Calculator Methodology

This page sets out the formulas and conventions behind our calculators so you can see exactly how each result is produced — and check it yourself. Every calculator is built as a pure calculation engine with its own automated tests, separate from the interface, which is how we keep results consistent across the site. For the standards and review behind this, see our editorial policy and review policy; for the rates and rules we rely on, see sources & references.

Loan EMI calculators

EMIs use the standard reducing-balance method, the same one banks and lenders use. The monthly instalment is:

EMI = P × r × (1 + r)ⁿ ÷ ((1 + r)ⁿ − 1)

where P is the principal, r is the monthly interest rate (annual rate ÷ 12 ÷ 100), and n is the tenure in months. The amortization schedule splits each instalment into interest (on the outstanding balance) and principal, so the principal column sums exactly to the loan amount. When the rate is zero, the EMI is simply principal ÷ months.

Investment calculators (SIP, Lumpsum, CAGR, SWP, XIRR)

• SIP uses the future value of a monthly annuity-due (each instalment compounds from the start of its month).
• Lumpsum uses annual compounding of a one-time investment.
• CAGR solves for the steady annual rate that links a start value to an end value over a whole number of years: (End ÷ Start)^(1 ÷ years) − 1.
• SWP is iterated month by month — the balance grows at the periodic rate, then the withdrawal is taken — so it correctly handles a corpus that runs out partway through.
• XIRR finds the annualised rate that sets the net present value of dated, irregular cash flows to zero, using the actual/365 day-count convention (the same as spreadsheet XIRR). It is solved numerically with Newton-Raphson and a bisection fallback for stability, and it reports when no valid rate exists rather than showing a misleading figure.

Savings calculators (FD, RD, PPF)

• FD uses quarterly compounding, the standard Indian fixed-deposit convention.
• RD treats each monthly deposit as compounding at the quarterly rate converted to an effective monthly rate.
• PPF uses annual compounding on yearly contributions (annuity-due), reflecting how PPF interest is credited.

Tax calculators (GST, Income Tax)

The GST calculator adds or removes GST at the statutory slabs and splits the tax into CGST + SGST (intra-state) or IGST (inter-state). The income tax calculator computes tax under both the old and new regimes and recommends the cheaper one: taxable income after deductions, then progressive slab tax, the Section 87A rebate (with marginal relief at the new- regime threshold), surcharge with marginal relief at the statutory thresholds, and a 4% health-and-education cess. Slabs, rebate limits and surcharge rates are versioned by financial year in a single typed data file, so a Budget change updates one reviewed source.

Rounding and assumptions

Calculators work in rupees and round results to the nearest rupee for display. They assume a constant interest rate over the period unless you change it, and they ignore product-specific costs such as processing fees, insurance, prepayment charges and taxes on returns unless a calculator states otherwise. Real figures from an institution can therefore differ slightly. Our results are estimates for planning, not an offer or a guarantee — see our disclaimer.

Testing

Each engine ships with unit tests that pin published reference values and guard the edge cases (zero rate, zero input, negative input, a corpus that depletes, cash flows with no solvable rate). Schedule tables are tested to reconcile with the headline result — the final year of a growth table equals the engine's maturity value, and a loan's principal column sums to the exact loan amount. If a change breaks any reference figure, the tests fail and the change doesn't ship.