Last updated:
The silent risk creep. How a buy-and-hold portfolio quietly becomes much riskier than you started.
Beta drift is the change in a portfolio's beta over time, driven by shifts in holdings, leverage, or the market correlation of existing positions. A portfolio that started with beta 0.8 and now has beta 1.4 has experienced significant drift — its risk profile changed even if you did not trade. Periodic rebalancing anchors beta to your target.
Beta drift = β_now − β_startBeta drift = βnow − βstartTwo sources of drift: (1) high-beta holdings rallied harder so they are a bigger fraction of your portfolio; (2) the beta of specific holdings itself changed over time.
Beta drift happens silently. Hold a 50/50 split of low-beta utilities and high-beta tech. If tech doubles and utilities stay flat, you are now ~67/33 tech/utilities — much higher beta than you set out with. You took on more risk without doing anything.
This is why rebalancing exists. Even if your strategy is "buy and hold," rebalancing once a year reverts beta drift toward your target risk profile.
Starting allocation: 50% VTI (beta 1.0), 50% TLT (beta −0.1). Starting portfolio beta: 0.45.
2 years later: VTI doubled (now 67%); TLT fell 10% (now 33%). New portfolio beta = 0.67 · 1.0 + 0.33 · −0.1 = 0.64.
Beta drift = 0.64 − 0.45 = +0.19. Your equity exposure crept up 19 percentage points of beta from gains alone.
Annual beta drift of more than ±0.15 in a buy-and-hold portfolio is a signal that rebalancing is overdue.
Upload a brokerage CSV (IBKR, Schwab, Fidelity, Robinhood, Coinbase, Kraken, Binance…) and get your Beta Drift computed on your real holdings — alongside 70+ other portfolio metrics. No signup, runs in your browser.
Open the Portfolio Beta Calculator →Industry guideline is annually for tax-efficient portfolios, or whenever any allocation drifts more than 5 percentage points from target. Tax-deferred accounts can rebalance more often.
It changes your risk profile silently. If you started with a conservative beta-0.7 portfolio and now have beta 1.1, you have taken on more market risk than you intended.
Yes — letting winners run is a deliberate strategy. But it should be acknowledged as taking on more risk, not assumed to be neutral.
Yes — current beta is shown alongside historical beta over multiple windows so you can see the drift.
Upload your brokerage CSV — Foliolytic computes Beta Drift plus 70+ other metrics using real historical prices, real Treasury yields, and real CPI data. Free, no signup, your data stays in your browser.
Analyze your portfolio free →