Portfolio rebalancing explained: why a mix drifts and how to read the gap
A portfolio is set up with a plan for how much risk it carries, but markets move the weights without asking anyone. Rebalancing is the name for noticing the gap between the plan and the mix as it is now, and deciding what, if anything, to do about it. This guide explains why a mix drifts, why ignoring the drift can make a fall deeper, and the usual ways a gap is closed.
Updated 15.10.2026
What rebalancing is
Rebalancing starts from target weights: how much of the portfolio is meant to sit in each class, region or sector. It compares those targets with the weights the portfolio has today and looks at the difference. Closing the difference, partly or fully, is what is called rebalancing.
It is a way of keeping risk close to a plan. It is not a forecast of which asset will do well, and it is not a way to chase gains.
Why a mix drifts toward more risk
Weights are not fixed. Whatever grows faster becomes a larger share, and whatever lags becomes a smaller one. A portfolio that began as half stocks and half bonds can, after a long stretch in which stocks rose faster, carry clearly more stock risk than it was designed for, although nobody decided that.
A position that grows quickly becomes a larger share of the whole, and one winner can come to dominate.
A class that rises for years takes a growing part of the portfolio, so the mix leans toward it.
Currency moves change the weight of foreign positions even when nothing was traded.
Deposits, withdrawals and dividends change the weights as well.
Why ignoring the gap can make a fall deeper
A drawdown is the fall from a peak to a later low. How deep it is for a portfolio depends on how much of it sits in the more volatile assets when the fall comes. A drifted mix has more in the more volatile class than the plan intended, so the same market move produces a deeper fall than the plan expected, and the person finds out at the worst time.
The effect works in both directions. A mix that drifted toward steadier assets carries less risk than planned and also less room to grow. No rule removes losses, and markets can fall right after a mix has been brought back to target. The purpose is to know the risk being carried and to keep it a choice, not to avoid every fall.
Common ways to close a gap
On a calendar: look at the mix at a fixed interval, for example once or twice a year, and compare it with the targets.
By threshold: act only when a class has moved a set distance away from its target weight, and leave smaller gaps alone.
With new money: direct new contributions and dividends toward the classes that are below target, so no existing position has to be sold.
In combination: check on a schedule and act only when a threshold has been passed.
What to check before changing anything
Costs: each trade can carry commissions and spreads, so a small gap may not be worth closing.
Tax: a sale can create a taxable gain in some countries, and holding-period rules can change when a gain is taxed.
The rules differ by country and are worth checking for your own case.The plan itself: the targets ought to come from your goals, horizon and tolerance for loss.
A model split is a reference for comparison, and it knows nothing about you.Concentration: one large position can matter more than a small gap between two classes.
Reading a gap in practice
Start with the largest gap and ask where it came from: market movement, or the way contributions were made. Look at it in money terms as well as in percentage terms, because a large gap in a small class can matter less than a small gap in a large one.
Allocaria shows the current mix next to a model split for a chosen risk level, and the difference between them, by class, region and sector. It places no orders and it does not tell anyone what to do with the difference.
Common questions
There is no single right interval. Common choices are a fixed schedule, once or twice a year, or a threshold such as a class drifting a set distance from its target. More frequent checks mean more trades and more costs, and less frequent checks let the drift grow.
Not reliably. Its main purpose is to keep the risk of the portfolio close to the level that was planned. Over some periods it helps returns and over others it lowers them.
No. Market timing tries to predict where prices will go. Rebalancing responds to the weights the portfolio has now and to the plan, and it makes no prediction.
No. New contributions and dividends can be directed to the classes that are below target, and a small gap can be left alone. Whether to act at all is a personal decision.
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General information, not investment, tax or legal advice, and not a recommendation about any security. Allocaria is a decision-support tool; it places no orders and holds no money.