How FinTools4U calculations work

This page documents the mathematical assumptions behind each calculator so you can interpret results with appropriate caution. All calculation engines run in your browser; we do not receive your inputs for computing results.

Shared principles

Unless a tool says otherwise, results are nominal (no inflation adjustment), pre-tax, and fee-free. Charts may aggregate to yearly points for readability even when the engine steps monthly. Nothing here is financial, tax, or legal advice.

Compound interest

Monthly compounding with end-of-month contributions: each month, balance = balance × (1 + annualRate/12/100) + monthlyContribution. Yearly tables show end-of-year snapshots. Scenario B runs the same model with different inputs for comparison.

FIRE calculator

FIRE number = annualExpenses ÷ (safeWithdrawalRate/100). Portfolio growth uses the same monthly contribution model as compound interest, projected for up to 60 years or until the balance reaches the FIRE number.

Coast FIRE calculator

FIRE number uses the same SWR formula. Coast number = FIRE number ÷ (1 + r)^yearsToRetirement with annual compounding and zero future contributions. Projected balance at retirement = currentSavings × (1 + r)^years. This differs intentionally from the monthly FIRE growth model so the coast threshold stays a clean closed-form estimate.

Mortgage payoff

Standard fixed-rate amortization for principal and interest. Monthly payment uses the classic formula M = P × [r(1+r)^n] / [(1+r)^n − 1]. Extra payments reduce principal after interest is charged. Property taxes, insurance, HOA, and PMI are excluded.

Debt snowball vs avalanche

Each month, interest accrues on every balance, minimum payments are applied, then remaining extra payment targets one debt: lowest balance (snowball) or highest APR (avalanche). Paid-off minimums roll into the next target. Hybrid strategies are not simulated.

What we intentionally exclude

Investment fees and employer matches stay out of scope. Inflation is applied only on the inflation calculator, as a rate you type; other tools stay nominal unless they say otherwise. Taxes, insurance, and PMI appear only when a calculator says it includes them as flat monthly add-ons — they are not estimated from an address, and they do not change the loan balance. Always verify important decisions with a qualified professional and your own statements.

Mortgage payment, schedule, refinance, and Price vs SAC

The payment and amortization tools use standard fixed-rate (Price) amortization: M = P × [r(1+r)^n] / [(1+r)^n − 1], with r = annual rate / 12. Optional tax, insurance, and PMI are added on top of principal and interest and are excluded from the balance. Refinance compares the remaining balance on that same formula with a new rate, term, and closing costs. Break-even months = closing costs ÷ monthly savings, only when the new payment is lower. Price vs SAC is the Brazilian pair: Price is the level payment above; SAC amortizes principal in equal installments (P / n) and adds interest on the declining balance. Both use a nominal annual rate divided by 12. None of these tools is financial advice.

Savings goal, emergency fund, real return, and home affordability

Savings goal and the nominal path of the inflation calculator use the same monthly compounding as the compound-interest tool: each month the balance grows by the annual rate divided by 12, then the contribution is added. The required monthly deposit solves that future-value formula for the payment. The emergency fund multiplies essential spending by the months of cover you choose and counts months until the balance reaches that target. Real return uses the Fisher equation, (1 + nominal) / (1 + inflation) − 1. Purchasing power divides the nominal balance by (1 + inflation)^years. Contributions in that model stay in nominal currency. Home affordability caps the housing payment at the lower of the front-end ratio and the back-end ratio after other debts, subtracts the tax and insurance you enter, and inverts the fixed-rate payment formula to a loan amount. Price is that loan plus the down payment. None of these tools is financial advice.

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