Calculator Methodology

Last updated: September 2026

Every RupeeVeda calculator uses deterministic, published financial mathematics — never an AI model or a hidden data feed — to turn your inputs into results. This page sets out the shared conventions and lists the formula each of our 19 calculators uses; each calculator page repeats its own formula alongside the tool.

Principles

  • Transparent: the formula is shown on every calculator page and summarised below.
  • Deterministic: the same inputs always give the same output; all math runs in your browser.
  • Tested: the code behind every formula is covered by an automated test suite that compares outputs with independently computed worked examples, run before each release.
  • Illustrative: results are estimates based on the assumptions you enter, not forecasts, offers or guarantees.

Shared conventions

  • Monthly rate i = annual rate ÷ 12 ÷ 100 unless a product compounds differently.
  • Instalments (SIP, step-up SIP, RD) are assumed at the start of each period; EMIs at the end of each month.
  • XIRR uses an actual/365 day count, matching spreadsheet behaviour.
  • Amounts are shown in Indian rupees rounded to the nearest rupee; internal calculations are not rounded until display.
  • Taxes, fees, exit loads, insurance and charges are excluded unless a calculator states otherwise.

Formula by calculator

  • SIP: FV = A × [(1 + i)n − 1] ÷ i × (1 + i) (future value of an annuity-due over n months).
  • Step-Up SIP: the SIP formula applied year by year, with the instalment raised by the step-up percentage after every 12 months.
  • Lumpsum: FV = P × (1 + r)t.
  • Goal planner: the SIP formula solved for A given a target FV, rate and horizon.
  • Retirement planner: expenses inflated to retirement age, corpus required for the withdrawal phase, then the SIP formula solved for the monthly saving needed after allowing for any existing corpus.
  • SWP: monthly simulation balance = balance × (1 + i) − withdrawal until exhausted; sustainable withdrawal reference = corpus × i.
  • Compound interest: A = P × (1 + r/n)n×t, extended period by period when recurring contributions are added.
  • Simple interest: SI = P × R × T ÷ 100.
  • CAGR: (End ÷ Start)1/years − 1.
  • XIRR: the rate r at which Σ CFk ÷ (1 + r)daysk/365 = 0, solved numerically (Newton–Raphson with a bracketed bisection fallback).
  • Inflation: future cost = today's cost × (1 + inflation)t; real value is the inverse.
  • FD: A = P × (1 + r/n)n×t with a selectable compounding frequency.
  • RD: each monthly instalment compounded quarterly for the months it remains invested, then summed.
  • PPF: annual compounding on the running balance, balance = (balance + contribution) × (1 + rate), using an editable rate assumption.
  • EMI, Home / Personal / Car loan EMI: EMI = P × i × (1 + i)n ÷ [(1 + i)n − 1] with a full reducing-balance amortisation schedule.
  • Loan prepayment: the amortisation schedule re-simulated with extra payments; interest and months saved are the difference between the two schedules.

Assumptions and limitations

All calculators hold the rate constant for the whole period, which real products rarely do: bank rates are reset on renewal, floating-rate loans move with the market, and equity returns vary every year. Results therefore illustrate a scenario rather than predict an outcome. They are designed for understanding and comparison — not as a substitute for figures confirmed by your bank, fund house or a qualified professional.

Default rates are editable assumptions

Every default rate is a starting point you can change. Market-linked defaults (such as the 12% used to seed the SIP calculator) are common illustrative figures, not expectations or promises. Rate- and rule-dependent defaults (for example PPF) are stored in a single configuration file with the date they were last verified against the official notification, and are labelled on the page as assumptions rather than current official rates. Verify any statutory figure with the primary source before relying on it.

How we test

Each formula has regression tests with known answers — for example the EMI on ₹10,00,000 at 8.5% for 240 months, a doubling over 5 years that must give a CAGR of 14.87%, or a ₹1,00,000 → ₹1,10,000 cash flow one year apart that must give a 10.00% XIRR. When a reader reports a calculation issue, we reproduce it, fix it and add a test so it cannot recur (see the Corrections Policy).