How we calculate

Our general approach

Every calculator uses standard, well-established financial formulas — the same time-value-of-money math used by lenders, planners, and finance textbooks. We favor clarity and reasonable defaults over false precision. Where a real-world outcome depends on factors we can't know (your exact credit terms, future market returns, tax situation), we say so and use transparent assumptions you can adjust.

Loans & mortgages

Monthly principal & interest uses the standard amortizing-loan formula:

M = P · [ r(1+r)ⁿ ] / [ (1+r)ⁿ − 1 ], where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments.

• Mortgage: we add property tax and insurance to estimate full PITI, and automatically include estimated PMI (~0.75% of the loan per year) when the down payment is under 20%. Extra and bi-weekly payments are simulated month-by-month to show real interest savings and payoff dates.
• Auto loan: we build the amount financed from vehicle price, destination, dealer discount, sales tax (on the taxable amount after trade-in), fees, trade-in equity, and down payment, then amortize it — including any extra monthly principal.
• Student loans: the Standard plan amortizes over your term; the Income-Driven estimate models a SAVE/PAYE-style plan at 10% of discretionary income (income above 150% of the federal poverty line) with forgiveness after 20 years. IDR figures are approximations of complex federal rules.

Retirement & income

• 401(k) / retirement: we grow your current balance and ongoing contributions (plus any employer match, capped at your stated limit) using monthly compounding at your assumed return. We also show an inflation-adjusted value and an estimated retirement income using the 4% rule. Markets are variable — actual returns will differ from any assumption.
• Annuity: payouts are derived from the present-value-of-an-annuity formula for your principal, rate, frequency, and payout period (fixed-period or life-expectancy based). Deferred annuities grow the principal tax-deferred before payouts begin. An optional annual fee is treated as a drag on the rate (subtracted from both the growth and payout phases). For a joint & survivor option we solve a two-phase payment — the full amount while both spouses are living (to the earlier life expectancy), then the chosen survivor percentage through the later life expectancy. We also flag the impact of inflation on fixed payments. These are educational estimates, not actuarial insurer quotes.

Debt & planning

• Debt payoff: we simulate both the avalanche (highest interest first) and snowball (smallest balance first) methods month-by-month with your extra payment, and compare total interest and time to debt-free.
• Home affordability: based on the lender-standard 28% front-end and 36% back-end DTI ratios, we solve for the maximum home price your income and debts support, and flag likely PMI.
• Rent vs. buy: we compare the net cost of buying (payments, tax, maintenance, minus home equity from appreciation) against renting (rent growth, minus investment growth on the money a renter would otherwise tie up), and find the breakeven year.
• Emergency fund: we total your essential monthly expenses, set 3/6/12-month targets, and project how long reaching your 6-month goal takes given your savings rate and HYSA interest.

Markets & crypto

Live prices, dividend yields, and the Fear & Greed index are pulled from third-party data providers and may be delayed, cached, or temporarily unavailable. Profit/loss, DCA, and portfolio tools apply standard arithmetic to the figures you enter or that we retrieve. Nothing here is a recommendation to buy or sell any asset.

Live data sources

Where we display live market figures, we source them from established financial-data providers:

• Dividend & stock prices/yields — Financial Modeling Prep.
• Cryptocurrency prices & market data — CoinGecko.
• Crypto "Fear & Greed" index — Alternative.me.
• Federal poverty guidelines (used in income-driven student-loan estimates) — U.S. Department of Health & Human Services.

Live data may be delayed, cached, or temporarily unavailable, and is provided for informational purposes only.

Assumptions & limitations

• Default rates and figures reflect general market averages at the time of writing and are fully editable — your results use your inputs.
• Tax treatment is simplified and does not account for your full personal tax situation.
• Poverty-line figures used in IDR estimates are based on recent U.S. federal guidelines and update periodically.
• Projections involving future returns or appreciation are illustrative — actual market performance will vary.

Found something off?

We take accuracy seriously. If a result looks wrong or an assumption seems off, please tell us at hello@capitalcalcs.com — we investigate every report.