Calculation Methodology
Last updated
All SteadyMath calculators run in your browser using one shared, open calculation library. This page summarizes how it works.
Shared conventions
- Loan payments use the standard amortization formula
M = P·r / (1 − (1 + r)−n)with a monthly rate r = annual rate ÷ 12. - Schedules are simulated month by month: interest = balance × r; the rest of the payment reduces principal.
- Compounding at other frequencies is converted to an equivalent monthly rate.
- Results are nominal (not inflation-adjusted) and pre-tax unless a page says otherwise.
Testing
Our automated test suite checks, among others: the $200,000 / 6% / 30-year payment of $1,199.10; $10,000 at 5% compounded annually for 10 years = $16,288.95; $100/month at 6% for 10 years = $16,387.93; 5% APR monthly = 5.116% APY; 2026 federal income tax on a $75,000 single salary ($7,670 under the standard deduction); the 2026 Social Security wage-base cap of $184,500; and the 2026 401(k) employee limit of $24,500. Tests run before every release.
2026 figures used
- Federal brackets and standard deduction ($16,100 single / $32,200 joint / $24,150 head of household): IRS.
- Social Security 6.2% up to $184,500; Medicare 1.45% plus 0.9% additional: IRS Publication 15.
- 401(k) deferral limit $24,500; catch-up $8,000 (ages 60–63: $11,250): IRS.
Calculator-specific notes
- Mortgage Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Understanding your Loan Estimate; CFPB: What is private mortgage insurance?; CFPB: What is a debt-to-income ratio?; CFPB: Buying a house — tools and resources.
- Compound Interest Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: Investor.gov (SEC): Compound interest calculator & explanation; FDIC: Deposit insurance.
- Auto Loan Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Auto loans — consumer tools.
- Credit Card Payoff Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Credit cards — consumer tools.
- Savings Goal Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: FDIC: Deposit insurance; CFPB: Regulation DD (Truth in Savings).
- 401(k) Retirement Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: IRS: 401(k) limit increases to $24,500 for 2026; IRS: Retirement topics — 401(k) contribution limits.
- Debt Snowball vs. Avalanche Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Credit cards — consumer tools.
- APR to APY Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Regulation DD (Truth in Savings); CFPB: Mortgage interest rate vs. APR.
- Inflation Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: U.S. Bureau of Labor Statistics: Consumer Price Index; Federal Reserve: Why does the Fed target 2% inflation?.
- Take-Home Pay Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: IRS: Tax year 2026 inflation adjustments; IRS Publication 15 (2026), Employer’s Tax Guide; IRS: Tax Withholding Estimator.
- Refinance Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Understanding your Loan Estimate; Federal Reserve: Selected interest rates (H.15).
- Rent vs. Buy Calculator: see the “Formula used” and “Limitations” sections on the page. Primary sources: CFPB: Buying a house — tools and resources; CFPB: What is a debt-to-income ratio?.
Known limitations
Calculators simplify reality: constant rates, no fees unless entered, and simplified tax treatment. The take-home pay tool estimates federal tax with the standard deduction only and a flat state rate. Always confirm with official documents such as a lender’s Loan Estimate or your employer’s payroll.