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How the numbers are made

Every formula, convention and source behind the calculators

A calculator is only as good as its conventions, and most of them are invisible. This page states ours — including the four corrections that changed answers on this site by a third or more.

Three rules that hold everywhere

  • No number is written into a page. Every formula lives in a tested module and every tax figure in a single source of truth with a citation. A rate cannot be right in one tool and stale in another, because there is only one of it.
  • Tests assert against the outside world, not against ourselves. Scheme rules, published bank and AMC illustrations, closed-form identities and the tax department’s own worked examples — not against whatever the code currently returns.
  • An unusable input shows “—”, never NaN. A calculator mid-edit should look unfinished, not broken, and never confidently wrong.

Tax figures are current for FY 2026-27 per Union Budget 2026 (no change to rates), verified 2026-08-13. Each rate-dependent tool carries its own “rates as of” strip so staleness is visible on the page rather than only here.

By tool family

Investment growth — SIP, lumpsum, compounding

FV = P × [((1 + i)ⁿ − 1) / i] × (1 + i) · FV = P × (1 + r)ⁿ

Conventions

  • A SIP instalment is invested at the start of the month and compounds monthly — the annuity-due convention AMFI and the AMCs use in their own illustrations.
  • A lumpsum compounds annually on the whole amount from day one.
  • Returns are the figure you enter, before exit load, expense ratio drag and capital gains tax. It should already be net of the fund’s expenses.
  • A step-up is applied on each anniversary, so each year’s instalments are compounded separately rather than through the level-SIP closed form.

What most calculators get wrong here

The portfolio tool solves a real money-weighted XIRR over the actual dated flows. A plain CAGR equals XIRR only when there was exactly one purchase; with later top-ups it overstates the return, and labelling it "XIRR" — as this tool once did — makes the error invisible.

SIP CalculatorLumpsum CalculatorCompound InterestStep-up SIPPortfolio CAGR

Deposits — FD, RD, PPF, EPF, NPS

A = P × (1 + r/n)^(n×t)

Conventions

  • An FD compounds quarterly by default, because that is what Indian banks actually do on a cumulative deposit. The effective annual yield is shown alongside the nominal rate — that is the number to compare across instruments.
  • A PPF year is (opening + deposit) × (1 + r), compounded annually, with the ₹1.5 lakh statutory cap enforced.
  • Interest rates are your input, not ours. EPFO declares its rate annually and small-savings rates are notified quarterly, so a projection at a single fixed rate is a scenario rather than a forecast.

What most calculators get wrong here

Two that matter. An RD compounds quarterly, not monthly like a SIP — modelling it monthly overstates the maturity value. And EPF diverts 8.33% of the employer’s share to the pension scheme, computed on wages capped at ₹15,000 rather than on actual basic; counting the whole employer 12% as EPF overstates a 33-year corpus by roughly a third.

Statutory figures from Employees’ Provident Fund Organisation, last verified . Source

FD CalculatorRD CalculatorPPF CalculatorEPF CalculatorNPS Calculator

Loans — EMI, prepayment, eligibility

EMI = P × i × (1 + i)ⁿ ÷ [(1 + i)ⁿ − 1]

Conventions

  • Reducing balance, the Indian standard: interest each month is charged on the balance still outstanding, so the interest share of a constant EMI falls over the tenure.
  • Processing fees, bundled insurance and GST on charges are excluded. Ask any lender for the annual percentage rate including all of them before comparing.
  • A "flat rate" is not comparable to a reducing-balance rate — the same number quoted flat is roughly twice as expensive.

Statutory figures from Reserve Bank of India, last verified . Source

Personal LoanCar LoanMortgage CalculatorPrepayment BenefitBuy vs Rent

Tax — income tax, capital gains, TDS, HRA

standard deduction → Chapter VI-A → slabs → 87A rebate (+ marginal relief) → surcharge (+ marginal relief) → 4% cess → round to ₹10

Conventions

  • The order above is the whole game. Applying the rebate after cess — a common shortcut — produces small persistent discrepancies against the department’s own worked examples.
  • Slab tables are keyed by financial year and a Budget change adds a new year rather than editing an old one, because a belated or revised return still has to compute the earlier year correctly.
  • Every figure is traced to a citation, and the AI tax assistant is fed from the same tables the calculators use, so the chat cannot drift from the arithmetic.
  • The governing statute matters for citations: the Income-tax Act, 2025 replaced the 1961 Act on 1 April 2026 and renumbers sections (80C→123, 80D→126, 87A→156), without changing any rate.

What most calculators get wrong here

Capital gains depend on the acquisition date, not only the holding period — Budget 2024 rewrote the rules from 23 July 2024 and three regimes coexist in any current return. And holding period is a date comparison, not a count of whole months: an asset held twelve months and a day is long-term, and rounding to completed months misclassifies exactly the boundary cases.

Income Tax CalculatorTax CalculatorLTCG / STCGCapital Gains TaxTax SavingTDS CalculatorHRA Calculator

Employment and retirement

gratuity = (last drawn basic + DA) × 15 × reckonable years ÷ 26

Conventions

  • Salary means basic plus dearness allowance, never CTC. Since basic is often 40–50% of CTC, using CTC roughly doubles the answer.
  • The divisor is 26 for an establishment covered by the Act and 30 outside it, and the rounding rule differs too.
  • Retirement projections are pre-tax and nominal unless the tool says otherwise; use the inflation calculator to restate a corpus in today’s purchasing power.

What most calculators get wrong here

Gratuity has a five-year qualifying period — waived only on death or disablement — and a part-year over six months rounds up inside the Act while only completed years count outside it. A calculator with no eligibility check quotes a confident figure to someone with three years’ service who will receive nothing.

Statutory figures from Ministry of Labour & Employment, last verified . Source

Gratuity CalculatorRetirement CalculatorRetirement CorpusLeave Encashment

What BullTimes will not do

Most Indian financial calculators are the top of a sales funnel: the number is real enough, and the point of it is the “invest now” button underneath. We do not have one, so this list is worth stating plainly.

  • Recommend a fund, a stock, an insurer, a bank or a broker. We name no product and rank none.
  • Take a commission, a referral fee or a placement payment from any financial institution. There is nothing to steer you towards, which is why nothing steers you.
  • Sell, rent or hand your inputs to anyone. Calculator inputs are not sent to our servers at all — they live in the page address, which is what makes a result shareable.
  • Present an assumption as a finding. Where a number depends on a rate you chose — expected return, inflation, a future interest rate — the tool says so.
  • Quote a rate we have not checked. If a figure has no recorded source and verification date, it does not appear.

And one thing we cannot do

None of this is financial advice, and none of it can be. Advice depends on your income, your obligations, your tax position and your appetite for a bad year — facts a calculator does not have. What these tools give you is arithmetic you can check, on assumptions you chose. The decision stays yours.

Found something wrong?

A stale rate or a bad convention is a real bug, and we would rather hear about it than have it sit there. Tell us what you were computing and what you expected.

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