Where our numbers come from

Two kinds of calculator

• History replays (backtests). The DCA, asset comparison, lump sum, buy-the-dip, worst-timing, Bitcoin, gold and inflation calculators, and every comparison page, use real monthly prices. They answer "what would have happened", not "what will happen".
• Planning math. Compound interest, FIRE, retirement and everyday-money calculators use the return, inflation and amounts you enter. They show what a steady assumption produces; real markets never move in a straight line.

The data

All series are monthly. Data runs through the latest month each source has published.

What each line measures

• S&P 500: S&P 500 monthly average level with dividends reinvested (an index fund, before its small fee).
• Gold: gold monthly average price in US dollars per ounce.
• Bitcoin: Bitcoin monthly average of daily closing prices on Coinbase, in US dollars.
• Real estate: S&P CoreLogic Case-Shiller U.S. National Home Price Index: home prices only, without rent, mortgage, upkeep or taxes.
• Bonds: total return of 10-year US Treasury bonds (interest plus price changes), modelled from the monthly 10-year yield.
• Ethereum: Ethereum monthly average of daily closing prices on Coinbase, in US dollars (staking rewards not included).
• T-bills: interest on 3-month US Treasury bills, a stand-in for cash in a money market fund.
• Oil: WTI crude oil monthly average spot price per barrel (oil funds that hold futures can return quite differently).

How a backtest works

1. One buy per month at that month's average price. Real buys on a given day land a little higher or lower; over years the difference evens out.
2. Income is reinvested where the asset pays it: S&P 500 dividends (you can switch this off), bond interest and T-bill interest. Gold, oil, crypto and home prices are price only.
3. Values at the end month use that month's average price.
4. Yearly return is the money-weighted return for monthly buys (it accounts for each deposit's timing) and the compound annual growth rate for a single lump sum.
5. Worst drop is the largest fall from a previous high, measured on monthly averages, so the worst single days were a little deeper.
6. No fees or taxes are included. Index funds charge small yearly fees, crypto apps often charge more per buy, and taxes depend on your account.

When the data updates

An automatic job checks every source on the 3rd and 17th of each month and publishes new months as soon as they appear. It refuses data that looks broken (missing months, impossible prices) and alerts us if a source goes quiet for more than four months. Short gaps of up to two months, such as the October 2025 CPI release that was never published, are estimated from the months around them and listed in the data.

Data last updated: Oct 2026.

Known limits

• Past returns do not predict future returns. A good period for one asset can be followed by a long bad one.
• Monthly averages smooth out the extremes of daily prices.
• Home prices exclude rent, mortgages, upkeep and property taxes, so they understate what owning a rental can earn and what it costs.
• Oil is the spot price; oil funds that hold futures can perform very differently.
• Bitcoin and Ethereum history is short (since 2015 and 2016) and extremely volatile.

Spotted a mistake?

We fix errors quickly. Write to bd@fatdca.com with the page and what looks wrong.

FatDCA is educational, not investment, tax or legal advice. See our terms.