What are the differences between calendar rebalancing and percentage-of-portfolio rebalancing, and which approach is better for different investor types?

Rebalancing discipline is essential for maintaining a portfolio's risk profile over time. The two primary approaches -- calendar-based and percentage-of-portfolio (threshold-based) -- differ in their trigger mechanisms, transaction costs, and ability to control risk.\n\nCalendar Rebalancing:\n\nRebalance at fixed time intervals (monthly, quarterly, annually) regardless of how far allocations have drifted.\n\nPercentage-of-Portfolio (Threshold) Rebalancing:\n\nRebalance only when any asset class weight deviates from its target by more than a specified threshold (e.g., +/- 5% of target weight or +/- 3 percentage points).\n\nComparison:\n\n| Criterion | Calendar | Percentage-of-Portfolio |\n|---|---|---|\n| Trigger | Fixed dates | Allocation drift |\n| Monitoring | Low (check at intervals) | High (continuous or daily) |\n| Transaction costs | Predictable | Variable; lower in calm markets |\n| Risk control | May allow large drift between dates | Tight; bounds drift precisely |\n| Trending markets | Rebalances even when drift is small (unnecessary cost) | Defers action when drift is small |\n| Volatile markets | May rebalance too infrequently | Triggers frequently; higher costs |\n| Implementation | Simple; calendar-based | Requires monitoring infrastructure |\n\nWorked Example:\n\nForestdale Foundation targets 60% equity / 40% fixed income on a $400M portfolio.\n\nScenario: A 6-month equity rally pushes equities to 67% ($268M) and fixed income to 33% ($132M).\n\nCalendar approach (quarterly):\nAt the end of Q1 (3 months in), equity was at 63%. No action. At Q2, equity is 67%. Rebalance: sell $28M equity, buy $28M bonds. The portfolio bore unintended equity risk for 3 months.\n\nThreshold approach (5% relative band):\nThreshold: 60% +/- 3pp = [57%, 63%]. When equity hit 63.1% at week 8, the system triggered rebalancing immediately. Sell $12.4M equity at that point. Drift never exceeded the band.\n\nThe threshold approach caught the drift 5 weeks earlier, reducing the period of unintended risk.\n\nHybrid Approach:\n\nMany institutional investors combine both: review the portfolio on a fixed schedule (quarterly) but also monitor for threshold breaches between review dates. This captures the simplicity of calendar-based governance with the risk control of threshold triggers.\n\nInvestor-Specific Recommendations:\n\n| Investor Type | Recommended Approach | Reason |\n|---|---|---|\n| Large pension fund | Threshold + calendar governance | Needs tight risk control; has monitoring infrastructure |\n| Individual investor | Calendar (quarterly) | Simpler; lower monitoring burden |\n| Taxable account | Calendar (annual) | Minimizes realization events |\n| Endowment | Threshold with wide bands | Lower turnover; long horizon tolerates drift |\n| Insurance company | Tight threshold | Regulatory constraints require precise ALM |\n\nPractice rebalancing scenarios in our CFA Level III question bank.