GST Invoice Calculator: CGST, SGST and IGST Split
Most GST mistakes on a sales invoice are arithmetic or tax-head mistakes: tax on the wrong base, CGST and SGST used for an interstate sale, or a round-off that does not match the return. This calculator shows every line so you can check what Tally Prime, Busy, Marg or Zoho Books produced.
Your numbers
Discount shown on the invoice reduces the taxable value.
Rate slabs changed in 2025. Confirm the rate for your HSN code before issuing the invoice.
Only for goods that still carry cess, on the taxable value.
Your results
Check the GST rate and place-of-supply rules for your goods and services. This is a calculation aid, not tax advice.
Invoice value vs trade discount
Sweeps Trade discount from 0 to 30, holding your other inputs fixed.
Automate invoice tax checks
Want invoice tax heads checked against place of supply automatically from your ERP data? Request an optional expert review.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter the quantity and the pre-tax rate, then the trade discount that appears on the invoice. Pick the GST rate for the HSN or SAC code and whether the place of supply is the same state as the supplier. The tool returns taxable value, the tax split and the rounded invoice total. Compare each figure with the printed invoice or the voucher in your accounting package. A one-rupee gap usually comes from round-off, anything larger points to a rate or base error.
The formula behind it
Taxable value is quantity times rate, less any discount shown on the invoice. For an intrastate supply the GST rate is split in two equal halves: CGST and SGST each equal taxable value times rate divided by 200. For an interstate supply IGST equals taxable value times rate divided by 100. Cess, where it applies, is a percentage of taxable value. The invoice total is the sum of all of these, rounded to the nearest rupee.
- Discounts given after the sale through a credit note need separate treatment under section 15 conditions.
- Freight and packing charged to the buyer usually form part of the taxable value.
- Exports and SEZ supplies follow different rules for IGST and LUT.
Common mistakes
The usual errors are applying the discount after tax, using CGST and SGST for a buyer in another state, using an outdated rate after a slab change, and letting the software round each line separately so the invoice total differs from the sum. Also check that the buyer GSTIN state code matches the place of supply you selected. When these differ, your GSTR-1 will not reconcile with the e-invoice portal data.
Frequently Asked Questions
Is the discount applied before GST?
Yes, for a trade discount shown on the invoice. GST is charged on the transaction value after that discount. Post-sale discounts have separate conditions and are usually handled through a credit note linked to the original invoice.
Why does the tool round the invoice value?
Most Indian invoices show a rounded total in rupees with a small round-off line. The round-off is not taxed. The tool rounds the sum of taxable value and tax, which is how Tally and Busy usually present it.
Can I use it for services?
Yes. Enter quantity as 1 and the service fee as the rate, then choose the SAC rate. Place of supply rules for services differ from goods, so confirm the state before choosing CGST/SGST or IGST.
Does it handle reverse charge?
No. Under reverse charge the buyer pays the tax directly and the supplier invoice shows no tax. Use the tool with a rate of 0 for the supplier view, and compute the liability separately.
If your invoices come from an ERP that mixes manual and automatic tax calculation, Netray can build a read-only check that flags invoices where tax heads do not match place of supply.
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