Our principles
- Standard formulas only. We use the same textbook formulas banks, lenders and payroll systems use. No proprietary “magic” numbers.
- Official data. Tax rates and limits come from the IRS, the Social Security Administration and each state's revenue department. Each calculator page links to its sources.
- Clear assumptions. Where we simplify (for example, ignoring local city taxes), we say so on the page.
- Your data stays with you. Calculations run in your browser. We never receive the numbers you type.
Loans: mortgages, auto loans and credit cards
The regular monthly payment uses the amortization formula M = P × r / (1 − (1 + r)^−n), where P is the amount borrowed, r is the annual rate ÷ 12, and n is the number of monthly payments. We then simulate the loan month by month: interest for the month is balance × r, your payment (plus any extra) is applied, and the remainder reduces the balance. Payoff dates, total interest and schedules all come from that simulation.
Credit card minimum payments are modeled as 1% of the balance plus that month's interest, with a $25 floor — a common issuer formula. Your card's exact formula may differ.
Debt payoff: snowball and avalanche
Your monthly budget is the sum of every minimum payment plus your extra amount, and it stays fixed. Each month every debt gets its minimum, and everything left over goes to the priority debt: the smallest balance (snowball) or the highest rate (avalanche). When a debt is paid off, its minimum rolls into the next target. “Minimums only” pays each debt its own minimum with no rollover, for comparison.
Take-home pay
We apply the 2026 federal income tax brackets and standard deduction published by the IRS to your income after pre-tax deductions, then add Social Security (6.2% up to the annual wage base), Medicare (1.45%, plus 0.9% above the additional Medicare threshold) and your state's income tax using its published brackets, deductions, exemptions and credits. Employee-paid state payroll taxes such as disability or paid-family-leave insurance are included where they apply.
We don't model itemized deductions, dependents and tax credits, local or city income taxes, or your specific W-4 choices, so your actual withholding can differ. The result is an annual estimate divided evenly across your pay periods.
Savings and compound interest
Growth is calculated monthly. Your chosen compounding frequency is converted to an exact equivalent monthly growth rate, (1 + rate ÷ n)^(n ÷ 12), so daily, monthly, quarterly and yearly compounding all give precise results. Deposits are added at the end of each month.
How often we update
We review tax figures each year when the IRS and states publish new numbers (usually October to January), and whenever laws change mid-year. Each calculator shows the date it was last updated. If you spot an error, please tell us — we fix confirmed errors quickly. Read more in our editorial policy.