Compute sustainable retirement withdrawals using historical backtesting against US market data (stocks, bonds, inflation, plus several alternatives).

Four CLI commands answer four different questions:

• constant-dollar: "If I withdraw X% per year (inflation-adjusted), how often does that survive my horizon?" (The 4% rule.)
• constant-percent: "If I withdraw X% of my current balance each year, how often does that survive?"
• dynamic-dollar: "Given my current balance and remaining years, how much can I sustainably spend this year?"
• dynamic-success: "Given my balance and a planned budget, how likely is that budget to survive my remaining years?"

Quick start

drawdown dynamic-dollar -b 850000 -a 67 -e 92 -p "us_stocks:60;us_bonds:40;"

dynamic-dollar

Inputs
  Current balance:      $850000
  Current age:          67.00
  End age:              92
  Remaining horizon:    25 years
  Target success:       80.0%

Results
  Raw calculated:       $43538
  Smoothed:             $43538
  Final budget:         $43538
  Success at budget:    80.0%
  SSA offset this year: $0
  Portfolio withdrawal: $43538
  Signal vs prior:      none

Commands

constant-dollar: fixed real-dollar withdrawal (the 4% rule)

Evaluate the success rate of withdrawing a fixed real dollar amount each year. The dollar amount is set at retirement as a percent of the initial portfolio, then adjusted nominally for inflation thereafter. This is the canonical 4% rule methodology.

Examples

The classic Trinity 4% / 30 years / 60-40 portfolio (short flags):

drawdown constant-dollar -w 4 -p "us_stocks:60;us_bonds:40;" -y 30 -r yearly

A more conservative FIRE scenario with 3.5% over 50 years, 100% stocks:

drawdown constant-dollar --withdrawal-rate 3.5 \
    --portfolio "us_stocks:100;" \
    --inflation us_inflation --years 50 --rebalance yearly

Pass/fail against a 95% success target:

drawdown constant-dollar --withdrawal-rate 4 --target-success 95 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --inflation us_inflation --years 30 --rebalance yearly

Flags

Required:

Short	Long	Meaning
-w	--withdrawal-rate <pct>	Withdrawal rate (percent of initial portfolio)
-p	--portfolio <spec>	Portfolio spec, e.g. "us_stocks:60;us_bonds:40;"
-y	--years <int>	Horizon length in years

Common:

Short	Long	Default	Meaning
-i	--inflation <series>	us_inflation	Name of an inflation series (embedded or user-supplied)
-r	--rebalance <method>	yearly	none | monthly | yearly
-sy	--start-year <year>	0	Earliest historical backtest start year (0 = use full data)
-ey	--end-year <year>	0	Latest historical backtest start year (0 = use full data)
-iv	--initial-value <dollars>	1000	Starting portfolio value
-t	--target-success <pct>	0	If > 0, output adds PASS/FAIL note vs this target
-j	--json	—	Emit JSON instead of text
-c	--csv	—	Emit CSV per-path output instead of text

Advanced:

Short	Long	Default	Meaning
-wf	--withdraw-frequency <n>	12	12 = yearly, 1 = monthly
-f	--fees <fraction>	0.0005	TER as a fraction (e.g. 0.0005 = 0.05%, typical broad-market index ETF)

---

constant-percent: fixed percent of current balance

Withdraw a fixed percent of the current (not initial) portfolio balance each year. The dollar amount fluctuates with the portfolio; no inflation adjustment to the percent itself. This is what many mistakenly believe the 4% rule is.

Examples

4% of current balance over 30 years on a 60-40 (short flags):

drawdown constant-percent -pct 4 -p "us_stocks:60;us_bonds:40;" -y 30 -r yearly

Same scenario but with a higher minimum spending floor (3.5% of initial):

drawdown constant-percent --percent 4 --minimum-floor 3.5 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --inflation us_inflation --years 30 --rebalance yearly

Flags

Required:

Short	Long	Meaning
-pct	--percent <pct>	Percent of current balance withdrawn each year
-p	--portfolio <spec>	Portfolio spec, e.g. "us_stocks:60;us_bonds:40;"
-y	--years <int>	Horizon length in years

Common:

Short	Long	Default	Meaning
-i	--inflation <series>	us_inflation	Name of an inflation series (embedded or user-supplied)
-r	--rebalance <method>	yearly	none | monthly | yearly
-sy	--start-year <year>	0	Earliest historical backtest start year (0 = use full data)
-ey	--end-year <year>	0	Latest historical backtest start year (0 = use full data)
-iv	--initial-value <dollars>	1000	Starting portfolio value
-mf	--minimum-floor <pct>	3.0	Minimum annual spending as a percent of initial portfolio. Prevents extreme drawdowns by setting a floor on what gets withdrawn even when the current balance is very low.
-j	--json	—	Emit JSON instead of text
-c	--csv	—	Emit CSV per-path output instead of text

Advanced:

Short	Long	Default	Meaning
-wf	--withdraw-frequency <n>	12	12 = yearly, 1 = monthly
-f	--fees <fraction>	0.0005	TER as a fraction (e.g. 0.0005 = 0.05%, typical broad-market index ETF)

---

dynamic-dollar: per-year sustainable withdrawal

A per-year point query. Given the current portfolio balance, current age, fixed end age, and a target success rate, find this year's sustainable annual withdrawal by historical backtesting over the remaining horizon. Designed to be re-run annually with updated balance and age.

Examples

The basic question: "I'm 67, have $850K, plan to live until 92, and want 80% success" (short flags):

drawdown dynamic-dollar -b 850000 -a 67 -e 92 -p "us_stocks:60;us_bonds:40;"

With Social Security starting at age 70 ($24K/year):

drawdown dynamic-dollar --balance 850000 \
    --current-age 67 --end-age 92 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --inflation us_inflation \
    --ssa-income 24000 --ssa-start-age 70

Next year's run, with smoothing to prevent year-over-year shocks:

drawdown dynamic-dollar --balance 820000 \
    --current-age 68 --end-age 92 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --inflation us_inflation \
    --ssa-income 24000 --ssa-start-age 70 \
    --smoothing 0.10 --prior-amount 43538

A more conservative target (90% success):

drawdown dynamic-dollar --balance 850000 \
    --current-age 67 --end-age 92 --target-success 90 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --inflation us_inflation

Flags

Required:

Short	Long	Meaning
-b	--balance <dollars>	Current portfolio balance
-a	--current-age <years>	Your current age (float allowed, e.g. 67.5)
-e	--end-age <years>	Planning end age (integer)
-p	--portfolio <spec>	Portfolio spec, e.g. "us_stocks:60;us_bonds:40;"

Common:

Short	Long	Default	Meaning
-i	--inflation <series>	us_inflation	Name of an inflation series (embedded or user-supplied)
-t	--target-success <pct>	80	Target success rate. The solver finds the highest WR that hits at least this success rate.
-r	--rebalance <method>	yearly	none | monthly | yearly
-si	--ssa-income <dollars>	0	Annual Social Security income (set to non-zero to enable).
-sa	--ssa-start-age <years>	0	Age SSA income begins. Required if --ssa-income > 0.
-s	--smoothing <fraction>	0	Max year-over-year change in spending, e.g. 0.10 = ±10%. Caps swings caused by market volatility. Requires --prior-amount.
-pa	--prior-amount <dollars>	0	Last year's spending budget. When set, populates the signal field (increase / hold / decrease) and is used by --smoothing if enabled.
-j	--json	—	Emit JSON instead of text
-c	--csv	—	Emit CSV per-path output instead of text

Advanced:

Short	Long	Default	Meaning
-sy	--start-year <year>	0	Earliest historical backtest start year (0 = use full data)
-ey	--end-year <year>	0	Latest historical backtest start year (0 = use full data)
-wf	--withdraw-frequency <n>	12	12 = yearly, 1 = monthly
-f	--fees <fraction>	0.0005	TER as a fraction (e.g. 0.0005 = 0.05%, typical broad-market index ETF)
-st	--solver-tolerance <dollars>	1	Binary-search stopping tolerance. Smaller = more iterations, more precision.

---

dynamic-success: success probability for a planned budget

The inverse of dynamic-dollar. Given the current portfolio balance, current age, fixed end age, and a planned annual spending budget, return the historical probability that budget survives the remaining horizon. Also reports the maximum sustainable budget at a comparison target success rate, so you can see your planned budget alongside the historically safe threshold.

Examples

The basic question: "I'm 67, have $850K, plan to live until 92, and want to spend $50K/year. How likely is that?" (short flags):

drawdown dynamic-success -b 850000 -a 67 -e 92 -bd 50000 -p "us_stocks:60;us_bonds:40;"

With Social Security starting at age 70 ($24K/year), checking a $60K total budget:

drawdown dynamic-success --balance 850000 \
    --current-age 67 --end-age 92 \
    --budget 60000 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --inflation us_inflation \
    --ssa-income 24000 --ssa-start-age 70

Checking a borderline budget vs a more conservative 90% comparison target:

drawdown dynamic-success --balance 850000 \
    --current-age 67 --end-age 92 \
    --budget 43000 \
    --portfolio "us_stocks:60;us_bonds:40;" \
    --target-success 90

Flags

Required:

Short	Long	Meaning
-b	--balance <dollars>	Current portfolio balance
-a	--current-age <years>	Your current age (float allowed, e.g. 67.5)
-e	--end-age <years>	Planning end age (integer)
-bd	--budget <dollars>	Annual planned spending budget
-p	--portfolio <spec>	Portfolio spec, e.g. "us_stocks:60;us_bonds:40;"

Common:

Short	Long	Default	Meaning
-i	--inflation <series>	us_inflation	Name of an inflation series (embedded or user-supplied)
-t	--target-success <pct>	80	Comparison target success rate. The output shows the max budget at this rate alongside your planned budget.
-r	--rebalance <method>	yearly	none | monthly | yearly
-si	--ssa-income <dollars>	0	Annual Social Security income (set to non-zero to enable).
-sa	--ssa-start-age <years>	0	Age SSA income begins. Required if --ssa-income > 0.
-pa	--prior-amount <dollars>	0	Last year's spending budget. When set, populates the signal field (increase / hold / decrease).
-j	--json	—	Emit JSON instead of text
-c	--csv	—	Emit CSV per-path output instead of text

Advanced:

Short	Long	Default	Meaning
-sy	--start-year <year>	0	Earliest historical backtest start year (0 = use full data)
-ey	--end-year <year>	0	Latest historical backtest start year (0 = use full data)
-wf	--withdraw-frequency <n>	12	12 = yearly, 1 = monthly
-f	--fees <fraction>	0.0005	TER as a fraction (e.g. 0.0005 = 0.05%, typical broad-market index ETF)
-st	--solver-tolerance <dollars>	1	Comparison binary-search stopping tolerance. Smaller = more iterations, more precision.

Output formats

All four commands support three output modes. --json and --csv are mutually exclusive.

Mode	Flag	Format
text	(default)	Human-readable Inputs / Results / Notes sections
JSON	--json	Flat JSON object: { "command": ..., "inputs": ..., "results": ..., "notes": [...] }. Snake-case keys, dollars as floats (no $ prefix), percentages as floats (e.g. 80.4, not "80.4%")
CSV	--csv	Per-historical-start-year tabular detail with columns start_year,start_month,success,terminal_value,total_withdrawn,worst_duration_months. Preceded by #-prefixed comment lines describing the scenario

Data sources

Historical data series available out of the box:

Series name	What it covers
us_stocks	US stock market
us_bonds	US bonds
us_inflation	US CPI
ex_us_stocks	Non-US developed-market stocks
gold	Gold
commodities	Broad commodities
cash	Cash equivalents

Data runs ~1871 through ~2025 (monthly). All seven series are embedded into the binary at compile time. You can run drawdown from any directory without co-located data files.

To use your own data, place a CSV named <series>.csv in stock-data/ next to where you run the binary, formatted as month,year,value (one row per month, no header). The loader falls back to file I/O for any series name that isn't embedded.

Build

Supports Linux (x86_64), macOS (arm64), and Windows (x86_64) — all 64-bit. Requires GCC 15 (or newer) and GNU Make.

• Linux: g++-15 from a package or the ubuntu-toolchain-r/test PPA
• macOS: g++-15 via Homebrew (brew install gcc@15)
• Windows: g++-15 via MSYS2 (pacman -S mingw-w64-x86_64-gcc make), run from a MINGW64 shell

(One unified Makefile; same flags across platforms.)

make                  # default: release_debug build
make release_debug    # builds release_debug/drawdown
make release          # fully optimized
make debug            # debug build
make all              # all three modes
make test             # build and run the unit test suite
make compile_commands # regenerate compile_commands.json via bear (optional)
make clean            # remove build directories

A tagged release (v* tag pushed to GitHub) builds portable binaries for all three platforms via .github/workflows/release.yml. Each binary is static-linked libstdc++/libgcc and stripped:

Platform	Artifact
Linux x86_64	drawdown-linux-x86_64
macOS arm64	drawdown-macos-arm64
Windows x86_64	drawdown-windows-x86_64.exe

Background

This tool was forked from Baptiste Wicht's swr-calculator and reshaped around three point-query commands suitable for ad-hoc retirement planning rather than analytical sweep tables.

The methodology and historical-backtesting approach trace back to the Trinity Study (Cooley, Hubbard, Walz 1998) and its many updates. See thepoorswiss.com/updated-trinity-study for an accessible overview.

Important caveats:

• These commands answer "could this strategy have survived past US market history?" They do not concretely answer the question "will it survive the future?" Markets can produce sequences worse than anything in the data.
• The simulation uses the actual historical sequence of returns and inflation. It is not parametric Monte Carlo. Order-of-returns risk is baked in.
• Fees, taxes, and account types (taxable / 401k / Roth) are abstracted away. The --fees flag captures expense ratios only.

License

MIT. See LICENSE.