Methodology

Every calculator on TryClearTallyis built on standard, publicly documented financial formulas — the same math used by lenders, financial institutions, and consumer finance regulators. Nothing here is a proprietary black box. This page explains the method behind each group of calculators, where the dated figures come from, and how we verify our own work.

How we check our own work

Two layers of automated tests run before anything ships. First, every calculator's math has a test suite whose expected values are derived by hand from the standard formula — never copied from the software — so the code is checked against the formula, not against itself. Second, the worked examples printed in our explainers are asserted against the calculators' own output, so a number quoted on a page can't quietly drift from what the tool actually returns. If you find a result that still looks wrong, tell us— accuracy reports are reviewed first.

Loan amortization

Used by the mortgage, auto loan, personal loan, mortgage payoff, and refinance calculators, and for the mortgage side of rent vs buy. The standard amortization formula gives the fixed monthly payment:

M = P × [r(1+r)ⁿ] / [(1+r)ⁿ− 1]

where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments. Each month, interest is charged on the remaining balance and the rest of the payment reduces principal — which is why early payments are mostly interest and late ones mostly principal. Extra payments go entirely to principal, shortening the schedule; the payoff and refinance calculators simply run this schedule twice and compare. The mechanics match the Consumer Financial Protection Bureau's mortgage resources.

Compound growth, investing & retirement

Used by the compound interest, investment return, savings goal, retirement, and 401(k) calculators. Balances compound monthly: each month the balance earns interest at the monthly rate, then that month's contribution is added, so contributions start compounding immediately. The investment return calculator additionally converts your nominal annual rate into an effective annual rate for the compounding frequency you choose; the savings goal calculator runs the same math in reverse, solving for the contribution that reaches a target by a date. We model a constant average return — no calculator can predict real market sequences, and we say so on those pages.

The 401(k) calculator caps employee contributions at the IRS elective-deferral limit ($24,500, plus a $8,000 catch-up from age 50 — 2026 IRS limits). Those figures live in a single dated configuration file updated when the IRS publishes new limits; see the IRS's 401(k) contribution limit guidance for the current rules.

Credit card & debt payoff

Each month, interest accrues on the remaining balance at APR ÷ 12, your payment covers that interest first, and the rest reduces the balance — repeated until zero. A payment that doesn't exceed the monthly interest charge never makes progress, and the calculator warns when that happens. The debt payoff calculator simulates several debts at once under a fixed budget: every debt gets its minimum, the leftover attacks one focus debt (smallest balance for snowball, highest APR for avalanche), and a cleared debt's payment rolls into the next. See the CFPB's credit card resources for how issuers calculate interest.

Home affordability & rent vs buy

The affordability calculator applies the classic 28/36 rule: housing costs capped at 28% of gross monthly income (front-end ratio) and all debt payments at 36% (back-end); the lower cap wins, and we solve algebraically for the largest loan whose payment — including property tax scaled to the home's value — fits under it. Rent vs buy compares netcost over your stay: total rent minus what your invested down payment would earn, against total ownership outflows (including closing and selling costs) minus the equity you'd walk away with. Both label their assumptions on the page.

Income tax, take-home pay & paychecks

Federal income tax applies the standard deduction for your filing status, then the progressive brackets. FICA is Social Security (6.2% up to the annual wage base) plus Medicare (1.45%, with a 0.9% surtax above a high-earner threshold); traditional 401(k) contributions reduce income tax but not FICA, while pre-tax health premiums reduce both. State tax uses each state's published bracket schedule or flat rate. Bracket figures, deductions, and wage bases come from a build-time dataset (2026 tax year (IRS figures)) sourced from the PayrollTax API and refreshed on a yearly schedule — nothing is fetched at runtime, and every tax page shows its tax year. Cross-check against the IRS's published rates and brackets and the SSA's contribution and benefit base. Country pages (UK, Canada, India, Australia) use each government's published rates for the tax year shown on the page. Paycheck conversion is plain arithmetic: hourly rate × hours × weeks, split into the standard US pay periods (52 weekly, 26 biweekly, 24 semi-monthly, 12 monthly), before taxes and deductions.

Live rate & market data

Where shown, default interest rates and the “Today's Rates” widget pull from the Federal Reserve Bank of St. Louis's FRED database and from Finnhub for market index and news data. This page re-fetches that same FRED data on every visit (cached for at most a day), so the figures below are live, not hand-typed:

• 30-year fixed mortgage average (MORTGAGE30US): 6.67%
• 15-year fixed mortgage average (MORTGAGE15US): 5.96%
• Effective federal funds rate (FEDFUNDS): 3.63%

Fetched August 18, 2026. See the Federal Reserve's open market operations page for how the federal funds rate is set. You can always edit any rate field on a calculator by hand.

Review process & how this content is made

Calculators and this page are reviewed by the ClearTally editorial team. Our calculator explainers and FAQs are drafted with AI assistance, then reviewed for accuracy against the standard formulas and sources cited on each page before publishing.Each calculator shows its own “last updated” date, and pages with dated data (tax years, contribution limits) label the year in the text. If a result here differs from your lender's figures, it's almost always fees, points, or underwriting details a calculator can't see — see our financial disclaimer— but if you think it's actually wrong, report itand we'll check it against the formula.