India GST Guide: Slabs, Mathematical Formulas, CGST/SGST Splits & Input Tax Credit
To add GST to a base price: GST Amount = (Base Price * GST%) / 100, and Gross Total = Base Price + GST Amount. To remove GST from an inclusive price: Base Price = Gross Price / (1 + GST% / 100), and the GST portion is Gross Price - Base Price.
The Goods and Services Tax (GST) is a unified, multi-stage tax system introduced in India in 2017. It replaced multiple indirect taxes such as excise, service tax, VAT, and luxury tax under a single comprehensive framework.
Tax Slabs in India: CGST, SGST, and IGST
GST rates are decided by the GST Council. Standard tax slabs are 5%, 18%, and a 40% rate on luxury goods (with 0% nil/exempt and 3% gold). Since 22 September 2025 the old 12% slab is gone and 28% applies only to a few tobacco products. Transactions are split based on location:
- Intra-State Transactions (Within State): Tax is split equally into CGST (Central GST) and SGST (State GST). For example, an 18% GST rate translates to 9% CGST and 9% SGST.
- Inter-State Transactions (Across States): Tax is levied as IGST (Integrated GST), which goes to the central pool before allocation.
The Mathematical Formulas
For tax exclusive calculation (adding tax):
- GST Amount = (Base Amount * GST%) / 100
- Total Price = Base Amount + GST Amount
For tax inclusive calculation (removing tax):
- Base Price = Total Price / (1 + GST% / 100)
- GST Portion = Total Price - Base Price
Worked Example: 18% GST Exclusive & Inclusive
Exclusive: Base Amount = ₹10,000, GST = 18%. GST Amount = (10,000 * 18) / 100 = ₹1,800. Gross Total = ₹11,800.
Inclusive: Gross Total = ₹11,800, GST = 18%. Base Price = 11,800 / (1 + 0.18) = 11,800 / 1.18 = ₹10,000. GST Portion = ₹1,800.
Slab-Specific GST Calculators
Use dedicated slab calculators with worked exclusive and inclusive formulas under current GST 2.0 rates (effective 22 Sept 2025):
- [5% GST Calculator](/tools/gst-calculator/gst-calculator-5-percent): Calculate essential commodities, packaged foods, and transport services.
- [18% GST Calculator](/tools/gst-calculator/gst-calculator-18-percent): Calculate standard commercial goods, software, consulting, and banking charges.
- [GST on Services in India](/guides/gst-on-services-india): Classification guide covering SAC codes, restaurant rules, hotel tariffs, and tax invoice compliance.
- [40% GST Calculator](/tools/gst-calculator/gst-calculator-40-percent): Calculate luxury, sin-tax, and aerated/demerit goods under the 2025 GST 2.0 slab.
Frequently asked questions
What is Input Tax Credit (ITC)?+
ITC allows businesses to deduct the tax they paid on inputs/purchases from the tax they owe on output sales, preventing tax cascading.
Who must register for GST?+
Businesses with annual turnovers exceeding ₹40 Lakhs (goods) or ₹20 Lakhs (services) are generally required to register for GST.
Is there a penalty for incorrect GST calculations?+
Yes. Emitting incorrect tax invoices or under-reporting tax liabilities can lead to interest penalties of 18% and audits under Section 74 of the GST Act.
Prashant Sharma · Founder & Technology Strategist
Prashant Sharma is the founder and technology strategist of Zynocode IT Solutions, specializing in web development, mobile applications, and generative AI workflows.