Educational tool. Not financial advice. Sources & methodology

Methodology & Data Sources

Every calculator on this site uses publicly available data and documented assumptions. This page explains where the numbers come from, what we assume, and where the limitations are. If you find an error or an outdated source, please report it.

How we calculate compound returns

The compound interest visualizer uses the standard compound interest formula: FV = PV(1 + r)^n + PMT × ((1 + r)^n - 1) / r, where FV is future value, PV is present value, r is the periodic return rate, n is the number of periods, and PMT is the periodic contribution.

The default annual return rate is 7%, which approximates the historical average real (inflation-adjusted) return of the S&P 500 over rolling 30-year periods. This is a commonly cited benchmark but is not a guarantee. Actual returns vary by asset allocation, time period, and market conditions. Source: SEC investor.gov.

Tax data

Federal income tax brackets are sourced from IRS Revenue Procedures published annually (typically October–November for the following tax year). Current data reflects the tax year noted in the calculator and our state-tax-2026.json source file.

State income tax brackets are sourced from each state’s revenue department or department of taxation. States with no income tax (Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, Wyoming) are handled as 0% state withholding. Data is updated annually when states publish new tables.

FICA withholding (Social Security 6.2%, Medicare 1.45%) uses current federal rates. The Social Security wage base is updated annually per IRS guidance.

Allowance norms

Age-based allowance data is sourced from the Till Financial annual report on family finances, which surveys thousands of U.S. families. We report the median and interquartile range when publicly available. Limitations: self-reported survey data with potential sampling bias toward digitally engaged families.

529 state benefits

State tax deduction and credit data for 529 contributions is sourced from individual state 529 program websites and cross-referenced with Savingforcollege.com. Benefits vary by state and change annually. We note the tax year for each state’s data. Not all states offer a tax benefit for 529 contributions.

Teen auto insurance estimates

Insurance cost ranges are sourced from industry actuarial surveys and consumer research (The Zebra, Policygenius, NAIC filings). Individual quotes vary significantly based on driving record, vehicle type, coverage level, location, and insurer. Our estimates are statewide medians for a teen driver added to a parent’s policy, not individual quotes.

When calculations are approximate

Always. Every calculator on this site produces estimates, not exact figures. Tax calculations do not account for deductions, credits, or filing status complexities beyond what the calculator explicitly models. Investment projections assume constant returns, which do not occur in real markets. Insurance and allowance figures are survey-based medians, not individual guarantees.

Each calculator includes an inline methodology note describing its specific assumptions and limitations. If a number matters for a real financial decision, verify it with a licensed professional.

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