Partial Rebalancing

Full rebalancing means selling and buying to restore each position to its target weight. Partial rebalancing means moving only halfway, or a third of the way, back to target. This tactic lets you blunt the worst drifts without incurring the full capital gains tax or transaction costs of a complete rebalance. It’s a pragmatic middle ground for taxable accounts that trades precision for tax efficiency.

The core idea

Suppose you hold a 60/40 equity-bond portfolio. After a bull market, equities have drifted to 72%, bonds to 28%. A full rebalance would sell 12% of equities and buy 12% of bonds, bringing you back to exactly 60/40. That sale crystallizes capital gains and incurs bid-ask spread costs.

A partial rebalance might sell only 6% of equities to buy 6% of bonds, landing you at 66/34. You’ve cut the drift in half. Your portfolio is less concentrated in equities than it would be with buy-and-hold, but more concentrated than a full rebalance would leave it. The capital gains tax bill is halved, as are transaction costs.

Over time, small drifts accumulate into acceptable ranges. You might execute a full rebalance once a decade when drift has become extreme, but rely on partial rebalances—or no rebalance at all—in normal years.

Why this matters for taxable investors

The core tension in rebalancing is simple: capital gains taxes are expensive. If you bought an equity position at $50,000 and it’s now worth $80,000, rebalancing to cut it back incurs a $30,000 taxable gain. At a 20% long-term capital gains rate, that’s a $6,000 tax liability, payable immediately.

In a 401(k) or other tax-deferred account, this friction disappears—you can rebalance freely. But in a taxable brokerage account, every rebalance trades drift risk (the chance that a concentrated portfolio loses more in a downturn) for tax risk (the certainty of paying capital gains tax now).

Partial rebalancing lets you manage drift without fully triggering the tax. You shrink the overweight position, reducing both its concentration risk and its unrealized gain, but not enough to cause a large tax event in a single year.

The partial rebalance formula

There’s no single rule. Common approaches include:

50% rebalance: Move 50% of the way back to target. If equities are 72% and target is 60%, a 50% rebalance brings you to 66%. You sell 6 percentage points of equities.

One-third rebalance: Move one-third of the way back. A one-third rebalance lands you at 64%—still above target, but closer.

Drift tolerance bands: Rather than rebalance by formula, you set bands around your target. For instance, “rebalance only if any allocation drifts more than 5 percentage points from target.” This is often combined with partial rebalancing: if equities reach 65% (5 points above the 60% target), you rebalance only partially—say, back to 62%—rather than all the way to 60%.

Combining partial rebalancing with new contributions

The most tax-efficient version of partial rebalancing uses new savings. Suppose you contribute $10,000 annually to your taxable account. Rather than rebalance existing holdings, deploy the contribution entirely to your underweight asset—bonds, in our example. This provides a pseudo-rebalance without any tax cost.

Over time, this passive rebalancing via contributions can substantially shift your allocation. A 60/40 portfolio drift to 72/28 can be nudged back toward 65/35 simply by directing two years of contributions to bonds. It takes longer than a single aggressive rebalance, but it avoids taxes entirely.

This approach is most practical in mid-career or pre-retirement accumulation phases, when annual contributions are large relative to portfolio balance. For near-retirees or those not adding savings, pure partial rebalancing is necessary.

The risk trade-off

Partial rebalancing does not eliminate drift—it merely slows it. If equities have soared to 72% and you only rebalance to 66%, they could easily drift to 75% in another strong bull year. You’ve deferred tax, but you’ve also deferred the restoration of your target risk profile.

This is acceptable if:

• You’re comfortable with temporary excess equity exposure.
• You’re rebalancing regularly enough that drift stays within a bounded range (never drifting more than 10 percentage points from target, say).
• You plan a full rebalance eventually, perhaps in a lower-income year or after harvesting tax losses.

It’s riskier if:

• You skip rebalancing for many years, allowing drift to accumulate unchecked.
• A major downturn arrives and your drifted portfolio is far riskier than you thought.
• Your personal circumstances change (you near retirement, for instance) and your target allocation changes, but you’ve never rebalanced the old drift away.

Combining partial rebalancing with tax-loss harvesting

A powerful pattern for taxable investors:

1. Over a bull market, partial rebalance to keep drift within bands (say, never more than 10 points from target).
2. If a downturn arrives, any losses can be tax-loss harvested. Sell the losing position, capture the tax loss, and redeploy into a similar asset.
3. Using tax losses, you can now rebalance without incurring net capital gains. The harvested loss offsets realized gains.

This shifts the timing of rebalancing toward downturns when tax losses are most plentiful. A portfolio that drifted 72/28 in a bull market can be reset to 60/40 during a crash by harvesting losses as you restore your target allocation. No tax bill, and you’ve rebalanced at attractive prices.

When partial rebalancing fails

Partial rebalancing is a delay tactic, not a long-term solution. If you use it perpetually without ever fully rebalancing, drift will eventually dominate. A 60/40 portfolio that drifts 5 percentage points per year will become 80/20 in four years. Partial rebalancing every year (moving back halfway) keeps it in a narrow band, but you’re doing significant work to avoid ever completing a rebalance.

At some point—typically every 5–10 years—a full rebalance is necessary. The tax bill is inevitable; partial rebalancing merely defers it. Some investors trigger a full rebalance when moving to a lower tax bracket, in a year of realized losses from other sources, or when rebalancing within a tax-deferred account can be done without friction.

Behavioral discipline

Partial rebalancing also has a behavioral edge. It’s far easier to execute a $5,000 rebalance trade (to move equities from 72% to 66%) than a $10,000 full rebalance (moving equities from 72% to 60%). Smaller trades feel less consequential and are psychologically easier to execute, even though they’re less precise.

For investors prone to procrastination or loss aversion—who find it hard to sell winners even for sound portfolio reasons—partial rebalancing is a practical stepping stone. You’re still rebalancing, still controlling drift, but in smaller increments that feel manageable.

See also