About and method
Data, calculation method and limits.
The calculators replay 97 calendar years of US market returns and CPI inflation. This page explains the sources, the order of operations and what the results leave out.
What this site is
FatFIRE Math publishes free calculators and guides for people planning an early retirement with relatively high spending. The site focuses on two questions: how long it took to build a target under past returns, and how a portfolio supported withdrawals once retirement began.
Calculators and guides require no account. Calculations run in the browser, and the values entered in a calculator stay on the device. The optional annual community membership uses an account for its small-group chat.
The data
One row per calendar year from 1928 to 2024, holding four figures:
- Stocks — the total return, price plus reinvested dividends, of a broad US large-cap index.
- Bonds — the total return of a constant-maturity 10-year US Treasury.
- T-bills — the return on rolled 3-month US Treasury bills, standing in for cash.
- Inflation — December-to-December CPI-U.
The return series follow the long-run US market dataset published by NYU Stern (Damodaran); inflation follows the US Bureau of Labor Statistics. Both are public, widely used, and easy to check against.
How a calculation runs
Each historical replay follows these steps:
- Pick a starting year.
- Take that year's actual returns, weight them by your allocation, and deflate the result by that year's actual inflation. That is the year's real return.
- Add the year's saving, or subtract the year's spending, and apply the real return to what is left.
- Move to the next calendar year and repeat.
- Repeat the calculation from each other starting year with enough data.
Every chart reports values in today's money. A portfolio shown as $3M has the purchasing power of $3M today. Retirement withdrawals keep the same real value from year to year.
What the record says
Compounded across the whole 97-year record, after inflation, the four mixes the site offers returned:
| Allocation | Real return a year |
|---|---|
| All stocks | 6.7% |
| 80 / 20 stocks and bonds | 6.0% |
| 50 / 50 stocks and bonds | 4.6% |
| Cash and T-bills | 0.3% |
Those are compound annual growth rates across 1928–2024, after CPI inflation, rebalanced yearly and before any tax or fee.
Limits
- No forecast. Historical outcomes show what occurred in this dataset. They do not assign probabilities to future returns.
- No tax model. Results are pre-tax. Account type, state tax, capital gains and Roth conversions are outside the calculation.
- No fees. Fund expenses and advisory fees are not deducted.
- US data only. Values use US dollars, US assets and US CPI inflation.
- Educational use. The results do not account for a household's full financial or legal circumstances.
Using the charts
The calculators are built to be shared. Every result has a link that carries your inputs, so sending someone the link sends them the exact chart you are looking at, and every result has a summary card you can download as an image and post.