Methodology

How the numbers are made.

The formulas, assumptions and testing behind every tool on the desk

Every FinanceOneDesk calculator runs on standard, textbook financial mathematics — the same formulas used by banks, actuaries and academic finance. Nothing is proprietary or hidden, and this page documents it: which methods each collection uses, the assumptions baked in, how we test the results, and — just as important — what the tools deliberately leave out. If you ever find a number that looks wrong, this page tells you exactly what it should be doing, and the contact address is always open for corrections.

The core engine: compound growth

Almost every projection on the site is built on compound interest, applied monthly. A balance B with monthly return i (the annual rate ÷ 12) and a monthly contribution c evolves as B → B·(1+i) + c, repeated month by month. Where a closed-form answer exists we use or verify against it: a lump sum grows to P·(1+i)ⁿ, and a stream of contributions to c·[(1+i)ⁿ−1]/i — the standard future-value-of-annuity formula. This drives the compound interest, investment, savings-goal, pension, net-worth and FIRE-family tools.

Loans and mortgages: amortization

Loan tools use the standard annuity payment formula: for principal L, monthly rate i and n months, the payment is L·i / (1−(1+i)⁻ⁿ). Each simulated month, interest accrues on the outstanding balance and the remainder of the payment reduces principal — which is why early payments are interest-heavy and overpayments made early save the most. The affordability tool inverts the same formula (payment → maximum loan), and applies the classic 28/36 underwriting rules: housing costs capped at 28% of gross monthly income, total debt service at 36%. The debt-payoff planner simulates every debt in parallel, month by month, rolling each cleared payment onto the next target (snowball: smallest balance first; avalanche: highest APR first).

Retirement and drawdown: the 4% framework

Retirement tools use the safe-withdrawal-rate framework from the "Trinity study" line of research: a retirement pot roughly equal to annual spending ÷ withdrawal rate (25× spending at the classic 4%). Drawdown simulations ("will my money last?") apply monthly growth, subtract inflation-adjusted spending, and report when — if ever — the pot empties. Where we show real (after-inflation) values, we deflate by compounding the inflation rate: a value V in t years is worth V/(1+inf)ᵗ today. We default to conservative long-run assumptions and let you change them.

Comparisons: fair-test design

Comparison tools are built to avoid loading the dice. The rent-vs-buy model gives both sides the same monthly budget — whoever pays less invests the difference, the renter invests the deposit and buying costs from day one, and the owner's equity is always counted net of selling costs. The pension-match tool runs the identical career twice, with and without the employer's contribution, so the gap is purely the match and its compounding. Insurance tools use the DIME framework (Debt, Income, Mortgage, Education) and present-value annuity maths for income replacement.

How we test

• Closed-form verification. Every simulation engine is checked against the exact algebraic answer where one exists — future values, annuity payments, remaining-balance formulas, required-monthly solvers. Deviations beyond rounding fail the check.
• Independent reimplementation. Engines are re-coded from the written specification in a second language and the outputs compared, so a bug in the site's code can't silently agree with itself.
• Property tests. We assert relationships that must always hold: the avalanche method never pays more interest than the snowball; a higher fee never leaves you richer; reaching a target implies the solver's monthly amount actually reaches it.
• Worked-example audits. Every example quoted in a tool's text is generated from the live engine, not typed by hand — if the model changes, the copy is re-checked against it.

Standing assumptions — and honest limits

Simplifications are what make a calculator usable, but you should know what they are. Unless a tool says otherwise: returns are steady (real markets swing year to year, and the sequence of returns matters in drawdown); inflation is constant; contributions happen monthly without gaps; and — critically — most taxes, product fees and country-specific rules are excluded, because they vary too much by jurisdiction to model honestly in a generic tool. The capital-gains tool asks for your rate and allowance for exactly this reason. Treat every output as a well-built estimate of direction and magnitude — a way to compare choices and see trade-offs — not as a prediction or a guarantee, and confirm big decisions with a qualified professional who knows your situation. See our terms for the formal version.

Currencies, rounding and display

The currency switcher ($, €, £) changes the symbol only — no exchange-rate conversion is applied, since every figure is yours. Displayed values are rounded for readability; calculations run at full floating-point precision underneath, so schedules and totals may differ from hand-rounded arithmetic by a cent or two.

Updates and corrections

Tools are re-audited whenever their engine changes, and this page is updated when methods do. Found something that looks off? Email financeonedesk@outlook.com with the inputs you used — verified corrections ship quickly, and we'd rather fix an error than defend one.