Portfolio Rebalancing: The 5/25 Rule Explained

Portfolio rebalancing is one of the simplest, yet most overlooked disciplines in wealth management. Left untouched, an initial 60/40 stocks/bonds allocation can, after several years of a bull market, quietly become 75/25 without the investor noticing — exposing the portfolio to far more risk than originally chosen.

This drift phenomenon isn't an anomaly: it's an inevitable mathematical consequence of the fact that asset classes never move at the same pace. Understanding this mechanism, and knowing when to act, is essential for any disciplined investor.

Why do allocations drift?

A portfolio is not a fixed snapshot: it's a set of assets each evolving at its own pace. If stocks climb 15% in a year while bonds stagnate at 2%, the relative share of stocks in the portfolio mechanically increases, even if no order was ever placed.

This drift accelerates over time and with the relative volatility between asset classes. The larger the performance gap between two portfolio components over a long period, the further the actual risk structure strays from the initially defined target structure.

The hidden risk of inaction

The danger isn't merely cosmetic. A portfolio drifting toward a higher-than-intended equity weighting becomes more volatile, more sensitive to market corrections, and potentially unsuited to the investor's actual time horizon or risk tolerance. Conversely, drift toward bonds can slow the long-term growth needed to reach certain financial goals.

The main rebalancing strategies

Several approaches exist for deciding when to rebalance a portfolio. Each answers the trade-off between discipline, cost, and operational simplicity differently.

Calendar-based rebalancing

The most common method rebalances at fixed intervals — monthly, quarterly, or annually — regardless of how large the observed drift is. Its simplicity is its main strength: it requires no active monitoring between scheduled dates.

Its downside is that it can trigger unnecessary rebalancing when drift is minimal, generating transaction costs and taxable events with no real benefit for risk management.

Threshold-based rebalancing

This approach triggers action only when the gap between actual and target allocation exceeds a predefined threshold, for example 5 percentage points. It is more responsive to actual risk but requires more regular portfolio monitoring.

The hybrid 5/25 rule

Popularized by institutions like Vanguard and echoed by Morningstar analysts, the 5/25 rule combines an absolute and a relative threshold. It states that rebalancing should be triggered as soon as either of the following conditions is met, independently of the other: absolute drift greater than 5 percentage points from the target weight, OR relative drift greater than 25% of the target weight itself.

This dual condition matters because it scales to each asset class. For a 40% target position, an absolute gap of 5 points (35% or 45%) is enough to trigger the alert. But for a smaller target position, say 5%, an absolute gap of 5 points would mean a full doubling — far too tolerant a threshold. That's where the 25% relative threshold comes in: for that 5% position, rebalancing triggers as soon as the weight reaches 6.25% (25% more than 5%), well before the absolute threshold would ever be hit.

Worked example: a drift and its correction

Consider a target portfolio of 60% stocks, 30% bonds, and 10% commodities. After eighteen months of sustained equity gains and stagnant bonds, the actual allocation shifts to 68% stocks, 25% bonds, and 7% commodities.

For stocks, the absolute gap is 8 percentage points (68% minus 60%), already exceeding the 5-point threshold. For commodities, the absolute gap is only 3 points, but the relative gap is 30% (a drop from 10% to 7% is a 30% relative loss of the target weight), exceeding the 25% threshold. Both asset classes therefore trigger rebalancing, while bonds, with an absolute gap of exactly 5 points, sit right at the boundary.

Rebalancing then means selling part of the equity position and reinvesting the proceeds into bonds and commodities until the weights return to the 60/30/10 targets. This mechanically sells what has risen most and buys what has lagged, systematically applying a sell-high, buy-low discipline.

The cost-versus-risk trade-off

Rebalancing too often

Overly frequent rebalancing multiplies brokerage fees, bid-ask spreads, and above all generates taxable events in non-sheltered accounts, since each sale potentially realizes a taxable capital gain. The marginal benefit for risk control then falls below the cumulative cost of these frictions.

Rebalancing too rarely

Conversely, overly infrequent rebalancing lets the portfolio drift toward an unintended risk profile for extended periods. In a market correction, a portfolio that should have been rebalanced long ago suffers a far harsher shock than one whose target allocation had been maintained.

The 5/25 rule aims precisely at this middle ground: it lets normal small market fluctuations breathe while imposing firm discipline as soon as drift becomes significant.

Cash-flow-based rebalancing

A more tax-efficient alternative, particularly suited to investors who regularly fund their portfolio, is to direct new contributions — monthly deposits, reinvested dividends — toward underweight asset classes rather than selling overweight ones.

A concrete example

Take the earlier portfolio with stocks at 68% instead of 60%. Rather than selling stocks to buy bonds, an investor contributing €1,000 a month can direct that entire new capital toward bonds and commodities until balance is restored, without triggering any sale or any capital gains tax.

This method is slower at correcting a large drift, since it depends on the pace of contributions, but it's particularly well suited to accumulation phases where regular deposits are large enough to progressively absorb allocation gaps.

Summary and next steps

Portfolio rebalancing is not a one-off operation but an ongoing discipline that protects the investor's chosen risk profile against the natural wear of time and markets. The 5/25 rule offers a robust, well-tested framework for objectively deciding when to act, avoiding both excessive reactivity and prolonged neglect.

To visualize the drift in your own allocation and simulate the impact of rebalancing under different rules, the portfolio analysis tools available at /outils let you test these scenarios on your own data, for purely educational purposes and never as personalized investment advice.