Portfolio Rebalancing: When, Why, and How
Left alone, a 60/40 portfolio drifts toward 70/30 in a long bull market, quietly raising your risk. Here's when and how to rebalance back to your plan.
Don't have time? Here's what you need to know:
- 1Markets drift your allocation off target — a 60/40 mix can quietly become 70/30, raising your risk uninvited.
- 2Rebalancing forces you to sell high and buy low on a rule, removing emotion from the decision.
- 3The three methods are calendar, threshold (e.g., the 5/25 rule), and cash-flow; a hybrid of annual checks plus a threshold works well.
- 4Rebalance in tax-advantaged accounts and use new contributions in taxable ones to avoid unnecessary capital-gains taxes.
Why Portfolios Drift Off Target
Rebalancing is the process of restoring your portfolio to its target asset allocation after market movements have pushed it out of line. The need arises because different assets grow at different rates. If you start at a 60% stocks / 40% bonds mix and stocks outperform over several years, your portfolio can quietly drift to 70/30 or higher — without you buying a single share. The allocation you chose has been replaced by one the market chose for you.
That drift matters because it silently increases your risk. A 60/40 investor who lets the mix climb to 75/25 during a long bull run is now far more exposed when the next downturn hits than they intended to be. Rebalancing is how you keep your actual risk in line with the risk you signed up for — it is fundamentally a risk-control discipline, not a return-maximizing trick.
Rebalancing Forces You to Buy Low and Sell High
The elegant thing about rebalancing is that it automates a behavior most investors find emotionally impossible. To return to your target, you sell some of what has gone up (trimming the asset that has become overweight) and buy more of what has lagged (topping up the underweight asset). That is, by definition, selling high and buying low — and doing it mechanically, on a rule, instead of trying to guess the market's mood.
After a stock-market crash, for example, rebalancing tells you to sell some bonds and buy more stocks precisely when stocks are cheap and frightening to own. After a euphoric run-up, it tells you to trim stocks when everyone wants more. The rule removes emotion from the decision, which is its greatest practical value. Over the long run, the main benefit of rebalancing is this disciplined risk control; any return boost is a secondary bonus that is not guaranteed.
Tip: Rebalancing's biggest payoff is behavioral: it forces you to act against the crowd — buying after crashes, trimming after booms — on a rule instead of a feeling.
Three Ways to Rebalance
There are three main approaches, and they can be combined. Calendar rebalancing means resetting to your targets on a fixed schedule — annually is the common choice. Threshold (or band) rebalancing means acting only when an asset drifts beyond a set tolerance, such as the popular '5/25 rule': rebalance when an allocation moves more than 5 percentage points in absolute terms or 25% of its target weight, whichever is smaller. Cash-flow rebalancing means steering new contributions and dividends toward whichever asset is underweight, nudging the portfolio back without selling anything.
For most investors, a hybrid works best: check once a year, but only trade if an allocation has drifted past your threshold. This keeps you from rebalancing needlessly when nothing has moved much, while still catching the big drifts that actually matter. Cash-flow rebalancing is the gentlest method of all, because directing fresh money to the laggard avoids selling — which means no transaction friction and, crucially, no taxable gains.
| Method | Trigger | Best for |
|---|---|---|
| Calendar | A set date (e.g., once a year) | Simplicity and a fixed habit |
| Threshold / band | Drift past a tolerance (e.g., 5/25 rule) | Responding to what markets actually do |
| Cash-flow | New contributions and dividends | Avoiding sales and taxes (growing accounts) |
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Rebalance Without an Unnecessary Tax Bill
How you rebalance matters as much as when. In a tax-advantaged account like an IRA or 401(k), you can buy and sell freely to reset your targets, because no tax is triggered — so do the bulk of your rebalancing there. In a taxable account, selling an appreciated fund realizes a capital gain, so prefer methods that avoid sales: direct new contributions and reinvested dividends toward the underweight asset, and only sell as a last resort.
Over-rebalancing is a real cost. Resetting too frequently — monthly, say — racks up transaction friction and, in taxable accounts, taxable events, all in pursuit of precision that does not improve outcomes. Annual checks combined with a drift threshold capture nearly all the benefit at a fraction of the cost and hassle. If you want help working through the mechanics, our guide to rebalancing your ETF portfolio walks through a full example.
Important: Rebalancing too often — monthly, for instance — adds transaction costs and taxes without improving results. Annual-plus-threshold captures nearly all the benefit.
Frequently Asked Questions
What does it mean to rebalance a portfolio?
Rebalancing means restoring your portfolio to its target asset allocation after market movements have shifted it. For example, if a 60/40 stock-bond mix drifts to 70/30 because stocks rose, you sell some stocks and buy bonds to return to 60/40. It keeps your actual risk level aligned with the one you originally chose.
How does rebalancing make me buy low and sell high?
To return to target weights, you trim whatever has risen (now overweight) and add to whatever has lagged (now underweight) — selling high and buying low by definition. After a crash, the rule tells you to buy more stocks when they're cheap; after a boom, to trim them. It mechanizes a discipline most investors struggle to follow on emotion alone.
Does rebalancing increase my returns?
Not reliably. Rebalancing is primarily a risk-control tool — it keeps your allocation, and therefore your risk, in line with your plan. It can add a small return bonus in choppy, mean-reverting markets, but in a long one-directional bull run it can slightly reduce returns by trimming winners. Treat the risk control as the main benefit, not a guaranteed return boost.
What is the 5/25 rule for rebalancing?
It's a threshold rule for deciding when to rebalance: act when an asset class drifts more than 5 percentage points in absolute terms, or more than 25% of its target weight, whichever is smaller. For a 10% holding, the 25% band (2.5 points) triggers first; for a 50% holding, the 5-point band does. It rebalances only when drift is meaningful.
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Alex Harrington
CFA Level II Candidate, Finance & Economics
Alex Harrington is an independent ETF researcher and personal finance writer with over 8 years of experience analyzing exchange-traded funds. A CFA Level II candidate with a background in economics, Alex has reviewed 800+ ETFs and helped thousands of beginners build their first investment portfolios through clear, jargon-free education.
This content is for educational purposes only and does not constitute financial advice. Past performance does not guarantee future results. Consult a licensed financial advisor before making investment decisions.