Portfolio rebalancing is the practice of periodically adjusting a portfolio's holdings back to a target asset allocation after market gains or losses cause it to drift.
Portfolio Rebalancing
Portfolio rebalancing is the process of periodically buying or selling assets within a portfolio to bring it back to its original or target asset allocation, such as a 60% stocks and 40% bonds split. Because different asset classes grow at different rates, a portfolio's actual weights naturally drift away from that target over time. If stocks post several strong years while bonds lag, the equity portion might climb from 60% to 70% of the total portfolio, even though the investor never actively chose to take on more stock exposure. Rebalancing corrects this drift by trimming the assets that have grown disproportionately large and reinvesting the proceeds into the assets that have fallen behind, restoring the intended mix.
The direction of the drift tells you what rebalancing is really doing. When an asset class's weight rises well above its target, it usually means that asset has outperformed recently and now represents a larger share of the portfolio's risk than the investor originally intended. Selling a portion of it and buying more of the underperforming asset effectively locks in some of the gains from the winner and adds to a position that may be relatively cheaper. This is why rebalancing is often described as a disciplined way of buying low and selling high, since it forces decisions based on target weights rather than on emotion or recent performance.
In practice, rebalancing is not free. Selling appreciated assets can trigger transaction costs and, in a taxable brokerage account, capital gains taxes, so investors need to weigh the benefit of staying close to target against the cost of trading too frequently. Many long-term investors rebalance only once or twice a year, or use new contributions and dividends to nudge the portfolio back toward target instead of selling existing holdings outright. The right frequency and method depend on account type, tax situation, and how much drift an investor is willing to tolerate before risk levels feel uncomfortable.
Example
Suppose an investor starts with a $50,000 portfolio allocated 60% to stocks ($30,000) and 40% to bonds ($20,000). Over the next two years, a strong stock market pushes the equity portion up to $42,000, while the bond portion grows more slowly to $23,000, bringing the total portfolio to $65,000. The new allocation is now roughly 65% stocks and 35% bonds, meaning the portfolio carries noticeably more equity risk than the investor originally intended, even though no new money was added.
To rebalance back to the 60/40 target on a $65,000 portfolio, the investor needs $39,000 in stocks and $26,000 in bonds. That means selling about $3,000 worth of stocks and using the proceeds to buy about $3,000 worth of bonds. This single trade restores the target allocation, locks in part of the stock market's recent gains, and reduces the portfolio's overall risk level back to what the investor originally planned for.
Practical Application
Rebalancing is a core part of how financial advisors and target-date retirement funds manage risk over time. In a 401(k) or IRA, rebalancing keeps a portfolio's stock-to-bond mix aligned with an investor's time horizon and risk tolerance, which is especially important as retirement approaches and the account can afford less volatility. Robo-advisors typically automate this process, monitoring allocations daily or monthly and executing trades automatically once drift exceeds a set threshold, removing the need for the investor to time the market manually.
Institutional investors, including pension funds and endowments, use rebalancing policies to enforce discipline across large, diversified portfolios spanning stocks, bonds, real estate, and alternative assets. A written rebalancing policy specifies target weights and tolerance bands in advance, so decisions are made systematically rather than reactively during periods of market stress. This structure helps prevent a fund from unintentionally becoming overexposed to whichever asset class has recently performed best, which is often the riskiest time to be overweight that asset.
Common Mistakes
A common mistake is rebalancing too frequently, reacting to every small market move instead of waiting for meaningful drift. In taxable accounts, frequent trading can trigger transaction costs and capital gains taxes that outweigh the benefit of staying precisely at the target allocation. Most investors are better served by checking their allocation on a set schedule, such as annually, or only acting once an asset class has drifted beyond a defined tolerance band, rather than trading every time the market moves.
Another mistake is ignoring rebalancing entirely and letting a portfolio drift indefinitely, which can quietly turn a moderate-risk portfolio into an aggressive one after a long stock market rally. Investors who never rebalance often don't realize how much their risk exposure has changed until a downturn hits and losses are far larger than expected. Sticking to a target allocation without periodic rebalancing defeats much of the purpose of choosing that allocation in the first place.
Investors also sometimes rebalance based on gut feeling about where markets are headed rather than on their predetermined target weights or tolerance bands, which turns a disciplined process into a form of market timing. Rebalancing works best as a mechanical, rules-based habit, not a judgment call made in the moment. Writing down target allocations and tolerance bands in advance removes emotion from the decision.
Finally, some investors forget to account for taxes and account type when rebalancing, selling winners in a taxable account when the same adjustment could have been made inside a tax-advantaged IRA or 401(k) instead. Coordinating rebalancing across all of an investor's accounts, rather than treating each account in isolation, can reduce unnecessary tax costs while still achieving the desired overall allocation.
Comparison
Dimension
Portfolio Rebalancing
Buy and Hold
Definition
Periodically adjusting holdings back to a target allocation
Holding original investments indefinitely without adjustment
How it's done
Selling outperforming assets and buying underperforming ones on a schedule or threshold
No trading after the initial purchase, regardless of drift
What it indicates/controls
Keeps portfolio risk aligned with the investor's original target
Risk level drifts freely with market performance over time
There's no single right answer, but most individual investors rebalance either on a fixed calendar schedule, such as once a year, or whenever an asset class drifts beyond a set tolerance band, such as 5 percentage points from its target. Calendar-based rebalancing is simple and easy to stick to, while threshold-based rebalancing reacts only to meaningful drift and can reduce unnecessary trading. Rebalancing too often adds transaction costs and, in taxable accounts, capital gains taxes, so many advisors recommend checking allocations quarterly but only actually trading once or twice a year unless drift is significant.
Does rebalancing guarantee better returns?
No. Rebalancing is primarily a risk-management tool, not a way to boost returns. Its main purpose is to keep a portfolio's risk level aligned with an investor's original goals and risk tolerance, rather than letting it drift toward whichever asset class has recently performed best. In some periods rebalancing improves returns by systematically buying low and selling high, but in a market with a strong sustained trend, it can also mean selling a winning asset before its rally fully plays out. The real benefit is more consistent risk exposure over time, not guaranteed outperformance.
Can I rebalance without selling any investments?
Yes, this is often called cash-flow rebalancing. Instead of selling existing holdings, an investor directs new contributions, dividends, or interest payments toward whichever asset class is currently underweight relative to its target. Over time, these incoming funds gradually nudge the portfolio back toward its target allocation without triggering a taxable sale of appreciated assets. This approach works especially well for investors who are still actively contributing to their portfolio, such as through regular retirement account deposits, though it may not fully correct large amounts of drift on its own.
What's the difference between calendar and threshold rebalancing?
Calendar rebalancing happens on a fixed schedule, such as quarterly or annually, regardless of how far the portfolio has actually drifted from target. Threshold rebalancing, sometimes called tolerance-band rebalancing, is triggered only when an asset class's weight moves beyond a predetermined range, such as plus or minus 5 percentage points, no matter when that happens. Threshold rebalancing tends to react more precisely to actual market movements and can reduce unnecessary trading during calm periods, while calendar rebalancing is simpler to plan around and doesn't require constant monitoring of the portfolio.
Is rebalancing worth it if it costs money in fees or taxes?
For most long-term investors, yes, because the goal of rebalancing is controlling risk, not avoiding all costs. A portfolio that's allowed to drift significantly can end up far riskier than an investor intended, which matters more than a modest amount of trading costs. That said, the costs are still worth minimizing: using tolerance bands instead of rigid calendar schedules, rebalancing inside tax-advantaged accounts first, and using new cash flows before selling appreciated assets are all ways to capture most of rebalancing's risk-control benefit while keeping fees and taxes low.