Where the formulas come from
Every calculator implements the standard, published form of its calculation, the same equations used by banks, spreadsheets and textbooks:
- Loan payments and EMIs use the annuity formula
P·r·(1+r)ⁿ ÷ ((1+r)ⁿ − 1)with monthly compounding, which is how virtually all consumer loans are quoted. - SIPs use the annuity-due convention (deposits at the start of each month), matching Indian platforms; US-style savings contributions use ordinary annuities (deposits at period end), matching American calculators. Where the convention differs between markets, the page says which one it uses.
- Fixed deposits compound quarterly, per Indian banking convention; PPF compounds annually on yearly deposits.
- XIRR solves the same actual/365 equation as Excel's XIRR function, using Newton-Raphson with a bisection fallback.
- Calendar arithmetic uses pure date mathematics (no timezones, no DST), with the clamped month convention used by mainstream date calculators, a 29 February birthday is observed on 28 February in non-leap years.
- Health estimates use Mifflin-St Jeor for BMR and WHO thresholds for BMI, with their limitations stated on the page.
How results are verified
The mathematics lives in a small, isolated code library covered by an automated test suite, several hundred assertions that run before any change ships. The tests pin results to external references: published bank EMI tables (₹50 lakh at 8.5% for 20 years = ₹43,391), the standard US mortgage example ($300,000 at 6% for 30 years = $1,798.65), Excel's documented XIRR example (37.34%), SIP figures every Indian platform agrees on (₹5,000 monthly at 12% for 10 years ≈ ₹11.62 lakh), and edge cases like leap-year birthdays and zero-interest loans.
The same reference numbers appear in each page's worked examples, so you can check any calculator by hand against its own documentation, and against your bank's.
What runs where
Every calculator runs in your browser. Numbers you type are never transmitted, logged or stored; there are no accounts and no server-side computation. You can load a page, disconnect from the internet, and keep calculating.
The limits, stated plainly
- Projections are not predictions. A SIP projection at 12% shows what that assumption implies, not what markets will deliver. Real returns arrive unevenly, and sequence matters.
- Rules change. Tax slabs, PPF rates, GST classifications and overtime laws are revised. Pages state the convention or rate they assume and when it was last verified; for filings and legal decisions, confirm against the current official source.
- Estimates, not advice. These tools compute; they do not know your situation. For consequential decisions, use them to prepare questions for a qualified adviser, not to replace one.
Corrections
If any calculator disagrees with a primary source, your bank's schedule, Excel, an official notification, please report it with the inputs you used. Verified discrepancies are fixed with priority, and the page's verification date is updated when re-checked.