When designing an investment portfolio, the first step is choosing the asset allocation: what percentage goes into equities, what goes into fixed income, and how each block is distributed geographically. That initial distribution is not arbitrary — it reflects the investor’s risk profile, time horizon, and concrete objectives.
The problem is that markets do not read the plan. Over time, some assets grow faster than others, and the original distribution shifts quietly. A portfolio designed with 70% in equities and 30% in bonds may have drifted to 80/20 after a year of strong stock market performance, without the investor having taken any active decision. The level of risk has increased passively, without anyone choosing it.
Rebalancing is the operation that corrects that drift. It means selling assets that have grown beyond their target weight and buying those that have fallen below, returning the portfolio to the designed distribution. It is not glamorous, does not require market analysis, and generates no headlines. Those are precisely the reasons many investors overlook it. It is also one of the elements that most clearly separates a methodically managed portfolio from one that is simply left to grow on its own.
Why the portfolio drifts over time
Drift is an unavoidable mathematical effect. If equities return 15% in a year and bonds return 3%, the relative weights change even without any intervention. The stock block gains percentage weight in the portfolio; the bond block loses it.
A concrete example illustrates the point. An investor starts with €10,000 split between €7,000 in a global index fund and €3,000 in a bond fund.
After a year in which equities rise 18% and fixed income rises 4%:
- Equities: €7,000 x 1.18 = €8,260
- Fixed income: €3,000 x 1.04 = €3,120
- Total: €11,380
New distribution: 72.6% equities and 27.4% fixed income. The portfolio has drifted 2.6 percentage points from its target without the investor doing anything.
If equity markets continue rising for several consecutive years — as happened notably between 2012 and 2021 — the drift can be far larger. A portfolio that started at 70/30 can reach 85/15 in a prolonged bull market, exposing the investor to a level of risk considerably higher than what they once considered appropriate for their situation.
The problem becomes apparent when a correction arrives. A 30% fall in equities hits an 85% equity portfolio far harder than a 70% equity portfolio. The protection that the fixed-income block was supposed to provide has been eroded by the market’s own momentum, not by any decision the investor made.
The lesson is not that equities are dangerous or that their allocation should be reduced. It is that the asset allocation makes sense in the context of the investor’s life stage, and that distribution needs to be maintained intentionally, not left to drift.
What rebalancing is and what it achieves
Rebalancing means adjusting the portfolio weights to return them to the target allocation. The operation can be carried out in two ways.
The first is to sell some of the assets that have grown above their target weight and use that money to buy those that have fallen below. Following the earlier example, the investor would sell enough equities to return to 70% and reinvest in fixed income until the 30% is restored.
The second — more tax-efficient — approach is to direct new periodic contributions towards the underweight assets, without selling anything. If the investor continues making monthly contributions, those can be directed entirely towards fixed income until the proportion is restored. No capital gains are realised, no tax event is triggered.
The purpose of rebalancing is not to predict which asset will perform better in the future, but to keep the risk level consistent with the agreed strategy. As a secondary effect, it produces a counterintuitive behaviour that proves advantageous: it forces the investor to sell assets that have risen the most and buy those that have fallen the most — to buy low and sell high in a systematic, mechanical way, without requiring any tactical analysis.
Rebalancing is not a market call. It is the discipline of returning the portfolio to the risk level the investor chose with careful thought, rather than letting market momentum decide it instead.
When and how to rebalance in practice
There are two main approaches to deciding when to rebalance.
The first is calendar rebalancing: a fixed date is established — once a year, twice a year — on which the portfolio is reviewed and adjusted regardless of how much it has drifted. It is simple to implement and avoids the temptation to act during moments of heightened volatility, when emotions can distort judgement.
The second is threshold rebalancing: a maximum tolerable deviation is defined — for example, 5 percentage points from the target weight of each asset — and action is only taken when that deviation is exceeded. If equities were meant to represent 70% and have reached 75% or fallen to 65%, rebalancing is triggered. Within that band, nothing is done.
Vanguard compared both approaches in a historical analysis and concluded that threshold rebalancing tends to be more efficient because it acts when deviations are significant and avoids unnecessary transactions during stable periods. In practice, combining both works well: review the portfolio once a year and act only if the deviation exceeds 5%.
The operational process is straightforward. Review the current percentages of each asset in the portfolio. Calculate the gap from the target. If the deviation exceeds the threshold, sell the excess of the overweight asset. Buy the underweight asset with the corresponding amount. If new contributions are available, direct them to the underweight asset before resorting to sales.
A practical detail worth noting: in many jurisdictions, switching between funds within a pension or tax-advantaged account does not trigger an immediate taxable event. Where the portfolio is structured around such accounts, rebalancing can be done with no immediate tax cost at all, simply by moving money from one fund to another within the same wrapper.
The tax impact of rebalancing
Selling assets that have gained in value triggers capital gains tax. In Spain, the current rates apply to the gain on a graduated scale: 19% on the first €6,000 of gain, 21% between €6,000 and €50,000, 23% between €50,000 and €200,000, and 27% on the portion above €200,000. Similar graduated structures apply in most European countries.
This means that each time assets are sold outside a tax-advantaged account to rebalance, a taxable event is created that can reduce the efficiency of the operation. The effect compounds if rebalancing is frequent and if accumulated gains are large.
There are three standard strategies to minimise the tax impact without abandoning rebalancing.
The first, as already noted, is to use new periodic contributions to rebalance passively. If the monthly savings flow is sufficient to offset deviations, the allocation can be maintained without selling anything and without triggering any tax.
The second is to use unrealised losses in other portfolio assets to offset the gains realised from selling winners. If the portfolio holds an asset at a loss, selling it and buying an equivalent fund allows that loss to be crystallised and offset against the rebalancing gain within the same tax year.
The third is to concentrate rebalancing activity within tax-advantaged vehicles such as pension plans, where switches between internal funds do not trigger immediate taxation. Within those vehicles, rebalancing is entirely free from a tax perspective.
The goal is not to avoid rebalancing out of fear of taxes — that would be letting the tax tail wag the strategic dog — but to design the process so that the tax cost is kept as low as possible while maintaining the intended allocation.
The most common mistake: rebalancing too much or too little
Both extremes produce equally poor results.
Rebalancing too frequently — monthly, or after every significant market move — generates transaction costs and an accumulated tax drag that can erode a meaningful portion of the advantage of maintaining the correct allocation. Moreover, in markets with a sustained upward trend, systematically selling the best-performing asset just as it still has momentum reduces returns without a clear strategic justification.
Rebalancing too infrequently — or not at all — allows the portfolio to drift indefinitely towards equities during bull markets, progressively and inadvertently increasing risk. An investor who did not rebalance between 2012 and 2021 may have arrived at the final stretch before retirement with a portfolio far more aggressive than their situation required at that point.
The balance lies in the annual review with a threshold for action. One year is sufficient time for significant deviations to accumulate. The 5% threshold filters out short-term market noise and only triggers rebalancing when the drift is real and relevant to the strategy.
An additional common mistake is confusing rebalancing with a tactical market call. Rebalancing from equities into fixed income does not mean believing that stocks are about to fall in the short term; it means that their current weight exceeds the risk level the investor has chosen to maintain consistently with their life stage. The decision is structural and strategic, not speculative.
Maintaining the allocation over time is, alongside choosing well-diversified assets and controlling costs, one of the quietest and most effective pillars of any long-term portfolio. It requires no particular talent or privileged information. It only requires a method and the discipline to revisit it once a year without being carried away by the market’s momentum.
