Tax data year: 2026 · Last reviewed September 2026
Data sources
- Federal income tax brackets and standard deduction (2026): IRS Revenue Procedure 2025-32.
- Retirement plan limits (2026): IRS announcement of Notice 2025-67.
- Social Security wage base (2026): $184,500, published by the Social Security Administration.
- State income tax classification and flat rates: Tax Foundation state income tax rates.
Formulas
Loan and mortgage payments
Payment = P × r ÷ (1 − (1 + r)−n), where P is the amount borrowed, r is the annual rate divided by 12 and n is the number of monthly payments. Schedules are built month by month: interest = balance × r, and principal = payment − interest. Extra payments reduce the balance immediately.
Compound interest
Each month: balance = balance × (1 + r) + deposit, where r is the monthly rate that adds up to your yearly return over twelve months, r = (1 + annual return)1/12 − 1. A 7% annual return therefore grows a balance by exactly 7% in a year. Deposits are made at the end of each month. The same rule applies to the savings goal and retirement calculators, and to an APY. Loan and credit card rates are different: an APR is a nominal yearly rate, so interest each month is the APR ÷ 12.
Federal income tax
Taxable income (income minus pre-tax deductions minus the standard deduction) is taxed in slices through the progressive brackets for your filing status.
Payroll taxes
Social Security: 6.2% of wages up to the wage base. Medicare: 1.45% of all wages, plus 0.9% of wages above $200,000 for withholding. Self-employed people pay both halves of Social Security (12.4%) and Medicare (2.9%) on 92.35% of net profit and may deduct half of that tax.
Credit card payoff
Interest each month = balance × APR ÷ 12. The payoff time also matches the closed-form result n = −ln(1 − B × r ÷ P) ÷ ln(1 + r).
Assumptions and limits
- Paycheck and self-employment estimates use the standard deduction and no tax credits.
- State tax is a flat percentage of wages. Graduated-rate states can be approximated with an effective rate you enter.
- Local (city or county) taxes and state payroll programs are not included.
- Investment and interest rates you enter are assumed constant. Real returns vary and are not guaranteed.
- The self-employment estimate does not include the qualified business income deduction.
How we test
The calculation code has an automated test suite. Tests check results against widely published examples, such as a $300,000 loan at 6% over 30 years costing $1,798.65 a month, and against independent mathematical identities, such as the closed-form payoff formula and the requirement that a fully paid loan ends at a zero balance. Tests also cover edge cases like zero interest, zero income and inputs above tax thresholds.
Updates and corrections
Tax figures are reviewed every year when the IRS publishes new inflation adjustments and whenever a law change affects a calculator. Each page shows when it was last updated. If you find an error, please contact us and we will investigate and correct it.