What Is Portfolio Rebalancing? Keeping Target Allocations

Portfolio rebalancing is the practice of periodically buying and selling holdings to bring a portfolio back to its original target allocation, after market moves have pushed it out of line. If you set a target of 60% stocks and 40% bonds, and stocks rally hard, that mix can drift to 75/25 without you doing anything at all — rebalancing means selling some of the stocks that grew and buying more bonds to restore the original 60/40 split.

Why drift happens on its own

A portfolio’s allocation isn’t static just because you never touch it. Different asset classes grow at different rates, and over time the winners simply become a larger share of the total by virtue of having grown more. This is a completely passive process — you don’t need to make a single trade for your risk profile to change out from under you.

The problem with letting that drift go unmanaged is that your original target allocation was presumably chosen for a reason — a specific level of risk you were comfortable with, tied to your goals and time horizon. As discussed in what is diversification, the whole point of holding uncorrelated assets is controlling overall portfolio risk. Left unchecked, drift undoes that: a portfolio that quietly becomes 75% stocks is meaningfully riskier than the 60% stock portfolio you originally chose, even though you never actively decided to take on more risk.

The mechanics: sell high, buy low, by design

Rebalancing has a structural feature worth naming explicitly: it forces you to sell the asset class that has gone up and buy the one that’s lagged. That’s the textbook definition of buying low and selling high — but done systematically, on a schedule, rather than based on a guess about where prices are headed next.

This is also precisely the discipline most investors struggle to maintain on their own. The asset class that’s outperformed feels like the one to keep holding; the laggard feels like the one to avoid. Rebalancing replaces that instinct with a mechanical rule, which is part of why it’s considered a risk-management practice rather than a return-boosting strategy — it doesn’t promise to beat the market, it promises to keep your risk where you intended it.

Two common triggers

There are two standard ways to decide when to rebalance:

• Calendar-based. Rebalance on a fixed schedule — quarterly or annually is common. Simple, predictable, and easy to automate, but it can mean either rebalancing when drift is negligible (wasting transaction costs) or letting significant drift accumulate between scheduled dates.
• Threshold-based. Rebalance whenever an allocation drifts beyond a defined band — for example, any asset class that moves more than five percentage points from its target. This responds to actual drift rather than the calendar, at the cost of requiring more frequent monitoring.

Some investors combine both: check on a schedule, but only actually trade if a threshold has been breached. This avoids the worst of each approach’s downside — unnecessary trading on a fixed schedule, or letting drift run unchecked between checks.

Costs to weigh against the benefit

Rebalancing isn’t free, and the costs matter more the more frequently you do it:

• Transaction costs. Every rebalancing trade has some cost, even if commissions are zero — bid-ask spread, market impact on large trades, or fund-level costs passed through to you.
• Taxes. In a taxable account, selling appreciated holdings to rebalance can trigger capital gains. Tax-loss harvesting is a related but distinct practice that can sometimes be combined with rebalancing to offset some of that tax impact — selling a losing position to harvest the loss can double as a step toward the target allocation.
• Retirement accounts avoid the tax problem. Rebalancing inside a tax-advantaged account like a 401(k) or IRA doesn’t trigger a taxable event, which is why many advisors recommend doing the bulk of rebalancing there when a portfolio spans both taxable and tax-advantaged accounts.

Rebalancing vs dollar-cost averaging

These two practices are often confused because both involve buying on a schedule, but they solve different problems. Dollar-cost averaging is about how you add new money to a portfolio over time, smoothing out the effect of entering at any single price point. Rebalancing is about managing money already invested, restoring a target mix regardless of whether new money is being added at all. A portfolio can use both simultaneously without conflict — dollar-cost averaging new contributions while periodically rebalancing the whole thing.

The takeaway

Portfolio rebalancing restores a portfolio’s target allocation after market moves push it out of line, by systematically selling assets that have grown into an oversized share and buying the ones that have lagged. It’s a risk-management discipline, not a return-chasing strategy — its job is keeping your actual risk exposure matched to the one you originally chose, weighed against the real transaction and tax costs of trading to get there.