Rebalancing restores your portfolio to its intended asset allocation after market drift pushes it off target. It prevents you from accidentally carrying more risk than you planned and mechanically enforces buying low and selling high. Annual rebalancing inside tax-advantaged accounts is usually the simplest, most effective approach for most investors.
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In January 2022, the S&P 500 fell roughly 20 percent in less than seven months.1 Investors who had never rebalanced their portfolios were sitting far heavier in equities than their original plan called for, because the 2020 and 2021 bull run had silently pushed a 70/30 stock-bond portfolio to something closer to 80/20. When the drop came, they lost more than they had agreed to lose.
That is not bad luck. That is drift.
Let's start with what drift actually is, because most explanations skip the mechanism and jump straight to the fix.
How a Portfolio Drifts Without You Doing Anything
Picture two buckets. You start with $100,000 split according to a target allocation: $70,000 in stocks and $30,000 in bonds.2 You have done your homework and decided that mix matches your timeline and your tolerance for a bad year.
Now let five years pass without touching it. Stocks return 8 percent a year, bonds return 4 percent. When you check in:
- Stocks: $70,000 compounded at 8% for five years = $102,797
- Bonds: $30,000 compounded at 4% for five years = $36,530
- Total: $139,327
Your stock weight has crept from 70 percent to 73.8 percent. That does not sound dramatic, but it compounds. Another strong run and you are at 78 percent. In a sustained bull market you can drift from 70/30 to 80/20 without making a single conscious decision to take on more risk. When the correction arrives, your losses are calibrated to the portfolio you have, not the portfolio you planned.
Rebalancing is how you close that gap. The SEC defines it simply: you bring the portfolio back to its original asset allocation mix.2 In practice that means selling some of the bucket that grew too large and buying more of the one that shrank.
Why This Is Psychologically Hard and Financially Useful
The act of rebalancing asks you to sell what has been winning and buy what has been lagging. That goes against every intuition you have. The natural instinct is to let the winners ride and avoid the losers. Rebalancing overrides that instinct with a rule, and the rule turns out to produce something valuable: a mechanical approximation of buy low, sell high.
Consider two portfolios starting at the same 70/30 split, running through a year where stocks return 8 percent, followed by a year where stocks drop 10 percent.
Portfolio A (no rebalancing): After year one, $75,600 in stocks and $31,200 in bonds (70.8/29.2). After year two's drop, $68,040 in stocks and $31,824 in bonds. Total: $99,864.
Portfolio B (rebalanced after year one): Trimmed back to 70/30 at $74,760 and $32,040. After year two's drop, $67,284 in stocks and $32,681 in bonds. Total: $99,965.
The gap is a hundred dollars on $100,000. That sounds trivial. Over 30 years and multiple market cycles, those small consistent corrections add up, and the bigger benefit is that your risk profile never wanders far from what you actually intended to carry.
When to Rebalance: The Two Methods
There are two approaches, and they go by the names time-based and threshold-based.
Time-based rebalancing means you pick a calendar date, every January 1 for instance, and you check your allocation then regardless of what markets have done. It is simple, predictable, and removes the temptation to tinker based on recent headlines. The downside is that markets can drift significantly between your scheduled dates.
Threshold-based rebalancing means you set a trigger: if any asset class drifts more than 5 percentage points from its target, you rebalance then, not on a schedule. This keeps your allocation tighter but demands more monitoring and can generate more transactions, which matters in taxable accounts.
For most investors, annual time-based rebalancing is the right call. The research on which approach produces better outcomes is genuinely mixed, and the simpler method is the one you will actually follow through on. Set the reminder, check once a year, move on.
The Tax Problem in Taxable Accounts
Here is where rebalancing gets more complicated, and where a lot of people either over-simplify or avoid it altogether.
In a 401(k) or a traditional IRA, you can buy and sell freely within the account because those transactions do not trigger a taxable event until you take a distribution.3 Rebalancing inside a 401(k) costs you nothing today. Same for a Roth IRA, where qualified distributions are tax-free entirely.4 The IRS rule here is straightforward: the tax advantages of these accounts extend to the trades you make inside them.5
In a taxable brokerage account, it is a different situation. When you sell shares that have appreciated, you realize a capital gain, and the IRS taxes it. The rate depends on how long you held the position: long-term gains (assets held over one year) are taxed at 0 percent, 15 percent, or 20 percent depending on your income, while short-term gains are taxed as ordinary income, which is typically higher.6
This does not mean you should skip rebalancing in taxable accounts. It means you should rebalance smarter.
The cleanest method is to use new contributions to rebalance rather than selling. Say your portfolio has drifted to 75 percent stocks when your target is 70 percent. You have $10,000 to invest this month. Instead of splitting it 70/30, you put the entire $10,000 into bonds. You have nudged the allocation back toward target without selling a single share or generating a single taxable event.
When the drift is large enough that new contributions cannot close it on their own, then yes, you sell. You pay the tax. But at that point the drift itself represents a larger risk problem than the tax bill you are trying to avoid, and the rebalancing benefit is almost always worth it. Just plan the sale in a low-income year if you can, where the long-term capital gains rate may fall to zero.6
What Your Target Allocation Should Actually Be
Rebalancing only makes sense if you have a target to rebalance to. That target should reflect two things: your time horizon and your ability to sit through a down year without flinching.
A rough framework: investors with 20 or more years before they need the money can carry 80 to 90 percent in equities. The volatility is real, but time absorbs it.1 Investors within ten years of a major goal, a retirement date or a home purchase, want more bonds and cash equivalents in the mix, because a bad year at the wrong moment genuinely matters. The classic age-based rule of thumb (put your age in bonds) is a starting point, not a law.
The retirement calculator below can help you stress-test a target allocation against realistic return assumptions. Run the numbers with your actual savings rate and timeline before locking in a split.
The Debate: Does Rebalancing Actually Improve Returns?
Let's be honest about what the evidence says. In a sustained bull market, rebalancing consistently trims your equity exposure just as stocks are climbing. The non-rebalancer beats you during that stretch. The research is genuinely mixed on whether rebalancing adds return or just reduces volatility.7
What is not mixed: rebalancing keeps your risk profile where you set it. Whether the stock-heavy drifted portfolio ends up outperforming over a 30-year period depends on which 30 years you happen to live through. What rebalancing guarantees is that the risk you carry is the risk you chose, and that matters a great deal when markets turn and you are deciding whether to stay invested or sell in a panic.
Overall, the case for rebalancing is not primarily a return argument. It is a discipline argument. Most investors do not underperform because they picked bad funds. They underperform because they bought high in excitement and sold low in fear.1 Rebalancing builds the opposite behavior into the structure of the portfolio, not into willpower.
The portfolio you end up with should look like the portfolio you intended. Rebalancing is the only way to make sure it does.
