You carefully designed your portfolio with a 70% stock / 30% bond allocation. Three years later, a strong stock rally has pushed you to 85%/15%. Your portfolio now carries significantly more risk than you intended. This is why rebalancing matters — and why neglecting it is a common and costly mistake.
What Is Portfolio Rebalancing?
Rebalancing is the process of restoring your portfolio to its target asset allocation after market movements have caused it to drift. When stocks rise faster than bonds, equities become a larger share of the portfolio than intended. Rebalancing means selling some equities and buying bonds to return to the 70/30 target.
Why Rebalance?
Risk Control
This is the primary purpose. Without rebalancing, a multi-year bull market in equities will push your portfolio to become predominantly stocks — just when valuations may be elevated and a correction more likely. Your portfolio becomes riskier at exactly the wrong time.
Systematic “Buy Low, Sell High”
Rebalancing forces you to sell assets that have risen (relatively expensive) and buy assets that have underperformed (relatively cheap). This disciplined, rule-based selling of recent outperformers and buying of recent underperformers can slightly improve long-term returns — though this effect varies by period and is not guaranteed.
Behavioral Discipline
Having a scheduled, rule-based rebalancing process removes emotional decision-making. During a crash, the rules say “buy more equities to restore the target” — exactly what emotion tells you not to do, but what evidence supports as the right action.
When to Rebalance: Two Approaches
Calendar Rebalancing
Rebalance at fixed intervals — once per year is the most common approach for individual investors. Simple, easy to implement, and works well for most portfolios. The exact calendar date matters less than consistency.
Threshold (Band) Rebalancing
Rebalance when an asset class drifts beyond a defined threshold from its target — typically 5-10 percentage points. If your target is 70% equities and stocks rise to 78%, you rebalance. This is more responsive to large market moves but requires more active monitoring.
Research suggests threshold rebalancing slightly outperforms calendar-only rebalancing, but the difference is small. For most investors, annual calendar rebalancing is perfectly adequate.
How to Rebalance: Step by Step
- Check your current portfolio allocation across asset classes
- Compare to your target allocation
- Calculate how much of each overweighted asset needs to be sold
- Calculate how much of each underweighted asset needs to be bought
- Execute the trades to restore the target allocation
Tax-Efficient Rebalancing Strategies
Selling appreciated assets to rebalance can trigger capital gains tax. Minimize this with these strategies:
- Use new contributions: Direct new monthly investments into the underweighted asset class, gradually restoring balance without selling
- Reinvest dividends strategically: Reinvest dividends from all assets into the underweighted class
- Rebalance within tax-sheltered accounts: No capital gains tax on trades inside pensions or ISAs — rebalance here freely
- Harvest losses: When rebalancing, sell underperforming assets to realize losses that offset gains from rebalancing sales
- Use tax-free allowances: Rebalance up to your annual capital gains tax-free allowance each year
Rebalancing in Practice: A Simple Example
Portfolio target: 70% global equity ETF / 30% bond ETF. Starting value: €100,000 (€70,000 equity / €30,000 bonds).
After one year: equities rise 15%, bonds rise 2%. New values: equity €80,500, bonds €30,600. Total: €111,100.
New allocation: equity 72.5%, bonds 27.5%. The 70% equity target has drifted to 72.5%.
To rebalance: target equity = 70% × €111,100 = €77,770. Need to sell €80,500 − €77,770 = €2,730 of equity ETF and buy €2,730 of bond ETF.
When Rebalancing May Not Be Necessary
If your portfolio has drifted less than 3-5% from targets, the cost of rebalancing (taxes, transaction fees) may outweigh the benefit. Small drift is tolerable. Only act when the portfolio meaningfully deviates from the intended risk profile.
Also, target allocations themselves should shift over time. As you approach retirement, your target equity allocation should decline — so what looks like “drift” may actually be a needed step toward your updated target.
Automating Rebalancing
Many robo-advisors and some brokers offer automatic rebalancing — restoring your target allocation either on a schedule or when bands are breached, without any action from you. If this service is available at low cost, it can remove the discipline requirement entirely.
Set your target allocation. Rebalance annually or when significantly off-track. Keep costs and taxes in mind. This simple discipline, maintained consistently, ensures your portfolio always reflects your actual risk tolerance — not whatever markets happened to do last year.