Portfolio rebalancing: when to do it and why it matters

Portfolio rebalancing: when to do it and why it matters

If you set up a portfolio with a target allocation and never touch it again, market movements will gradually shift it into something you did not plan. A portfolio that starts as 60% equities and 40% bonds can drift to 75% equities after a strong bull run, taking on more risk than you originally intended without you making a single decision.

Portfolio rebalancing is the process of bringing your allocation back to its target. It sounds simple, but the timing, method, and frequency involve real tradeoffs between risk control, transaction costs, and tax efficiency.

What rebalancing does

When asset classes perform differently, their weights in your portfolio change. Rebalancing corrects this drift by:

• Selling assets that have grown above their target weight
• Buying assets that have fallen below their target weight

The result is a portfolio that reflects your original risk and return intentions, not whatever the market has pushed it towards.

This has a counterintuitive effect: rebalancing forces you to systematically sell high and buy low. You trim positions that have run up and add to those that have lagged. Over long periods, this discipline can improve risk-adjusted returns relative to a portfolio left to drift.

Three rebalancing methods

1. Calendar rebalancing

You rebalance on a fixed schedule: quarterly, semi-annually, or annually. This is the simplest approach and requires no ongoing monitoring.

Advantage: predictable, low effort, easy to stick to. Disadvantage: you may rebalance when drift is minimal (wasting transaction costs) or fail to rebalance when drift is large (if the schedule does not coincide with a volatile period).

2. Threshold rebalancing

You rebalance whenever any asset drifts beyond a set tolerance band. For example, if equities exceed 65% or fall below 55% in a 60/40 portfolio (a 5% band).

Advantage: responds to actual drift, not an arbitrary calendar date. Disadvantage: requires continuous monitoring; in trending markets it can trigger frequent trades.

3. Hybrid rebalancing

You check on a fixed schedule (e.g. quarterly) but only rebalance if a threshold has been breached. This combines the discipline of a calendar approach with the efficiency of threshold-based triggers.

This is the method most commonly recommended in academic and practitioner literature for long-term investors.

A worked example

A portfolio is set with a 70/30 target: 70% equities and 30% bonds.

After one year, equities represent 75.8% of the portfolio (€9,100 / €12,000). The portfolio has drifted 5.8 percentage points above the equity target.

To rebalance:

• Target equity value: 70% x €12,000 = €8,400
• Sell: €9,100 - €8,400 = €700 of equities
• Buy: €3,600 - €2,900 = €700 of bonds

The portfolio is back to 70/30 and the investor has implicitly sold equities at a high and added to bonds at a relative low.

When rebalancing makes sense and when it does not

The cost of not rebalancing

A portfolio that is never rebalanced is not a passive strategy: it is an implicit active bet on whatever has performed best. In a long equity bull market this looks like a good decision. In a sharp correction, an over-weight equity position amplifies losses relative to what the investor originally accepted.

Research on US market data from 1926 to the present consistently shows that unmonitored portfolios drift toward higher equity concentration over time. The volatility of an unbalanced portfolio exceeds what its original target allocation would predict.

Rebalancing and transaction costs

Rebalancing has costs: brokerage fees, bid-ask spreads, and, outside tax-advantaged accounts, capital gains tax on the positions you sell.

Two ways to reduce friction:

1. Rebalance with new contributions. If you invest regularly, direct new money to underweight assets instead of buying and selling. This avoids selling and its associated costs entirely.
2. Widen your tolerance bands. A 10% band (rebalance only when drift exceeds 10 percentage points) trades some precision for lower turnover. For many investors, this is the better tradeoff.

Common mistakes

1. Rebalancing too frequently Monthly rebalancing in normal market conditions generates transaction costs without proportionally improving risk control. Annual or threshold-based rebalancing is sufficient for most long-term portfolios.

2. Ignoring tax consequences Selling appreciated assets to rebalance triggers taxable gains. In taxable accounts, consider rebalancing only through new contributions, or harvesting losses to offset gains.

3. Treating all assets the same Volatile assets like small-cap equities or commodities will breach tolerance bands more often than stable assets. Set bands proportional to each asset’s typical volatility.

4. Rebalancing based on short-term views Rebalancing is a mechanical discipline, not a market timing tool. If you are deviating from the schedule because you expect equities to keep rising, you are no longer rebalancing: you are speculating.

FAQ

How often should I rebalance my portfolio? Annual rebalancing with a 5-10% threshold band is the standard recommendation for long-term retail investors. Check quarterly; only rebalance if a threshold has been breached.

Does rebalancing improve returns? Not necessarily in absolute terms: it depends on market conditions. In trending markets, rebalancing reduces returns by trimming winners. Its main benefit is risk control: it prevents your portfolio from drifting into a risk profile you did not intend to carry.

What is a tolerance band in rebalancing? A tolerance band defines how far an asset can drift before triggering a rebalance. A 5% band on a 60% equity target means you rebalance if equities reach 65% or fall below 55%.

Should I rebalance inside a tax-advantaged account? Yes. Tax-advantaged accounts (pension, ISA, or equivalent) remove the capital gains cost of selling. Rebalance freely within them without worrying about the tax drag that affects taxable accounts.

Can I use backtesting to evaluate a rebalancing strategy? Yes. Running a backtest with a defined rebalancing rule (calendar or threshold) against the same portfolio without rebalancing shows the historical impact on volatility, drawdown, and CAGR for your specific allocation.

Test your rebalancing strategy

Run a free backtest on Wallible to see how your portfolio would have performed with and without rebalancing rules applied to real historical data.

Try Wallible’s free portfolio backtesting tool

Related guides

• Maximum Drawdown: how to measure and interpret portfolio losses
• Value at Risk (VaR) 95% and 99%: practical guide
• Sharpe Ratio: meaning, formula, and good values