Portfolio rebalancing explained: why and how to realign your allocations over time

Portfolio rebalancing is the process of periodically restoring a portfolio to its intended asset allocation after market movements have caused the actual weights to drift from the targets. It is a fundamental discipline of systematic portfolio management—one that controls risk, enforces a sell-high-buy-low dynamic, and maintains the diversification properties the portfolio was designed to deliver.

What portfolio rebalancing is

When a portfolio is first constructed, each asset or asset class is assigned a target weight. As prices change over time, the actual weights diverge from those targets. An asset that appreciates significantly grows to represent a larger share of the portfolio than intended; one that falls shrinks below its target. Without rebalancing, a portfolio set at 60% equities and 40% bonds after a sustained equity bull market may drift to 80% equities and 20% bonds—a materially different risk profile from the one the investor chose.

Rebalancing corrects this drift by selling the assets that have grown above their targets and using the proceeds to buy those that have fallen below. This is a mechanical, systematic process: it does not require a market view or a prediction about future performance. It simply enforces adherence to the original allocation framework.

The two most common rebalancing approaches are calendar rebalancing (restoring target weights at fixed time intervals—monthly, quarterly, or annually) and threshold rebalancing (triggering a rebalance when any weight deviates from its target by more than a specified amount, such as 5 percentage points). Hybrid approaches combine both: rebalance on a schedule, but only if deviations exceed a threshold.

How it works

The mechanics of rebalancing are straightforward. At the rebalancing date, the portfolio manager calculates the current weights of each asset, identifies those above and below their targets, and executes trades to bring all weights back to target—or as close as transaction costs and minimum trade sizes allow.

In a taxable account, rebalancing realises capital gains on the assets sold, creating a tax cost that must be weighed against the benefit of restoring the target allocation. Tax-efficient rebalancing directs new cash contributions towards underweight assets, harvests losses where available, and delays realisation of gains where the deviation is modest. In a tax-sheltered account, this constraint does not apply and rebalancing can be executed more frequently.

An important consequence of rebalancing is its contrarian character. By selling assets that have risen above target and buying those that have fallen below, a rebalancing discipline systematically reduces exposure to recent winners and increases exposure to recent losers. Over time, this has historically produced a modest return enhancement relative to a non-rebalanced portfolio—sometimes called the rebalancing bonus—though this effect depends on mean-reverting price behaviour and is not guaranteed.

What the evidence shows

Perold and Sharpe (1988, Dynamic Strategies for Asset Allocation) provided the foundational analysis of rebalancing as a constant-mix strategy, showing that it performs best in oscillating markets (where mean reversion creates opportunities to buy low and sell high) and worst in trending markets (where selling winners and buying losers reduces returns relative to a buy-and-hold position).

Ilmanen and Maloney (2015, Portfolio Rebalancing, Part 1: Strategic Asset Allocation) reviewed the evidence across multiple time periods and asset classes and found that calendar-based rebalancing consistently reduced portfolio volatility and improved risk-adjusted returns relative to drift portfolios, with the benefit most pronounced over longer holding periods where weights drift most substantially. Threshold-based rebalancing produced similar or slightly better results at lower transaction cost.

The frequency question—how often to rebalance—matters more for transaction costs than for risk control. Annual rebalancing has been shown to capture most of the risk-reduction benefit of more frequent approaches while minimising costs. Monthly rebalancing adds little incremental benefit in most multi-asset portfolios, though in highly volatile environments the benefit of tighter weight control increases.

Limitations and trade-offs

Rebalancing is not costless. Each rebalance incurs transaction costs—bid-ask spreads, commissions, and market impact—which reduce net returns. The optimal rebalancing frequency balances the risk-reduction benefit of tight weight control against the cumulative cost of executing trades. In illiquid markets or with small portfolios, transaction costs can erode the rebalancing benefit significantly.

In strongly trending markets—such as a prolonged equity bull run—a rebalancing discipline will trim the best-performing asset and underperform a buy-and-hold approach. Self-directed investors should be psychologically prepared for this: rebalancing is a risk-management practice, not a return-maximisation strategy, and its benefits are most visible over full market cycles that include both upturns and drawdowns.

Portfolio rebalancing in pfolio

pfolio rebalances portfolios systematically according to the method selected by the investor. The rebalancing rules and frequency are described in the rebalancing your portfolio help article. All three construction methods—mean-variance optimisation, Hierarchical Risk Parity, and equal weight—apply rebalancing to maintain alignment with the target allocation over time.

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