US Calculator Methods, Formulas and Sources
This page brings together the formulas, primary sources, editable inputs and review dates used by the United States calculators.
Calculation basis
Federal rule-based calculators use the 2026 tax year and the effective dates shown on each page. Stable loan, payoff and compound-interest formulas use the values entered by the visitor.
| Source | Scope |
|---|---|
| IRS Revenue Procedure 2025-32 | 2026 federal tax brackets and standard deductions |
| IRS Publication 15 (2026) | Social Security and Medicare payroll rates and wage base |
| IRS General Instructions for Forms W-2 and W-3 (2026) | federal and FICA wage reporting for traditional salary deferrals |
| IRS Additional Medicare Tax | filing-status thresholds and employee rate |
| IRS — temporary individual deductions | qualified tips, overtime, eligible seniors and qualifying new-car loan interest excluded from this model |
| IRS Notice 2025-67 / 2026 retirement limits | 401(k) employee, catch-up and combined contribution limits |
| Freddie Mac Primary Mortgage Market Survey | dated national 30-year fixed mortgage reference |
| Consumer Financial Protection Bureau — total mortgage payment | principal, interest, tax, insurance and other housing costs |
| Federal Reserve G.19 Consumer Credit | dated auto-loan, personal-loan and credit-card references |
| Consumer Financial Protection Bureau — interest rate and APR | contract interest rate, covered fees and APR comparison |
| Federal Reserve Bank of St. Louis / FDIC — national savings rate | dated national savings-rate reference |
| Federal Reserve — longer-run inflation goal | editable inflation starting assumption |
| Tax Foundation — 2026 state and local sales taxes | national planning context for the editable combined rate |
Calculator methods and sources
Federal Income Tax Calculator 2026
Taxable income equals combined pre-deferral employment wages minus entered traditional 401(k)/403(b) deferrals and the selected 2026 standard or itemized deduction. Federal income tax uses the seven statutory marginal bands. Social Security and Medicare use the pre-deferral wages because those traditional salary deferrals remain part of their wage bases. IRS Revenue Procedure 2025-32: 2026 federal tax tables
US Mortgage Calculator
Principal and interest use fixed-rate monthly amortization. Property tax and homeowners insurance are divided by 12; mortgage insurance, HOA dues and extra principal are added monthly. Extra principal reduces the balance after scheduled interest and can shorten the payoff period. Freddie Mac: current Primary Mortgage Market Survey
Auto Loan Calculator
Sales tax equals the entered taxable amount multiplied by the local combined rate. Amount financed equals vehicle price minus down payment and trade-in credit, plus sales tax and financed fees. Fixed monthly amortization uses the entered annual contract interest rate. The dated Federal Reserve G.19 average APR provides the editable starting reference. Federal Reserve G.19: consumer credit terms
Personal Loan Calculator
The entered contract rate drives fixed monthly amortization. A financed fee joins principal before interest is calculated; a withheld fee reduces cash received and is added to total cost. Extra payments reduce principal and the final payment is limited to the amount due. Federal Reserve G.19: 24-month personal-loan rates
Credit Card Payoff Calculator
Each month adds balance × annual rate ÷ 12 and new charges, then subtracts the fixed payment. The final payment falls to the amount due. The comparison path uses the greater of $35 or monthly interest plus 1% of balance as an indicative minimum. Federal Reserve G.19: credit-card accounts assessed interest
401(k) Calculator 2026
Employee contribution is salary × contribution percentage, capped at compensation and the $24,500 elective-deferral limit plus the applicable catch-up. The general catch-up is $8,000 from age 50; ages 60–63 use $11,250. Employer match applies only to the entered match-eligible salary percentage and respects the modeled combined limit. IRS Notice 2025-67: 401(k) limits for 2026
Compound Interest Calculator
The entered effective annual rate is converted to an equivalent monthly rate. Growth is applied each month before the month-end contribution. Today’s value divides the future amount by the entered annual inflation factor over the selected years. Federal Reserve Bank of St. Louis / FDIC: national savings rate
US Sales Tax Calculator
For a pre-tax amount, tax equals price × rate and total equals price plus tax. For a tax-inclusive total, pre-tax price equals total ÷ (1 + rate) and tax is the difference. Tax Foundation: 2026 state and local sales-tax rates
Editable inputs
Interest rates, property costs, local taxes, taxable purchase amounts, employer matching rules and savings assumptions can be replaced with figures from a quote, contract, pay statement or local authority.
Review dates
- Federal tax, payroll and retirement limits are reviewed for every tax year.
- Market references are reviewed when their cited series publishes a newer observation.
- State or local rules are reviewed when the relevant jurisdiction changes a rate or tax base.
Last verified: 25 July 2026. Use the contact page to report a discrepancy.
Using the estimates
Use the breakdown to compare scenarios, then match the chosen inputs to the applicable tax form, Loan Estimate, account disclosure, plan document, purchase order or local tax record.