Every portfolio starts with a target mix, and every market move quietly changes it. This article explains what rebalancing is, the three standard ways to do it, and why the tax and trading costs it can trigger deserve as much attention as the allocation itself. It draws mainly on US regulator guidance, with UK capital gains tax rules as a contrasting example.
Why a portfolio drifts away from its target
An investor who chooses an asset allocation, say a split between stocks and bonds, is deciding how much risk to carry. The US Securities and Exchange Commission (SEC) describes asset allocation as dividing a portfolio among categories such as stocks, bonds and cash, with the right mix depending largely on time horizon and ability to tolerate risk.
Markets do not respect that choice. When stocks rise faster than bonds, stocks become a larger share of the whole, and the portfolio carries more risk than its owner originally intended. The SEC’s own illustration involves stock holdings growing from 60% to 80% of a portfolio after a strong market. Nothing was bought or sold in that scenario. The risk simply changed on its own.
What is rebalancing, and what is it meant to do?
According to the SEC’s beginners’ guide to asset allocation, diversification and rebalancing, rebalancing means bringing a portfolio back to the desired asset allocation mix. The goal is risk control, not return maximization. A rebalanced portfolio holds the level of risk that was chosen at the start, rather than whatever level recent performance happens to have produced.
The SEC also notes that rebalancing can have a “buy low, sell high” character, because it involves cutting back on assets that have recently done well and adding to those that have lagged. That is a side effect worth understanding, but it is not guaranteed to improve returns. If a winning asset keeps winning, trimming it can mean giving up gains in the short run. This connects to a point made in our explanation of diversification: diversification is a structural choice, and rebalancing is the maintenance that keeps the structure intact.
A worked example with simple numbers
The arithmetic below is illustrative only. It assumes a $100,000 portfolio split 60% stocks and 40% bonds, with no fees or taxes.
| Step | Stocks | Bonds | Total | Stock share |
|---|---|---|---|---|
| Starting point | $60,000 | $40,000 | $100,000 | 60.0% |
| After a year (stocks +30%, bonds +2%) | $78,000 | $40,800 | $118,800 | about 65.7% |
| Target at 60/40 of $118,800 | $71,280 | $47,520 | $118,800 | 60.0% |
To return to 60/40, the portfolio needs to move $6,720 from stocks into bonds ($78,000 minus $71,280, which equals $47,520 minus $40,800). The drift in this example is modest. Over several years of uneven returns, it can be much larger.
The three standard ways to rebalance
The SEC guide lists three approaches:
- Sell and reallocate. Sell investments from the over-weighted categories and use the proceeds to buy investments in the under-weighted ones.
- Add new money to the laggards. Buy new investments in the under-weighted categories without selling anything.
- Redirect ongoing contributions. Alter contributions so more flows to the under-weighted categories until the portfolio is back in balance.
The second and third methods avoid selling, which matters because selling can trigger costs. They are only practical for investors who are adding money regularly, and they work slowly if the drift is large relative to new contributions.
How often should a portfolio be rebalanced?
The SEC guide describes two common schools of thought among financial experts. One is calendar-based: rebalance at regular intervals, such as every six or twelve months. The other is threshold-based: rebalance only when an asset class’s relative weight rises or falls by more than a pre-set percentage.
The guide adds that rebalancing tends to work best when done on a relatively infrequent basis. That is partly practical. Each rebalance can involve trading costs, and a portfolio that is adjusted constantly may spend more on those costs than it gains in risk control. Fees compound against investors just as returns do, so a routine that trades often has a price.
| Approach | Trigger | Main strength | Main drawback |
|---|---|---|---|
| Calendar | Fixed date, such as every 6 or 12 months | Simple and predictable | May trade when drift is trivial, or wait while drift is large |
| Threshold | Allocation moves beyond a set percentage | Acts only when drift matters | Requires regular monitoring |
Neither approach is universally better. Both are ways of deciding in advance, so that the decision is not made in the middle of a market panic or a rally.
Does rebalancing trigger tax?
It can, and this is where location matters. The SEC guide advises considering whether the chosen rebalancing method will trigger transaction fees or tax consequences before acting.
United States. According to the Internal Revenue Service (IRS), a capital gain on an asset held one year or less is short-term, and one held for more than a year is long-term. For tax years beginning in 2025, the IRS lists long-term capital gains rates of 0%, 15% and 20%, depending on taxable income. Selling an appreciated holding in an ordinary taxable brokerage account is a taxable event. How gains are treated inside tax-advantaged retirement accounts is a separate set of rules that this article does not cover.
United Kingdom. According to GOV.UK, for shares from 6 April 2026 the capital gains tax rates are 18% for gains within the basic-rate Income Tax band and 24% above it for basic-rate taxpayers, and a flat 24% for higher and additional-rate taxpayers. The page lists a tax-free allowance of £3,000.
Illustrative UK arithmetic: suppose rebalancing realizes a gain of £4,000 in a taxable account, and the £3,000 allowance is otherwise unused. The taxable gain is £1,000. At 18% the tax is £180, and at 24% it is £240. The same sale inside a tax-sheltered wrapper may work differently, and the UK and US rules above are not interchangeable.
What are the alternatives to doing it manually?
The SEC guide also describes lifecycle or target-date funds, which automatically shift toward a more conservative mix of investments as they approach a particular year in the future, such as a fund labeled “Retirement Fund 2030.” These funds handle rebalancing inside the fund, which removes the decision from the investor. The trade-off is that the investor accepts the fund’s glide path and its charges rather than choosing their own. Compare the costs carefully, as with any fund.
Redirecting contributions is another low-friction route. An investor who is saving every month can often steer new money toward the under-weighted category, which keeps the portfolio near its target without any selling. Steady contributions also drive long-term results, so this method supports two goals at once.
The details of a rebalancing rule matter less than having one. An investor with no rule tends to decide case by case, and those decisions are often shaped by recent headlines: trimming a falling asset out of fear, or letting a rising one run because it feels safe. A written rule, whether calendar-based or threshold-based, replaces that improvisation with a decision made calmly in advance. It also gives the investor a clear moment to check whether the target allocation itself still fits their goals and time horizon, which is a separate question from whether the portfolio has drifted.
Common misunderstandings about rebalancing
- It is not a way to beat the market. It controls risk relative to a chosen plan. Whether it improves returns depends on how markets behave afterward.
- It is not market timing. A rebalancing rule responds to the portfolio’s drift, not to a forecast.
- It does not remove loss. A portfolio back at 60/40 can still fall in value.
- It is not free. Costs and taxes can reduce the benefit, especially for small drifts.
Rebalancing is a cost-versus-risk decision
Rebalancing comes down to a trade-off: letting risk drift without limit on one side, and paying trading costs and possibly tax on the other. Regulator guidance points to a measured middle: a pre-set rule, infrequent action, a preference for methods that avoid unnecessary selling, and awareness of the tax rules in the investor’s own country. The right target mix and schedule depend on individual goals and circumstances.
This article is general information, not personalized financial advice. Tax rules, rates and allowances change and differ by country; check current rules or consult a qualified professional.
Sources: SEC; IRS; GOV.UK
