How Our Calculators Work
The formula, data source and assumptions behind every calculator on TopFund — nothing is a black box.
Every calculator on TopFund runs a standard, published financial formula — the same ones used by banks, financial planners and regulators. We don't use proprietary or hidden models. Below, for each calculator, we explain exactly how the number on screen is arrived at, so you can verify it yourself or understand what's really happening behind the inputs.
Data Sources & Assumptions
- Statutory rates — PPF interest, income tax slabs, HRA/gratuity rules, GST rates — are taken from official government notifications (Income Tax Department, EPFO, GST Council) and updated when they change.
- Return rates, inflation and other market inputs you enter (e.g. "12% expected return") are your own assumptions, not guarantees — mutual fund and market returns are never fixed and can be negative in any given year.
- The What-If calculator is the one exception: it uses actual historical NAV data from AMFI (via mfapi.in), not an assumed return rate.
- All compounding calculators use standard monthly or quarterly compounding conventions matching how banks and AMCs actually compute returns, not simplified annual approximations.
Why You Can Trust These Numbers
Standard, published formulas
Every formula below is the same one taught in finance textbooks and used by banks — nothing custom or opaque.
Reviewed against source data
Statutory figures are checked against AMFI, EPFO, Income Tax Department and GST Council publications — see our Editorial Policy.
No hidden assumptions
Every input you can't see is disclosed here — so a number never appears from a formula you can't check.
Estimates, not guarantees
Anywhere you enter an expected return or inflation rate, the output is a projection based on your assumption — actual results will differ.
Formula & Logic, By Calculator
SIP Calculator
Every month, you invest a fixed amount into a mutual fund. Each instalment buys units at that month's NAV, so you automatically buy more units when prices are low and fewer when prices are high (rupee-cost averaging). Over time, your invested amount and its returns compound together, which is why SIP corpus growth accelerates sharply in the later years.
Try the calculator →Lumpsum Calculator
You invest the entire amount on day one, and it starts compounding immediately at your assumed annual return rate. Unlike a SIP, there's no averaging of entry price — your entire investment grows (or falls) with the market from the very first day, so the entry timing matters more.
Try the calculator →Goal Planner Calculator
You enter your target amount, the number of years you have, and your expected return rate. The calculator works backward from the future value formula to solve for the exact monthly SIP that will reach that goal — optionally after inflating the goal's present-day cost to what it will actually cost in the future.
Try the calculator →CAGR Calculator
You enter your investment's starting value, ending value, and the number of years between them. CAGR smooths the entire journey — including any ups and downs along the way — into a single constant annual growth rate that would take you from the starting value to the ending value.
Try the calculator →EMI Calculator
Your loan amount, interest rate and tenure are plugged into the standard amortization formula to compute a fixed monthly instalment. Each EMI is split between interest and principal — in the early years most of it is interest, and that mix shifts toward principal as the loan matures.
Try the calculator →Rent vs Buy Calculator
The calculator projects total cost over your holding period for both paths: renting (rent + escalation + investing the down payment you save) versus buying (EMI + taxes + maintenance + property appreciation), then compares final net worth under each scenario.
Try the calculator →Life Insurance Calculator
Using the income-replacement method, the calculator multiplies your annual income by a standard cover multiple, adds your outstanding loans/liabilities, and subtracts your existing savings and investments — arriving at the term cover your family would need to stay financially secure without your income.
Try the calculator →Home Affordability Calculator
Based on the RBI-guided FOIR rule (your total EMIs shouldn't exceed ~40% of take-home pay), the calculator finds your maximum affordable EMI, converts that into a maximum loan amount at your chosen interest rate and tenure, then adds your down payment to give the total home price you can afford.
Try the calculator →Step-Up SIP Calculator
Like a regular SIP, but your monthly instalment increases by a fixed percentage every year (usually matching your expected salary increment). Later years contribute larger instalments, so total invested amount and final corpus both grow faster than a flat SIP of the same starting amount.
Try the calculator →SWP Calculator
You specify your invested corpus, a fixed withdrawal amount per interval, and an expected return rate. Each period, the withdrawal is deducted and the remaining balance continues to earn returns — the calculator projects month by month whether your corpus grows, holds steady, or depletes, and if so, when.
Try the calculator →Retirement Planning Calculator
You enter your current age, retirement age, monthly expenses and expected inflation. The calculator inflates your expenses to their future value at retirement, applies a standard corpus multiple (25-30x annual expenses) to size your target, then solves for the monthly SIP needed to reach it.
Try the calculator →PPF Calculator
Your annual PPF contribution earns interest at the current government-notified rate (7.1%), compounded annually, for the full 15-year lock-in. The calculator projects your year-by-year balance so you can see exactly how the maturity value builds up.
Try the calculator →Mutual Fund vs FD Calculator
The calculator compounds your investment amount in an equity mutual fund (taxed as LTCG only on redemption) and in an FD (taxed annually at your income slab) over the same period, then shows the post-tax final value and effective post-tax return for each side by side.
Try the calculator →FIRE Calculator India
The calculator inflates your current annual expenses to their value at your target FIRE age, applies a 25-30x corpus multiple (the inverse of a safe withdrawal rate), and solves for the monthly SIP needed to reach that number by your chosen early-retirement age — using India-specific inflation, not US defaults.
Try the calculator →Direct vs Regular Mutual Fund Calculator
The only real difference between a fund's direct and regular plan is the expense ratio (regular plans bake in distributor commission). The calculator compounds the same investment at both expense ratios over your holding period and shows the rupee gap that accumulates purely from that cost difference.
Try the calculator →FD Calculator
Your deposit amount is compounded at the bank's quoted annual interest rate, using the compounding frequency banks actually use (usually quarterly), for your chosen tenure — giving you the exact maturity amount and total interest earned.
Try the calculator →GST Calculator India
You enter a price and a GST rate, and the calculator either adds GST on top of a base price (exclusive pricing) or extracts the embedded GST from a total price (inclusive pricing) — both directions, using India's standard GST formula.
Try the calculator →RD Calculator
Each monthly instalment earns interest for the remaining time it stays deposited, compounded quarterly like a bank RD, and the calculator sums up every instalment's individual growth to arrive at the final maturity value.
Try the calculator →Gratuity Calculator
Under the Payment of Gratuity Act formula, the calculator multiplies 15/26 of your last drawn basic salary (plus DA) by your total years of service, giving the gratuity amount you're entitled to on leaving after 5+ years of continuous service.
Try the calculator →HRA Calculator
The calculator computes all three components of the HRA exemption rule — actual HRA received, rent paid minus 10% of basic salary, and 50%/40% of basic salary for metro/non-metro cities — and takes the lowest of the three as your tax-exempt HRA.
Try the calculator →Income Tax Calculator India
You enter your income and deductions, and the calculator applies both the new regime slabs (with the Section 87A rebate) and the old regime slabs (after standard deductions, 80C, HRA, home loan interest) side by side, so you can directly compare your final tax liability under each.
Try the calculator →NPS Calculator
Your monthly NPS contribution grows at your chosen blended equity/debt return rate until retirement. At 60, the calculator applies the standard 60% tax-free lump sum withdrawal and shows the remaining 40% that goes toward a mandatory annuity for your monthly pension.
Try the calculator →What If Calculator
You pick a real mutual fund, an investment type (SIP or lumpsum), and a start date. The calculator pulls that fund's actual historical NAV data (via AMFI/mfapi.in) and replays exactly what your money would be worth today — no assumed return rate, just real historical performance.
Try the calculator →Money Health Score
You answer a few questions about your emergency fund, savings rate, debts, insurance cover and retirement savings. Each area is scored against a standard financial-planning benchmark, and the five scores combine into a single Money Health Score out of 100.
Try the calculator →Personal Finance Timeline
You toggle on the life goals that apply to you (emergency fund, marriage, home, education, retirement) with a target year for each. The calculator computes the monthly SIP required for every enabled goal independently, then adds them up into one combined monthly total.
Try the calculator →Inflation Calculator
You enter today's cost of something and an assumed annual inflation rate, and the calculator projects what that same cost will be after your chosen number of years — or works in reverse, showing what a future rupee is worth in today's purchasing power.
Try the calculator →Loan Prepayment Calculator
You enter your existing loan details plus a one-time or recurring extra payment. The calculator re-runs the amortization schedule with that prepayment applied entirely to principal, and shows exactly how much interest you save and how many months earlier the loan closes.
Try the calculator →STP Calculator
Your lumpsum sits in a source fund (usually debt/liquid) and a fixed amount is transferred out at regular intervals into a destination equity fund. The calculator simulates both sides — the source fund depleting and the destination fund's SIP-style growth — to project your combined position at the end of the transfer period.
Try the calculator →XIRR Calculator
You list every cash flow (each SIP instalment as an outflow, your current value as an inflow) with its actual date. The calculator solves iteratively (Newton-Raphson) for the single annualized rate that makes the net present value of all those irregular, dated cash flows equal to zero.
Try the calculator →Limitations to Understand
- •Calculators that project future returns assume a constant annual rate — real markets are volatile, and actual year-to-year returns fluctuate.
- •Calculators do not account for taxes unless explicitly stated (e.g. the MF vs FD and Direct vs Regular calculators do include tax).
- •Statutory calculators (HRA, gratuity, income tax, GST) reflect rules as of the "Last updated" date on each page — always confirm current rules for a filing decision.
- •These tools are for planning and education, not personalised financial advice. See our Disclaimer.