What is portfolio rebalancing?
Lesson 46 · practical guide
Rebalancing restores a portfolio toward target allocations after prices or contributions change. It keeps risk connected to the plan, but creates costs, taxes and the possibility of selling a winner.
What you will learn
Rebalance a portfolio to its intended risk without turning it into constant trading.
Key terms
- Target allocation — the percentage each asset class is intended to represent.
- Drift band — a threshold that triggers review or rebalancing.
- Tax cost — the possible tax created by selling or exchanging an asset.
Follow these steps
- Set targets from your goal and risk capacity before markets move.
- Choose a calendar or drift band, such as a review every six or twelve months.
- Use new contributions first where possible to reduce selling and costs.
- Check taxes, spreads and concentration before each rebalance.
Practical advice
Rebalancing is a risk-control decision, not a prediction that the sold asset must fall. Infrequent, rule-based reviews are usually easier to follow than reacting to every move.
Long-form explainer
The full guide
Read this lesson as a chapter: understand the mechanism, test the assumptions and apply the idea with a clear risk limit.
Rebalancing restores the intended risk mix
A target allocation is the portfolio structure you chose before markets moved. Drift occurs when returns, contributions or withdrawals make one holding larger or smaller than intended. Rebalancing can reduce concentration, but selling may create tax, spread and timing costs. The process should therefore be scheduled or triggered by a meaningful band, not by every daily movement.
A drift band defines how far an allocation can move before review. Wider bands reduce trading and costs but allow more risk to accumulate; narrower bands control drift more tightly but create more activity. The right choice depends on volatility, taxes, liquidity and the importance of the target.
Use new money before selling when possible
Contributions or withdrawals can often move the portfolio toward its target without realising as many gains. When a sale is necessary, compare the risk reduction with the tax and execution cost. Document the reason, the price, the allocation before and after, and any exceptions.
Rebalancing is not a prediction that the sold asset will fall or the bought asset will rise. It is a way to keep the portfolio aligned with the risk you deliberately selected. Review whether the target still fits the goal before changing the target itself.
What usually goes wrong
The first mistake is rebalancing on a feeling rather than a rule, which usually means selling what is uncomfortable and buying what is reassuring. That is market timing wearing a respectable name. The second error is rebalancing too often. Frequent adjustments generate costs and, in taxable accounts, realised gains, and the risk reduction achieved rarely justifies either.
People also forget that rebalancing requires selling something that has done well, which is emotionally difficult precisely when it matters most. Avoiding it lets the portfolio drift into whatever recently rose, so the risk being taken is no longer the risk that was chosen. The third mistake is ignoring tax and fees in the calculation; in some accounts, directing new contributions toward the underweight asset achieves most of the effect without triggering either. The final failure is rebalancing into an asset whose case has actually broken. The rule assumes the original allocation still makes sense, and that assumption deserves reviewing rather than automating.
How it works
Choose target weights, review schedule and tolerance bands before the next market move. Calendar and threshold rules can both work; new contributions may rebalance without selling.
First check whether the target still fits the goal and loss capacity. Rebalancing an unsuitable allocation only preserves an unsuitable plan. Include spreads, commissions, tax and liquidity.
Worked example
Numbers make an idea concrete. Here is a small, illustrative one — not a forecast.
A 60/40 portfolio becoming 75/25 after a rally can be restored by selling the overweight side or directing new contributions to the underweight side.
Before you act
Run through these questions before you commit any money or make a decision based on this lesson:
- What are targets and bands?
- When will I review?
- Can contributions reduce trading?
- Does the target still fit?
Practice this lesson
Reading is a start; doing the exercise is what makes the idea stick.
Create a three-asset rule and test 20% rise and 30% fall scenarios including fees and the local tax question.
Further reading
Educational content only: This guide is not personal financial, legal or tax advice. Markets involve risk, including the possible loss of capital. Verify current rules, fees and product availability in your country.