Fixed BTC or Fixed Fiat?

Retirement planning tools for Bitcoin typically present a single drawdown mode: sell a fixed fiat amount monthly to cover expenses. We test this against the alternative of selling a fixed BTC fraction monthly across 50 overlapping 4-year windows in the post-$5,000 price era (October 2017 to March 2026).

Fixed BTC produces higher total income in 47 of 50 windows (94%) with a median income premium of +160% and always survives the full window. Fixed fiat exhausts the stack before the window ends in every test case regardless of starting BTC size up to 10 BTC, surviving the full 48 months in only 72% of windows even at a 10 BTC stack.

The Withdrawal Rate Problem

Bengen (1994) established the 4% rule for equity retirement portfolios: a retiree can withdraw 4% of their initial portfolio value annually, adjusted for inflation, with high probability of surviving a 30-year retirement. The rule assumes a diversified equity/bond portfolio and is calibrated to the worst historical sequence-of-returns in US market data (the 1966 cohort, retiring at the market peak before the stagflation decade).

Bitcoin introduces a structural asymmetry that Bengen’s framework does not address: the asset appreciates in a power law pattern rather than stochastically around a mean. An asset with a structural growth trend creates a tension in the withdrawal rule.

Fixed fiat drawdown (the Bitcoin analog of the 4% rule) implicitly bets that selling variable amounts of an appreciating asset converges to a stable income stream. Fixed BTC drawdown bets that the asset will appreciate enough that selling a constant fraction of the stack produces rising fiat income over time. For a zero-growth asset, the two strategies are identical. For an asset with structural appreciation, fixed BTC will dominate.

The Inverse-DCA Problem

Fixed fiat drawdown creates what we term the inverse-DCA problem. Dollar-cost averaging (DCA) accumulation works by buying more units when prices are low and fewer when prices are high, lowering the average cost basis. Fixed fiat drawdown inverts this: it sells more units when prices are low (to raise the required fiat amount) and fewer when prices are high.

This systematically destroys wealth during bear markets and undersells during bull markets — precisely the opposite of optimal behaviour for an appreciating asset.

Fixed BTC drawdown avoids the inverse-DCA problem by decoupling the sell volume from the price. A retiree selling 1/N of their stack per month sells the same quantity regardless of whether price is $20,000 or $200,000. Income varies; the drawdown schedule does not. This is the natural drawdown rule for an asset with a structural growth trend.

Results

We test across 50 overlapping 48-month windows in the post-$5,000 price era (98 monthly observations from October 2017 to March 2026). Both strategies start with 1.0 BTC and target $3,000/month in expenses. Strategy A (Fixed Fiat) sells $3,000 / price each month. Strategy B (Fixed BTC) sells 1/48 of the starting stack each month.

Fixed BTC survives every window. Fixed fiat survives none. The 5.7-month average survival of fixed fiat on a 1 BTC stack reflects the asset’s volatility: at $66,848 per BTC, a 1 BTC stack generates only $3,000/month for approximately 22 months in a flat market. Bear market price declines force higher sell volumes, exhausting the stack far sooner.

The three windows where fixed fiat marginally outperforms all begin in January–March 2021 — the peak of the 2021 bull market (prices $29,000–$45,000). The maximum fixed fiat advantage in these windows is 3.7%. These windows include the 2021–2022 bear market in which fixed BTC sold through the decline at constant volume, while fixed fiat reduced sell volume as prices fell. The advantage evaporates in any window that starts earlier or later.

Fixed Fiat Survival by Stack Size

The question for a fixed fiat retiree is not whether the strategy dominates — it does not — but what stack size makes it viable.

Even at 10 BTC ($668,480 at current prices), fixed fiat fails to survive the full 4-year window in 28% of test cases. Fixed fiat is not a viable retirement drawdown strategy for Bitcoin at any reasonable stack size in the current price regime.

Minimum Viable Stack for Fixed BTC

For fixed BTC drawdown, the question is not survival but income adequacy. Because fixed BTC always survives (it depletes at a fixed rate), the relevant metric is average monthly income and its distribution.

A stack of approximately 4.2 BTC produces an average monthly income of $2,968 — essentially $3,000/month — over 4-year windows in the post-$5k era. The P10 income of $1,926/month represents the worst-case monthly experience in the worst 10% of months: still meaningful income, though below the $3,000 target. This is the minimum viable stack for a $3,000/month average income target under fixed BTC drawdown at current prices.

Comparison to Bengen’s 4% Rule

Bengen’s (1994) 4% rule requires $900,000 in assets to generate $3,000/month ($36,000/year) at a safe withdrawal rate. Bitcoin offers dramatically better capital efficiency.

The floor freedom threshold — the point at which Bitcoin floor growth alone covers expenses — requires approximately 1.63 BTC at current market price ($66,848). This is an 8.3× capital efficiency advantage over the Bengen rule.

The fixed BTC minimum viable stack of 4.2 BTC ($280,762 at current prices) represents a 3.2× capital efficiency advantage for a retiree who targets $3,000/month average income rather than floor freedom. Capital requirements are price-sensitive and should be recalculated quarterly.

When Fixed Fiat Is Appropriate

Fixed fiat is appropriate when:

(1) Fixed nominal obligations. The retiree has fixed nominal obligations (rent, loan payments) that cannot accommodate variable monthly income.

(2) Short time horizon. The retiree’s time horizon is less than 2 years and the stack is large relative to the monthly draw.

(3) Cycle peak entry with anticipated prolonged bear. The retiree enters at cycle peak and anticipates a prolonged bear market, in which case fixed fiat limits BTC sold at low prices — though at the cost of income during the peak itself.

In all other cases, fixed BTC dominates. The income certainty of fixed fiat is a psychological benefit, not a financial one: the certainty comes at the cost of systematically selling more BTC during bear markets and less during bull markets, producing lower lifetime income and faster stack depletion.

Product Design Implication

Default mode: Wealth Preservation (fixed BTC). For all users entering the retirement distribution phase, the default drawdown mode should be fixed BTC. Display variable fiat income with the current month’s expected sale and a 12-month income projection based on current power law trend.

Optional mode: Income Certainty (fixed fiat). Available for users who require fixed nominal income. Display the accelerated stack depletion rate and the explicit trade-off: certainty today in exchange for lower lifetime income and higher depletion risk. Flag when the stack falls below the minimum viable threshold for the current price.

The choice between modes should be made at the time of first use, with a clear explanation of the trade-off framed not as a preference but as a financial decision with quantifiable consequences.