Indian GST Calculator (2025/2026: Add / Remove GST)

Mathematical Derivation & Variable Explanations: The Central Goods and Services Tax Act 2017 (CGST) and integrated state statutes establish a multi-stage destination-based consumption tax. Calculations partition into forward tax addition and backward (reverse) tax extraction.

• Forward GST Adding Formulation: Given a net base price $P_{\text{net}}$ and statutory tax percentage $R \in \{5\%, 12\%, 18\%, 28\%\}$:
  $$\text{GST Amount} = P_{\text{net}} \times \frac{R}{100}, \quad P_{\text{gross}} = P_{\text{net}} + \text{GST Amount} = P_{\text{net}} \times \left(1 + \frac{R}{100}\right)$$
• Reverse GST Extraction Formulation (Gross to Net): When recovering the net taxable price from a GST-inclusive invoice price $P_{\text{gross}}$:
  $$P_{\text{net}} = \frac{P_{\text{gross}}}{1 + \frac{R}{100}}, \quad \text{GST Amount} = P_{\text{gross}} - P_{\text{net}} = P_{\text{gross}} \times \left(\frac{R}{100 + R}\right)$$
• Intra-State vs Inter-State Partitioning: Under Section 7 and 8 of the IGST Act:
  $$\text{Intra-State:} \ \text{CGST} = \frac{\text{GST}}{2}, \ \text{SGST/UTGST} = \frac{\text{GST}}{2}; \quad \text{Inter-State:} \ \text{IGST} = \text{GST}$$
• Input Tax Credit (ITC) Mechanics: Under Section 16, registered businesses can deduct taxes paid on business purchases (input tax) from their tax liability on outward sales (output tax), preventing cascading tax-on-tax inflation.
• Input Tax Credit (ITC) & Reverse Charge Mechanism (RCM): Under Section 16(2) of the CGST Act, registered businesses can claim Input Tax Credit only if the tax invoice is uploaded by the supplier in GSTR-1 and auto-populated in GSTR-2B. Under Section 9(3) and 9(4), certain goods and services (such as Goods Transport Agency services, legal services by advocates, and director fees) attract Reverse Charge Mechanism (RCM), requiring the service recipient to pay GST directly to the exchequer. Furthermore, electronic invoicing (e-invoicing) is mandatory for entities exceeding ₹5 Crore annual aggregate turnover.
• Composition Scheme & GST Thresholds: Small traders and manufacturers with annual turnover up to ₹1.5 Crore (₹75 Lakh for special category northeastern states) can opt for the Composition Scheme, paying a flat 1% to 5% turnover tax without claiming ITC. The mandatory registration threshold under Section 22 is ₹40 Lakh for goods (₹20 Lakh in special category states) and ₹20 Lakh for service providers.