Rebalancing Bands: The Math of When to Rebalance

Pick any asset mix — say 60% stocks and 40% bonds — and markets will immediately begin dismantling it, because the two assets grow at different rates by design. Rebalancing is the maintenance work that restores the target, and it forces the behavior every investor endorses in the abstract and resists in practice: selling what has done well and buying what has lagged. The practical question is when to do it. Two schools answer differently — the calendar school (rebalance every January) and the band school (rebalance when an allocation drifts past a threshold). This article works the band method with real arithmetic so you can see exactly what it triggers and what it is worth.

The Setup: A $100,000 Portfolio at 60/40

Assumptions: a $100,000 portfolio at 60% stocks / 40% bonds — $60,000 and $40,000 respectively. Over one year, stocks gain 25% (a strong year, but one that has happened often historically) and bonds are flat. No contributions or withdrawals. The arithmetic:

One good year and the allocation has drifted from 60/40 to 65.2/34.8. Notice the mechanism: drift compounds. The portfolio now holds $15,000 more stocks than its design intends, so the next year's stock move hits harder than the plan calls for — risk has crept up without a single decision being made.

The Band: What ±5 Percentage Points Means

A band (or "threshold") rule says: rebalance only when an asset's weight moves more than a fixed distance from target. The common choice is ±5 percentage points of the portfolio for major asset classes — so this 60/40 portfolio triggers when stocks reach 65% or fall to 55%. Some investors use relative bands instead (±10% of the weight itself, i.e. 60% becomes a trigger range of 54–66%); the absolute band is stricter and triggers more often. In the example above, stocks at 65.2% have crossed the 65% line, so the rule fires:

• Sell $6,000 of stocks ($75,000 → $69,000).
• Buy $6,000 of bonds ($40,000 → $46,000).
• Result: $69,000 / $46,000 = exactly 60/40 of the $115,000 portfolio.

Notice what the band method did not do: it did not trade during the many months the allocation sat at 61% or 63%. That restraint is the entire argument for bands over calendars — you trade rarely, and when you do, the drift is large enough that the trade matters. Research on rebalancing frequency has repeatedly found that the exact method matters less than people assume for long-run returns, but bands tend to produce fewer trades than calendar rebalancing at similar risk control, which reduces costs and (in taxable accounts) tax drag.

What the Discipline Was Worth in the Example

Here is the same story continued one more year, with a nastier chapter: stocks fall 10%, bonds again flat. Compare the rebalanced portfolio against one left to drift:

The rebalanced portfolio ends $600 ahead — not because rebalancing is magic, but because the band forced a $6,000 sale of stocks at the elevated price before the fall. The same mechanism cuts the other way: in a year when stocks kept climbing, the rebalanced portfolio lags the unrebalanced one, because it systematically trims the winner. Rebalancing is not a return-enhancement strategy; it is a risk-control strategy that happens to harvest some volatility premium along the way. Any article promising that rebalancing "boosts returns" is selling the wrong headline.

Practical Details That Change the Math

• Cash flows are free rebalancing. Direct new contributions into whichever asset is underweight and most drift corrects itself with zero selling. In a $115,000 portfolio drifting toward 65% stocks, directing $1,000 of monthly savings entirely into bonds rebalances several percentage points a year without a single taxable sale.
• Taxable accounts change the calculus. Selling appreciated index funds realizes capital gains. In taxable space, rebalance with flows first, rebalance inside tax-advantaged accounts where possible, and let the bands be wider (some investors use ±10% relative) to reduce the number of taxable events. Gains harvesting considerations and loss offsets interact here — see the capital gains explainer.
• Check bands annually, act only when crossed. The right cadence for a band portfolio is a calendar reminder to measure — quarterly or monthly — and to trade only when a band has actually been breached. Measuring is free; trading is not.
• Correlated assets drift together. Bands apply per asset class. A portfolio of overlapping funds can breach bands internally while the top-line stock/bond split looks fine — which is why the target should be defined across the whole portfolio, not per fund.

A 5-point band on a 60/40 portfolio triggered one trade — selling $6,000 of a recent winner — after a 25% run-up in stocks. Whether that trade helps or hurts over the next year depends on what stocks do next; that it keeps the portfolio's risk at the level you chose does not depend on anything.

Choose the method you will actually follow: a band rule that fires rarely and predictably beats a sophisticated rule that gets abandoned the first time it sells something uncomfortable. The arithmetic above is reproducible in four spreadsheet cells, and the same framework extends to any target mix. For the reasoning behind choosing a mix in the first place, see the stock/bond allocation guide; for how often people talk themselves out of good rules, see the behavior gap piece.

This guide is general educational information, and it is neither financial, tax, nor legal advice. It recommends no specific security, fund, or provider, and it describes no individual's situation. All balances, returns, thresholds and outcomes above are simplified illustrations with the assumptions stated; they are not forecasts and do not depict any real investor's result. Rebalancing rules, tax treatment of gains, and account types vary by jurisdiction and change over time. Consult a licensed financial professional about your own circumstances before acting.