Why a GST calculator in a calendar app?
OIOA is about everyday life in India, and for many people everyday life includes a quick tax calculation: a shopkeeper checking a bill, a freelancer preparing an invoice, a family checking whether a quoted price includes GST. Rather than switching to another app, you can do the sum in OIOA. Like the holiday calendar and reminders, the GST calculator is the same for every community.
The two calculations
Almost every GST question comes down to one of two sums.
Adding GST to a price (exclusive). You know the price before tax and want the tax and the total.
- GST amount = price × GST rate
- Total = price + GST amount
For example, a ₹1,000 service at 18% has ₹180 of GST, so the total is ₹1,180.
Finding the GST inside a price (inclusive). You know the final price, which already includes GST, and want to know how much of it is tax.
- Price before tax = total ÷ (1 + GST rate)
- GST amount = total − price before tax
For example, if ₹1,180 includes 18% GST, the price before tax is ₹1,180 ÷ 1.18 = ₹1,000, and the GST is ₹180.
A common mistake is to take 18% of ₹1,180 (₹212.40) as the tax. That is wrong, because the 18% was charged on the pre-tax price, not the total.
Text version of this diagram
Exclusive (price before tax): GST = price × rate. ₹1,000 × 18% = ₹180, total ₹1,180.
Inclusive (price already includes tax): base = price ÷ (1 + rate). ₹1,180 ÷ 1.18 = ₹1,000, so the GST inside is ₹180.
Within a state, GST is split equally into CGST and SGST; between states it is charged as IGST.
CGST, SGST and IGST
GST is one tax, but on a bill it is shown in parts depending on where the sale happens:
- Within one state (intra-state): the GST is split equally between the Centre and the state. At 18%, that’s 9% CGST (Central GST) and 9% SGST (State GST). Union territories without a legislature use UTGST instead of SGST.
- Between states (inter-state): the full GST is charged as IGST (Integrated GST), for example 18% IGST.
The total tax is the same either way; only the split changes.
The current slab structure
GST rates were restructured with effect from 22 September 2025. Most goods and services now fall in a 5% or 18% slab, with a 40% rate for a small set of luxury and “sin” goods. Some items are exempt, and a few have special rates. The earlier 12% and 28% slabs were folded into this structure.
Rates for particular goods and services can change at GST Council meetings, and the correct rate depends on the exact classification (HSN or SAC code) of what you’re selling. OIOA’s calculator does the arithmetic; choosing the right rate for an item is up to you, so check it against the official rate list. For background, read GST calculation basics.
Rounding
GST amounts are usually rounded to the nearest paisa, and invoice totals are often rounded to the nearest rupee. If you’re preparing an invoice, follow the rounding your accounting software or tax professional uses, and show CGST and SGST (or IGST) separately.
Works offline
Like everything else in OIOA that doesn’t involve other people, the GST calculator runs entirely on your phone. It works in a shop with poor signal, on a train, or anywhere else. See offline use.
Indicative results
The calculator does the arithmetic accurately, but tax rules are detailed and change from time to time. Treat results as indicative, and check them against official sources (the GST portal and CBIC notifications) or with a tax professional before filing returns or issuing invoices. The disclaimer explains this in full.
Quick reference
| You know | You want | Do this |
|---|---|---|
| Price before tax | GST amount | price × rate |
| Price before tax | Total | price × (1 + rate) |
| Total including GST | Price before tax | total ÷ (1 + rate) |
| Total including GST | GST amount | total − total ÷ (1 + rate) |
| GST amount (intra-state) | CGST and SGST | each is half the GST amount |
If something about the calculator could be clearer, or you’d like it to do more, please tell us.
Common questions
How do I find the GST inside a price that already includes tax?
Divide the price by (1 + rate). For 18%, divide by 1.18; the difference is the GST. For example, ₹1,180 ÷ 1.18 = ₹1,000, so the GST is ₹180.
Is 18% GST on ₹1,000 equal to ₹180?
Yes. 1,000 × 0.18 = 180, so the total is ₹1,180.
When do I show CGST and SGST rather than IGST?
Within one state, split the GST equally into CGST and SGST. For sales between states, charge the full amount as IGST. The total tax is the same either way.
Can I use the calculator for filing returns?
Use it for quick checks. For returns and invoices, rely on your accounting software and a tax professional, and confirm current rates on the official GST portal. The results are indicative, as the disclaimer explains.
Does the calculator need the internet?
No. It runs entirely on your phone.