We believe in complete transparency. Below, you'll find the exact formulas and methodology our calculators use. Every formula is sourced from official tax authorities and standard accounting practices. No black boxes, no guesswork—just math.
🇮🇳 GST Calculator (India)
India's Goods and Services Tax (GST) is a multi-rate system with slabs at 5%, 12%, 18%, and 28%. Our calculator supports both inclusive and exclusive GST calculations and automatically splits the result into CGST (Central) and SGST (State) components.
Formulas Used
Exclusive GST (adding GST to a base price):
GST Amount = Base Amount × (GST Rate / 100)
Total = Base Amount + GST Amount
CGST = GST Amount / 2
SGST = GST Amount / 2Inclusive GST (extracting GST from a total):
Base Amount = Total Amount / (1 + GST Rate / 100)
GST Amount = Total Amount − Base Amount
CGST = GST Amount / 2
SGST = GST Amount / 2Source: GST Council of India (gstcouncil.gov.in). Rates are updated when the GST Council announces changes.
🇬🇧 VAT Calculator (UK)
The UK's Value Added Tax (VAT) standard rate is 20%. Our calculator lets you add VAT to a net amount or remove VAT from a gross amount, with support for custom rates.
Formulas Used
Add VAT:
VAT Amount = Net Amount × (VAT Rate / 100)
Gross Total = Net Amount + VAT AmountRemove VAT:
Net Amount = Gross Amount / (1 + VAT Rate / 100)
VAT Amount = Gross Amount − Net AmountSource: HM Revenue & Customs (gov.uk/vat-rates).
🇺🇸 Sales Tax Calculator (USA)
Sales tax in the United States varies by state, county, and sometimes city. Our calculator lets you input your specific local tax rate to get accurate results.
Formulas Used
Add Tax:
Tax Amount = Subtotal × (Tax Rate / 100)
Total = Subtotal + Tax AmountRemove Tax:
Subtotal = Total / (1 + Tax Rate / 100)
Tax Amount = Total − SubtotalSource: State-level tax rates are referenced from official state revenue departments and the Tax Foundation.
→ Try our Sales Tax Calculator
📊 Profit Margin Calculator
Profit margin is one of the most important metrics for any business. It tells you what percentage of your revenue is actual profit after costs.
Formula Used
Profit = Revenue − Cost
Profit Margin (%) = (Profit / Revenue) × 100This is the standard net profit margin formula used by accountants and financial analysts worldwide.
→ Try our Profit Margin Calculator
⚖️ Break-Even Calculator
The break-even point tells you exactly how many units you need to sell before your business starts making a profit. It's critical for pricing strategy and business planning.
Formula Used
Break-Even Point (units) = Fixed Costs / (Selling Price per Unit − Variable Cost per Unit)
Break-Even Revenue = Break-Even Units × Selling Price per Unit→ Try our Break-Even Calculator
💰 Markup Calculator
Markup is the amount added to the cost price to arrive at the selling price. Understanding markup helps you set competitive yet profitable prices.
Formula Used
Markup Amount = Cost × (Markup % / 100)
Selling Price = Cost + Markup Amount🏷️ Discount Calculator
Whether you're running a sale or negotiating a deal, our discount calculator shows you the exact savings and final price.
Formula Used
Discount Amount = Original Price × (Discount % / 100)
Final Price = Original Price − Discount AmountOur Accuracy Commitment
Every calculator on Rapid Invoice is tested with real-world scenarios before deployment. We cross-verify results against manual calculations and trusted accounting software. If you ever find a discrepancy, please report it to us—we take accuracy very seriously.
Important: Our calculators provide estimates for educational purposes. For official tax filings and complex financial decisions, please consult a qualified accountant or tax professional.
Last reviewed: February 2026