Portfolio rebalancing is the practice of selling some of what has grown and buying more of what has lagged, to bring a portfolio back to its original target mix of stocks, bonds, and other assets. Vanguard's 2022 research on multi-asset rebalancing found that annual rebalancing is optimal for most investors who are not actively harvesting tax losses, and that 80% to 90% of the benefit of staying rebalanced comes from remaining exposed to the long-run return of stocks and bonds, not from the mechanics of the trades themselves.
The need for rebalancing comes from a simple fact: different assets grow at different rates. The U.S. Securities and Exchange Commission's Office of Investor Education and Advocacy explains it this way in its investor guidance: over time, some investments in a portfolio will grow faster than others, so a mix that started at, say, 60% stocks and 40% bonds can drift to 70% stocks and 30% bonds after a strong run in equities. That drift is not a mistake. It is compounding doing what compounding does. But it leaves a portfolio holding more risk than the investor originally chose to carry.
What Is Portfolio Rebalancing, and Why Does a Portfolio Drift?
Rebalancing is the act of bringing a portfolio back to its original asset allocation mix, according to Investor.gov, the SEC's investor-education site. A portfolio drifts because asset classes do not move in lockstep. A multi-year stretch of strong stock returns will raise the stock allocation's share of the total portfolio even if the investor never buys or sells anything, simply because stock holdings gained more in dollar terms than bond holdings did.
Left alone, that drift compounds on itself. A portfolio that starts moderate can gradually become aggressive, carrying more downside risk than the investor signed up for when they set the original mix. Rebalancing does not stop drift from happening between checkpoints; it resets the mix at the checkpoints the investor or their plan chooses.
What Are the Two Main Ways to Rebalance?
The SEC's guidance describes two broad approaches. The first is calendar-based: rebalancing on a fixed schedule, such as every six or twelve months, regardless of how far the portfolio has drifted. The advantage, per the SEC, is that the calendar acts as a built-in reminder, so investors do not need to watch allocations continuously.
The second is threshold-based: rebalancing only when an asset class's weight moves more than a set percentage away from its target, decided in advance. Vanguard's research modeled this approach against calendar rebalancing and combinations of the two, and found a threshold of roughly 1% to 3%, paired with periodic monitoring, produces results close to more complex, continuously monitored strategies for most portfolios.
There is also a third, lower-friction method the SEC describes: directing new contributions toward whichever asset class has fallen below its target, rather than selling anything at all. For an investor still adding money regularly — through a workplace retirement plan, for instance — this can rebalance a portfolio gradually without triggering a single sale.
How Often Should an Investor Rebalance?
Vanguard's analysis, published in October 2022 and covering multi-asset portfolios, found what its researchers described as a hump-shaped pattern in outcomes: rebalancing too rarely (every two years or never) and rebalancing too often (monthly or daily) both produced worse risk-adjusted outcomes than a middle ground. For investors not doing active tax-loss harvesting, the research identified annual rebalancing as the point that best balanced staying close to the target allocation against unnecessary trading.
That finding does not mean monthly rebalancing is harmful in every case, only that in Vanguard's modeling it did not add enough benefit to offset the extra trading it required. The same research found annual rebalancing meaningfully outperformed monthly rebalancing specifically during high-volatility stretches, in the risk-adjusted metric Vanguard used to compare strategies.
Does Rebalancing Trigger Taxes?
It can, and the mechanism runs through ordinary capital-gains rules. Per the IRS, if an investor sells an appreciated holding they have owned for more than one year, the gain is taxed as a long-term capital gain; if held one year or less, it is taxed as a short-term gain at ordinary income rates. For 2025, the IRS lists long-term capital-gains rates of 0%, 15%, or 20% depending on taxable income, with narrower categories such as collectibles taxed as high as 28%.
This is why the SEC's guidance specifically tells investors to consider, before rebalancing, whether the method they choose will trigger transaction fees or tax consequences, and to talk with a tax adviser about ways to limit the cost. Rebalancing inside a tax-advantaged account, such as an IRA or a 401(k), does not create a taxable event when assets are sold and repurchased within the account. Rebalancing in a taxable brokerage account can create one, because selling an appreciated position realizes a gain the IRS taxes in that year. Directing new contributions to underweight assets, rather than selling appreciated ones, is one way investors and their advisers commonly avoid realizing gains solely for the purpose of rebalancing — though the right approach for a given tax situation is a question for a tax professional, not a general rule.
What Does Rebalancing Actually Change — Return or Risk?
The evidence points mainly to risk control, not return enhancement. Vanguard's research attributed the large majority of rebalancing's measured benefit — 80% to 90% — to staying invested and exposed to the underlying returns of stocks and bonds, rather than to any edge from the timing of the trades. In other words, rebalancing's main job is keeping a portfolio's risk level where the investor set it, not generating extra performance.
Past research findings describe what happened in the specific periods and models Vanguard studied; they are not a projection of what any individual portfolio will earn going forward, and none of this is a recommendation to buy, sell, or hold any specific investment. Rebalancing is a maintenance decision about risk, made with reference to an investor's own goals, time horizon, and tax situation — the kind of decision an investor typically works through with a financial or tax professional rather than a one-size-fits-all rule.
Frequently Asked Questions
Do I have to sell anything to rebalance?
No. The SEC describes three methods: selling overweight holdings to buy underweight ones, buying more of the underweight asset with new cash, or directing future contributions toward the underweight asset until the mix evens out.
Is more frequent rebalancing always safer?
Not according to Vanguard's modeling. Its research found both very infrequent and very frequent rebalancing performed worse on a risk-adjusted basis than an annual approach, for investors not doing tax-loss harvesting.
Does rebalancing guarantee better returns?
No. This article is general investing education, not a recommendation to buy or sell any security, and past research findings are not a projection of future results. Rebalancing manages risk exposure; it does not promise a particular outcome.
Can rebalancing trigger a tax bill?
It can, if it involves selling an appreciated position in a taxable account. Per the IRS, the gain is taxed as long-term or short-term depending on the holding period, at rates that depend on the investor's income. A tax adviser can address an individual's specific situation.
For a related investing perspective, read How the Wash-Sale Rule Actually Works, and What Counts as Substantially Identical.
For more context, read How Often Should You Rebalance a Long-Term Portfolio?.
