Rebalancing

Portfolios drift. Small differences in returns compound into sizable allocation shifts. Imagine a $100,000 portfolio targeting **60$60,000 to $75,000. Bonds grow from$40,000 to $42,000. Total becomes$117,000 and the stock weight becomes $75,000 /$117,000 = 64.1% rather than 60. A 4.1 percentage point drift occurred in one year. Repeated year-after-year outperformance can produce a 10-15 percentage point drift over 3-5 years. That drift matters. A 60/40 portfolio typically exhibits historical annualized volatility near 7-10%. Moving to 70/30 can raise volatility to roughly 9-12% depending on correlations. Higher volatility increases short-term drawdowns. If your plan expects 3-6 months of expenses in a cash buffer and a 60/40 glide path, a shifted allocation may raise the chance of a 20-30% drawdown in a 1-3 year period. Worse, many investors react emotionally when the unexpected happens. Without rules, people tend to sell winners after gains and buy winners after losses - the opposite of systematic rebalancing. IF an investor lets allocations drift by more than 5-10 percentage points AND retirement date is within 5-10 years, THEN portfolio risk may materially exceed planned tolerance BECAUSE expected returns and volatility are functions of allocation. Another consequence is tax drag. Selling winners in a taxable account can crystallize capital gains taxes of 15-23% for long-term gains depending on income, which reduces the net benefit of a rebalance. Ignoring rebalancing creates invisible risk increases, behavioral pitfalls, and possible tax surprises. This section identifies the cost side of that trade-off with concrete numbers so later rules can compare costs and benefits.

Start with definitions. Rebalancing is the act of buying or selling assets so each asset class returns to its target weight. Two common methods exist: calendar rebalancing and threshold rebalancing. Calendar rebalancing trades on a fixed schedule - for example every 6 or 12 months. Threshold rebalancing trades whenever an asset class moves outside a band - for example plus-or-minus 3-7 percentage points from target. Key formulas. Current weight of asset i is $w_i = V_i / V_{total}$ where $V_i$ is the asset value and $V_{total}$ is portfolio value. Amount to trade to reach target weight $t_i$ is $T_i = t_i \cdot V_{total} - V_i$. Example with numbers. Suppose target $t_{stocks}=0.60$ and $V_{total} =$120,000 after returns. Current stocks $V_{stocks} =$78,000. Then $T_{stocks} = 0.60 \cdot 120,000 - 78,000 =$72,000 - $78,000 = -$6,000. Negative means sell $6,000 of stocks. Transaction cost and tax estimates matter. If trade commissions are$0-$10 per trade and bid-ask spreads are 0.01-0.10$6,000 sale with a 20% tax rate impose a tax bill near $1,200 if gains equal full sale amount, though in practice only the realized gain portion is taxable. Turnover comparisons. A threshold rule of 5 percentage points typically triggers 5-15$new contributions to buy the underweight asset class. For example, if monthly contributions equal $1,000 and current allocation is 65/35 while target is 60/40, directing new contributions of$1,000 per month entirely to bonds reduces drift without taxable sells. This method trades off speed of correction versus immediate tax payments and requires regular discipline. The formulas above plus examples give the arithmetic to compute trades, turnover, and a first-order estimate of tax impact.

Problem-first: investors need rules that balance tax cost, transaction cost, behavioral control, and drift. Here is a compact decision tree you can apply to most portfolios. IF the account is tax-advantaged - for example a 401(k) or IRA - AND internal fund options let you trade without tax consequences, THEN prefer rebalancing inside those accounts BECAUSE trades do not realize capital gains and tax drag is avoided. IF the account is taxable AND expected realized capital gains exceed $5,000 in a trade (example threshold), THEN consider alternatives such as directing$new contributions to the underweight asset or rebalancing using tax-loss harvesting BECAUSE selling winners would create an immediate tax bill of roughly 15-23% on long-term gains. IF drift is less than 3-7 percentage points, AND transaction costs are $5-$50 per trade with spreads below 0.2%, THEN letting small drift persist may be acceptable because expected reduction in future realized gains may exceed the cost saved from immediate rebalancing. IF the investor requires strict risk control - for example a retiree within 0-5 years of planned withdrawals - AND target volatility must stay within +/-2 percentage points of plan, THEN rebalance more often such as quarterly or when drift hits +/-3 percentage points BECAUSE small allocation shifts materially change expected drawdowns in that horizon. Practical steps to implement. Step 1 - pick target bands: narrow bands like 1-3 percentage points keep allocations tight but raise turnover; wider bands like 5-10 percentage points reduce turnover but increase drift. Step 2 - assign accounts: use tax-advantaged accounts first for rebalancing trades; use taxable accounts only when tax-efficient methods are unavailable. Step 3 - use contributions and withdrawals: direct new contributions to underweight asset classes and make withdrawals from overweight classes where feasible to rebalance without selling in taxable accounts. Step 4 - quantify trade-offs: estimate expected tax on a sell using your marginal capital gains rate and compare to transaction costs plus the value of restored risk alignment; if tax cost is greater than expected benefit over a 2-5 year horizon, delaying rebalancing may be preferable. This framework makes trade-offs explicit so choices depend on numbers rather than vague instincts.

What it does not handle well. First, concentrated or illiquid positions. If 40% of your portfolio is employer stock worth $200,000 and you target 60/40 across total assets, simple rebalancing math fails because selling large blocks can incur price impact or company-specific lockup penalties. IF a position is illiquid OR subject to trading restrictions, THEN rebalancing triggers additional costs beyond taxes BECAUSE price impact and regulatory limits can materially raise execution cost. Second, tax complexity beyond capital gains. The framework above assumes a flat long-term capital gains rate of 15-23$5,000, proportional fixed costs such as $4 commission or$1 mutual fund trading fee can represent 0.1-0.5% of assets per trade, making frequent rebalancing uneconomic. Limitations summary. This lesson does not model individual tax brackets, state taxes, employer stock restrictions, or margin and options strategies. It also assumes liquid, widely traded ETFs or mutual funds with spreads under 0.2%. IF any of those conditions do not hold, THEN this framework requires modification BECAUSE taxes, illiquidity, or special rules materially change the maths and trade-offs.

Prerequisites: This lesson builds on Asset Allocation (d3) - see /money/asset-allocation-d3 where we covered how stocks/bonds split drives 90%+ of portfolio variance and age-based glide paths. It also assumes familiarity with Target-date funds as a default option - see /money/target-date-funds for trade-offs between passive glide paths and manual rebalancing. Downstream concepts unlocked by mastering rebalancing include Tax-loss harvesting and timing - /money/tax-loss-harvesting which requires knowing when to deliberately realize losses while rebalancing, Withdrawal sequencing and tax-efficient retirement drawdown strategies - /money/withdrawal-strategies which rely on keeping allocations aligned across taxable and tax-advantaged accounts, and Glide-path customization for retirement risk control - /money/glide-paths where small allocation errors compound into materially different retirement outcomes.