If you set up an investment portfolio with a target allocation — say, 70% stocks and 30% bonds — and then leave it alone for a few years, market returns will drift it away from those targets. A strong stock market might push your allocation to 85% stocks and 15% bonds, taking on more risk than you intended. A market downturn might push it the other way. Rebalancing is the process of periodically restoring your portfolio to its target allocation by selling what has grown beyond its target and buying what has fallen behind. It sounds straightforwardly sensible — and it mostly is — but the details of how, when, and how often to rebalance matter more than most investors realise.
Why Rebalancing Matters
The primary purpose of rebalancing is risk management, not return enhancement. When you set a target asset allocation, you’re defining the level of portfolio volatility and risk exposure that’s appropriate for your time horizon, financial situation, and risk tolerance. If your portfolio drifts significantly away from that target, you’re taking on more or less risk than you intended — and either outcome is a problem. An 85% stock allocation when you planned for 70% means your portfolio will decline more severely in a bear market than your plan assumed. A 55% stock allocation when you planned for 70% means your portfolio will grow more slowly than your plan requires to reach your retirement goals.
Rebalancing also enforces a mild version of “buy low, sell high” — not through market timing, but mechanically. When stocks have risen significantly relative to bonds, rebalancing sells some stocks (which are now relatively expensive) and buys bonds (which are now relatively cheap). When stocks have fallen significantly, rebalancing sells bonds and buys stocks at lower prices. This systematic approach to trimming winners and adding to laggards has modest but real effects on long-term risk-adjusted returns, though the magnitude is debated and depends heavily on the specific market conditions and rebalancing approach used.
Calendar vs. Threshold Rebalancing
There are two main approaches to deciding when to rebalance. Calendar rebalancing means reviewing and restoring your portfolio on a fixed schedule — annually, semi-annually, or quarterly — regardless of how much drift has occurred. Threshold rebalancing means rebalancing only when your allocation drifts beyond a defined band — for example, rebalancing whenever any asset class deviates more than 5 percentage points from its target. Research comparing these approaches consistently finds that threshold-based rebalancing slightly outperforms calendar-based rebalancing in risk-adjusted terms, primarily because it avoids unnecessary trading in periods when drift is minimal while ensuring prompt correction when significant drift has occurred.
For most individual investors, annual calendar rebalancing — combined with a threshold check (rebalance annually unless a major market move has created significant drift earlier) — provides an adequate balance between maintaining target allocation and minimising transaction costs and complexity. More frequent rebalancing increases trading costs without proportional benefit; less frequent rebalancing allows meaningful allocation drift that can significantly affect risk exposure over multi-year bull or bear markets. Annual rebalancing coinciding with your regular financial review is a practical and well-supported approach for most portfolios.
The Tax Cost of Rebalancing in Taxable Accounts
Rebalancing in a taxable brokerage account generates taxable events when you sell appreciated assets to restore your target allocation — a cost that’s often overlooked in discussions of rebalancing benefits. Selling appreciated stock to buy bonds, for example, triggers capital gains taxes on the appreciation, which reduces the after-tax return of the portfolio even if the pre-tax case for rebalancing is sound. For portfolios held primarily in taxable accounts, this tax cost can meaningfully reduce or even eliminate the rebalancing benefit, particularly for investors with large unrealised gains in equity positions.
Several strategies reduce the tax cost of rebalancing in taxable accounts. Tax-loss harvesting — selling positions at a loss to offset gains elsewhere in the portfolio — can generate losses that offset the gains from rebalancing sales, reducing the net tax impact. Directing new contributions to underweight asset classes rather than selling overweight ones accomplishes rebalancing without triggering any sales and therefore without tax consequences. Rebalancing preferentially in tax-advantaged accounts — selling and buying within your IRA or 401(k) where no annual tax applies — keeps the taxable account allocation more stable and reduces the frequency of taxable rebalancing events there. Most financial advisors suggest locating more volatile assets in tax-advantaged accounts partly for this reason: it concentrates the necessary rebalancing activity in accounts where it has no annual tax cost.
Target-Date Funds: Automatic Rebalancing Built In
For investors in target-date funds — which are most people with 401(k) plans invested in their plan’s default option — rebalancing happens automatically and invisibly. The fund manager maintains the target allocation internally, rebalancing continuously as needed without any action required from the investor. This is one of the significant conveniences of target-date funds: they eliminate the rebalancing task entirely while maintaining a disciplined allocation. The absence of visible transactions also means investors don’t face the behavioural challenge of selling appreciated assets (which can feel counterintuitive) or the decision fatigue of determining when and how much to rebalance.
How Much Does Your Allocation Actually Need to Drift Before Rebalancing?
Research on optimal rebalancing thresholds generally suggests that bands of 5 to 10 percentage points from target are appropriate for most investors — wide enough to avoid excessive trading in normal market fluctuations, narrow enough to catch meaningful allocation drift before it significantly alters portfolio risk. A 70/30 portfolio might use a threshold of rebalancing when stocks drift above 80% or below 60% — a 10-point band that would have triggered rebalancing during the major market moves of 2020, 2022, and several other periods without generating unnecessary trading in calmer years.
Tighter thresholds produce more frequent rebalancing, higher trading costs, and more tax events, with diminishing marginal improvement in risk management. Wider thresholds reduce trading costs and tax events while allowing more allocation drift. The specific threshold is less important than having one and consistently applying it. An investor who never rebalances is making a different but equally valid point — accepting drift in exchange for zero trading costs — but should understand that a portfolio started at 70/30 in 2013 would have been approximately 88/12 by 2022 without rebalancing, taking on substantially more risk than the original allocation intended. Whether that additional risk was appropriate for that investor’s circumstances is a personal question, but it’s worth answering deliberately rather than discovering accidentally during the next major market downturn.
Rebalancing in Practice: A Simple Example
To make rebalancing concrete: imagine a $200,000 portfolio targeting 70% stocks ($140,000) and 30% bonds ($60,000). After a strong year for equities, stocks have grown to $165,000 and bonds to $62,000 — a total of $227,000. The new stock allocation is 73% ($165,000 ÷ $227,000), drifting 3 percentage points from the 70% target. If your threshold is 5 percentage points, no rebalancing is needed yet. If stocks continue rising and reach $185,000 while bonds remain at $62,000, the total is $247,000 and the stock allocation has reached 75% — 5 points above target, triggering a rebalance. You’d sell approximately $12,350 in stocks and buy $12,350 in bonds to restore the 70/30 target. In a tax-advantaged account this is straightforward. In a taxable account, you’d consider whether new contributions could accomplish the same rebalancing without triggering sales, or whether the tax cost of selling is worth paying to restore the target allocation. The discipline of working through this calculation annually keeps your portfolio aligned with the risk level you’ve chosen rather than drifting toward whatever asset class happened to perform best recently.
The Behavioural Case for Rebalancing Beyond the Numbers
Beyond the mathematical rationale, rebalancing serves a valuable behavioural function: it creates a structured, rule-based process for selling what has recently done well and buying what has recently done poorly — essentially the opposite of the momentum-chasing and panic-selling that characterise poor investor behaviour. An investor who has pre-committed to rebalancing annually doesn’t need to make a judgment call about whether markets have peaked or bottomed; they simply restore the target allocation regardless of recent market narrative. This pre-commitment removes the emotional content from what would otherwise be difficult decisions — it’s not “I think stocks are overvalued,” it’s “stocks are above my threshold and I rebalance to target.” The psychological benefit of a rules-based process — reduced anxiety, reduced temptation to time the market, reduced dependence on confidence in one’s own market predictions — complements the risk management rationale for rebalancing and produces better real-world investor behaviour over full market cycles.
Rebalancing is one of the few portfolio management activities that has clear theoretical and empirical support, imposes modest ongoing effort, and systematically reinforces the disciplined behaviour — maintaining a target allocation regardless of recent market performance — that separates investors who achieve their financial plans from those who don’t. Whether you rebalance annually, semi-annually, or on a threshold basis matters less than whether you rebalance at all, consistently, according to a pre-committed rule rather than a reactive judgment about market conditions.
