Investing Fundamentals

Portfolio Rebalancing: Why, When, and How

4 min read

What Rebalancing Actually Does

Rebalancing restores your portfolio to its target asset allocation. If you set out to hold 70% stocks and 30% bonds, but a strong stock rally pushes you to 78/22, rebalancing means selling some stocks and buying bonds to get back to 70/30.

It feels counterintuitive: you're selling what's working and buying what isn't. But this discipline is exactly what controls risk and, over time, can modestly boost returns.

Why Bother?

In March 2009, the S&P 500 had fallen 57% from its 2007 peak. An investor who started 2007 at 70/30 was now closer to 40/60 by March 2009—bonds had become the dominant holding simply because stocks had cratered.

An investor who rebalanced in March 2009 (selling bonds to buy stocks) captured the subsequent decade-long bull market. One who let the portfolio drift missed the recovery on a portion of their capital that sat in bonds earning 2–3% while stocks compounded at 15%+.

Rebalancing forces you to buy low and sell high mechanically, without emotion. It systematically harvests the tendency of asset classes to mean-revert.

Two Approaches: Calendar vs. Threshold

Calendar-based rebalancing is the simplest. Pick a date—annually, semi-annually, or quarterly—and rebalance on that schedule regardless of market conditions. Annual rebalancing each January or on your birthday catches enough drift to matter without generating excessive trading.

Advantages: predictable, automatable, low effort. Disadvantages: you may rebalance when it's barely needed, or fail to rebalance during a mid-year crash that demands action.

Threshold-based rebalancing triggers a trade whenever an asset class drifts beyond a set band. A 5% absolute threshold is common: if your target is 70% stocks, rebalance when stocks exceed 75% or dip below 65%. For smaller allocations (say, 10% REITs), a 25% relative band (2.5 percentage points) works better.

Advantages: responds to market events in real time, no unnecessary trades. Disadvantages: requires more monitoring, harder to automate without a financial advisor.

Tax-Aware Rebalancing

In retirement accounts (IRAs, 401(k)s), rebalancing is tax-free. Sell whatever you need; no consequences.

In taxable brokerage accounts, every sale is potentially a taxable event. Strategies to minimize the tax hit:

Use new contributions to buy underweight assets instead of selling overweight ones. If bonds are underweight, direct your next $5,000 contribution entirely to bonds.
Use dividends from stocks to buy bonds rather than reinvesting them.
Harvest losses: if any position is underwater, sell those specific shares to realize a tax loss, then use the proceeds to buy what you need. You get the rebalance plus a tax deduction.
Rebalance across accounts: treat all accounts as one portfolio. Place tax-inefficient assets (bonds, REITs) in tax-sheltered accounts and do the selling there. Keep stocks in taxable accounts where you can use contributions and dividends to rebalance incrementally without realizing gains.

How Often Is Enough?

The data suggests annual rebalancing with a 5% threshold band captures roughly 90% of the benefit for a fraction of the effort. Vanguard's research found that quarterly rebalancing didn't meaningfully improve risk-adjusted returns over annual rebalancing, while monthly rebalancing actually reduced returns due to momentum effects.

For most investors, checking once a year is plenty. If the market drops 20% mid-year, that's worth an unscheduled look—but don't let calendar discipline become an excuse for obsessive monitoring.

Automate What You Can

Target-date funds and robo-advisors rebalance automatically. If you hold individual ETFs, many brokerages now offer automatic rebalancing on a schedule. Fidelity, Schwab, and M1 Finance all support this. Set it, then log in once a year to verify the target allocation still matches your life situation.