GST is split equally into CGST (central) and SGST (state). At 18%, that's 9% + 9%. On our ₹25,000 quote, ₹2,250 CGST and ₹2,250 SGST.
- Sale to another state (inter-state): the whole amount is IGST. At 18%, that's ₹4,500 IGST on the same quote.
The total tax is the same either way. Only the label changes. The GST Calculator shows the CGST and SGST split for sales within a state and the IGST figure for sales between states, so you can copy the right lines onto your invoice.
A quick way to remember it: if your state and the customer's "place of supply" are the same, use CGST + SGST. If they're different, use IGST.
Which rate should you pick?
The calculator lets you choose from the current slabs (such as 5%, 18% and 40%) as well as older rates, which helps when you're checking an invoice issued before a rate change.
What it can't do is tell you which rate applies to your product or service. That depends on the HSN or SAC code and the current notifications. If you're unsure, check the official GST portal or ask your CA once, then write the rate down next to each product in your price list so you're never guessing again.
Practical tips for sellers and freelancers
Decide whether your price list is inclusive or exclusive, and say so. "₹999" on a marketplace listing usually includes tax. "₹999 + GST" in a B2B quote does not. Mixing the two across channels is the fastest way to lose money without noticing.
Work backwards from the customer's budget. When someone gives you an all-inclusive number, remove GST first, then decide what you can deliver for the base amount.
Check percentages and discounts separately. If you're giving a 10% discount, apply it to the base price before adding GST. The Percentage Calculator is handy for working out the discount itself, and then you run the discounted figure through the GST Calculator.
Keep a simple record. For every invoice, note the taxable value, the rate, and the CGST/SGST or IGST amounts. When it's time to file returns, you'll thank yourself.
Planning a big purchase on a loan? Work out the GST-inclusive price first, then use the EMI Calculator on that full amount so your monthly instalment isn't a surprise.
Common mistakes to avoid
1. Subtracting 18% from an inclusive price. This is the big one. ₹1,180 minus 18% is ₹967.60, not ₹1,000. GST was added on the base price, so you have to divide by 1.18 to get back to it. Using the Remove GST option avoids this every time.
2. Charging CGST + SGST on an inter-state sale (or the reverse). The total is the same, but the invoice is wrong, and the buyer may not be able to claim credit properly. Check the place of supply before you bill.
3. Splitting the rate wrongly. CGST and SGST are each half of the total rate. At 18%, it's 9% + 9%, not 18% + 18%.
4. Applying GST before the discount. Discount first, then tax, unless your specific case is different and your CA says otherwise.
5. Using an outdated rate. Rates for many items have changed over the years. If you're re-using an old quotation template, confirm the rate is still current.
6. Rounding each line differently. Round consistently, either per line or on the total, and match what your accounting software does. Small paise differences add up across many invoices.
A quick worked example
A Kanpur shop sells a product for ₹2,360 including 18% GST to a customer in Lucknow (same state, Uttar Pradesh).
- Base price: ₹2,360 ÷ 1.18 = ₹2,000
- GST: ₹360
- Split: ₹180 CGST + ₹180 SGST
If the same product went to a customer in Delhi, it would be ₹360 IGST instead, with the same ₹2,000 base price.
Wrapping up
GST maths is simple once you know which direction you're going: multiply to add, divide to remove. Keep the base price, the rate and the CGST/SGST or IGST split visible on every quote and invoice, and you'll avoid most disputes before they start.
The GST Calculator is one of the free tools at PKD Ventures, alongside percentage, EMI and other everyday calculators. Bookmark it for the next time a customer asks, "So what's the final price?"
Note: this guide explains general GST arithmetic. For the correct rate and treatment of your specific product or service, check the official GST portal or consult a chartered accountant.
