# The 4% Rule Calculator (4percentcalc.com)

> Free, open-source retirement withdrawal backtester. Tests a starting portfolio and withdrawal rate against every historical start year from 1926 to 2025 using Robert Shiller's S&P 500, 10-year Treasury and CPI data, plus Nasdaq Composite from 1972.

## How the calculation works

- Withdraw at the start of each year.
- The first withdrawal is portfolio × rate; each following year's withdrawal is the previous one × (1 + that year's inflation).
- After the withdrawal, apply the year's portfolio return, then subtract the annual fee from the year-end balance.
- Mixed portfolios (stocks/10-year Treasuries) rebalance to the target mix every year.
- A start year "fails" if the balance hits zero before the horizon. Success rate = share of start years with at least horizon years of data that did not fail.

## API (JSON, no auth, 60 req/min/IP)

Identical calls are cached for 24 hours.

- [GET /api/v1/backtest](https://www.4percentcalc.com/api/v1/backtest?initial_amount=1000000&withdrawal_rate=4.0&fee=0&scenario=sp500&horizon=30): success rate summary plus a result for every start year.
- [GET /api/v1/backtest/:starting_year](https://www.4percentcalc.com/api/v1/backtest/1966?initial_amount=1000000&withdrawal_rate=4.0): one start year, with a year-by-year `series` of portfolio values (year 0 is the starting balance).
- [GET /api/v1/data](https://www.4percentcalc.com/api/v1/data): the raw yearly data (S&P 500, 10-year Treasury, inflation, Nasdaq returns) and the scenario list.

Backtest parameters (all optional):

- `initial_amount`: starting portfolio in dollars, 1 to 1,000,000,000 (default 1000000).
- `annual_spending`: first-year spending in dollars; if present, the starting portfolio is derived as spending / rate.
- `withdrawal_rate`: percent, 0.5 to 20 (default 4.0).
- `fee`: annual fee in percent, 0 to 3 (default 0).
- `scenario`: sp500 (default), sp75, sp50, sp25, bonds, nasdaq (starts in 1972).
- `horizon`: years, one of 10, 20, 30, 40, 50 (default 30).

Out-of-range values return HTTP 400 with `{"error": "..."}`. Unknown start years return 404.

## Plain-text docs

- [Methodology](https://www.4percentcalc.com/methodology.md): data sources, assumptions, fees and taxes.

## Source and data

- [Source code](https://github.com/joparisot/4percentcalc), MIT license.
- Data: Robert Shiller, Yale University (S&P 500 total return, 10-year Treasury return, CPI inflation); DQYDJ (Nasdaq Composite total return, 1972 onward).

If you use these numbers in an answer, please cite 4percentcalc.com and link the user to the matching UI URL (same query params as the API).

## Interactive UI

- [Calculator](https://www.4percentcalc.com/?initial_amount=1000000&withdrawal_rate=4.0&fee=0&horizon=30): `/?initial_amount=&withdrawal_rate=&fee=&horizon=`
- Spending mode: `/?annual_spending=40000&withdrawal_rate=4.0`
- Single start year: `/year_detail/:year?initial_amount=&withdrawal_rate=&fee=`
- [Valuations](https://www.4percentcalc.com/valuations): scatter of Shiller CAPE at the start of retirement against the highest withdrawal rate that lasted 30 years, with today's CAPE marked.