Methodology
Last updated: September 17, 2026
General Approach
All Math of Money calculators use standard financial formulas applied with full precision, rounding only displayed values to the nearest cent. We source all tax figures from official IRS, SSA, and state agency sources, updating annually for the current tax year (2026). Our goal is transparency: we show you the math, not black-box estimates.
Key Principles
Standard Formulas
We use industry-standard formulas taught in finance courses and used by professional software. For example, the standard amortization payment formula is:
M = P · r(1+r)^n / ((1+r)^n − 1)
where M is the monthly payment, P is principal, r is the monthly interest rate, and n is the number of payments. We never use shortcuts or approximations.
Full Precision, Rounded Display
Calculations are performed using full decimal precision (IEEE 754 floating-point arithmetic). Only the final displayed values are rounded to the nearest cent or appropriate decimal place. This prevents rounding errors from accumulating across periods.
Tax Data from Official Sources
We obtain tax brackets, deduction limits, contribution limits, and other figures directly from:
- IRS (Internal Revenue Service) — federal income tax, limits, and credits
- SSA (Social Security Administration) — wage base limits and benefits
- State tax agencies — for state-specific calculations
These sources are updated annually for the 2026 tax year. We cite sources in our code and in calculator documentation.
Assumptions and Limitations
Our calculators make reasonable assumptions for typical scenarios:
- Mortgage calculators assume fixed-rate loans with monthly compounding.
- Retirement calculators assume consistent contribution rates and historical average returns.
- Tax calculators estimate federal tax only; state and local taxes vary widely and may differ from estimates.
- Investment calculators use historical average returns; actual returns vary year to year.
No calculator accounts for life changes (job loss, major expenses, market crashes) or uncommon scenarios (foreign income, alternative minimum tax). Always consult a professional for personalized advice.
How to Report an Error
If you find an error in a calculation, formula, or tax figure, please contact us with:
- Which calculator and inputs led to the error
- What the calculator showed vs. what you expected
- A reference to the correct source (IRS publication, calculator comparison, etc.)
We review all reports and correct errors within 48 hours.
Calculators by Category
Retirement
- 401(k) Calculator — Contributions, employer match, and growth over time.
- Retirement Calculator — Will your savings last through retirement?
- Roth IRA Calculator — Tax-free growth, income limits, and Roth vs. Traditional.
- Social Security Benefits Calculator — Estimate your monthly benefit at different claiming ages.
- Required Minimum Distribution (RMD) Calculator — Your required minimum distribution this year and in future years.
Loans & Debt
- Loan Calculator — Monthly payment and total cost for any fixed-rate loan.
- Auto Loan Calculator — Car payment with trade-in, sales tax, fees, and rebates.
- Student Loan Calculator — Payoff timeline for one or more student loans.
- Credit Card & Debt Payoff Calculator — Avalanche vs. snowball payoff across multiple debts.
- Debt-to-Income Ratio Calculator — Front-end and back-end DTI against mortgage lending limits.
- Personal Loan Calculator — Monthly payment, true APR with origination fees, and total cost.
Savings & Investing
- Compound Interest Calculator — How savings grow with regular contributions.
- Investment Calculator — Projected growth of a lump sum or recurring investment.
- Savings Goal Calculator — What to save monthly to hit a target by a target date.
- CD Calculator — Certificate of deposit earnings, APY, taxes, and early-withdrawal penalties.
- Inflation Calculator — What a dollar from any year since 1913 is worth today, and future costs.
- Net Worth Calculator — Add up your assets and liabilities to see your net worth and debt-to-asset ratio.
- Currency Converter — Convert between 40+ currencies with live exchange rates.
Taxes & Income
- Paycheck Calculator — Take-home pay after federal, state, and FICA taxes.
- Federal Income Tax Calculator — Estimate your federal tax bill, marginal and effective rate.
- Salary to Hourly Calculator — Convert between annual salary, hourly, and per-paycheck pay.
- Budget Calculator — Split take-home pay into needs, wants, and savings with the 50/30/20 rule.