Methodology and data sources
How the calculators work and where every number comes from
Two kinds of calculation
Projections apply one fixed annual return to your starting amount and monthly contributions, compounded monthly. This is the standard compound interest formula. Real returns change every year, so a projection shows what a steady average would produce, not what will happen. The S&P 500 calculator starts at 10% because the index returned 10.0% a year from 1928 to 2025 with dividends reinvested.
Historical backtests replay what actually happened. They apply each calendar year's real return in order, crashes included. The data is annual, so each year's return is spread evenly across its twelve months; results can differ slightly from a calculation that uses daily prices.
Data sources
- S&P 500, bonds, T-bills, gold and inflation, 1928–2025: Aswath Damodaran, NYU Stern, "Historical Returns on Stocks, Bonds and Bills". Inflation is the FRED CPI series included in that file. Retrieved 2026-09-27.
- Nasdaq-100, 1987–2025: Nasdaq via FRED (series NASDAQ100). Price returns only; the index's dividend yield has been under 1%.
- ETF fees, yields and distributions, from each issuer's own page, retrieved 2026-09-27:
How the numbers are checked
Tables on calculator pages and in guides are computed from the datasets above when the page is built, not typed by hand, so a data update changes them everywhere at once. Automated tests check the computed averages against the figures Damodaran publishes in the same file, and spot-check the Nasdaq-100 series against calendar-year returns published by Nasdaq.
The datasets are refreshed once a year, after each full calendar year of data is published. Fees and yields are dated where they appear.
Limits
Past returns don't predict future ones. Results are before taxes unless you choose an account type, and before fees unless you enter them. Wealth Sim is an educational tool, not financial advice; see the disclaimer.
Who is responsible
Wealth Sim is built and maintained by Oscar Gallego Ruiz. If you find a wrong number, please report it; corrections are made on the page and the date below is updated.
Last reviewed: