↳ Methodology
How we build trustworthy tools
Every calculator on ToolBook uses published, peer-reviewed formulas with cited sources. We never approximate when an exact formula exists.
Our approach
We use the exact textbook or regulatory formula for every calculation. When a government body publishes the canonical method (RBI for deposits, CBDT for tax slabs, WHO for BMI), we implement that method and cite it below.
All calculation logic lives in pure TypeScript functions with no UI dependencies. The same math that renders on the web page can be unit-tested, audited, and reused independently.
We review formulas and source citations when regulations change (e.g., new tax slabs, updated PPF rates) and update the tool within one working week of official notification.
Formula sources by engine
EMI / Loan Amortization
Tools: EMI Calculator, Loan Repayment Calculator, Mortgage Calculator
EMI = P x r x (1+r)^n / ((1+r)^n - 1)
- Reserve Bank of India (RBI) Master Directions on Interest Rate
- Standard amortization formula used across US CFPB mortgage disclosures
SIP / Monthly Contribution Compounding
Tools: SIP Calculator, Compound Interest Calculator
FV = P x ((1+r)^n - 1) / r x (1+r)
- SEBI mutual fund compounding methodology
- Standard future value of annuity formula (Brealey, Myers & Allen, Principles of Corporate Finance)
Fixed Deposit
Tools: FD Calculator
A = P x (1 + r/n)^(n*t)
- RBI guidelines on calculation of interest on deposits
- Compound interest formula (standard banking practice)
Recurring Deposit
Tools: RD Calculator, Savings Goal Calculator
Quarterly compounding per RBI guidelines
- India Post Office RD interest calculation methodology
- RBI circular on recurring deposit compounding
PPF (Public Provident Fund)
Tools: PPF Calculator
Annual compounding at government-notified rate (currently 7.1% p.a.)
- Ministry of Finance, Government of India, PPF Scheme 2019
- Section 80C of Income Tax Act, 1961
India Income Tax
Tools: Income Tax Calculator
Slab-based progressive taxation
- Income Tax Act, 1961 (as amended by Finance Act 2025)
- CBDT notifications for FY 2025-26 and FY 2026-27 tax slabs
Gratuity
Tools: Gratuity Calculator
Gratuity = (Basic + DA) x 15/26 x Years of service
- Payment of Gratuity Act, 1972 (Section 4)
HRA Exemption
Tools: HRA Calculator
Minimum of: actual HRA, 50%/40% of salary, rent paid minus 10% of salary
- Section 10(13A) of Income Tax Act, 1961
- Rule 2A of Income Tax Rules
BMI
Tools: BMI Calculator
BMI = weight (kg) / height (m)^2
- World Health Organization (WHO) BMI classification
Calorie Estimation
Tools: Calorie Calculator
Mifflin-St Jeor equation for BMR, activity multipliers for TDEE
- Mifflin MD, St Jeor ST, et al. (1990). American Journal of Clinical Nutrition
Limitations
Our tools provide estimates based on the inputs you provide and the formulas cited above. They do not account for fees, taxes (unless the tool is specifically a tax calculator), or market fluctuations that affect real-world outcomes.
For financial decisions, we recommend consulting a qualified financial advisor who can assess your complete situation. See our disclaimer for full details.