Rebalancing a Three-Fund Portfolio

Rebalancing a three-fund portfolio means bringing your domestic stock, international stock, and bond holdings back to their target allocation after prices have moved. The process is mechanical: calculate how far each piece has drifted from target, decide whether drift is large enough to rebalance, then compute exact trade sizes to restore balance.

Setting Up the Scenario

Let’s walk through a concrete example. Suppose you have a three-fund portfolio targeting:

• 40% US stock (via a total-market index fund)
• 20% international stock (via an international index fund)
• 40% bonds (via a bond index fund)

Your portfolio is worth $100,000 today. On day one, you allocate:

Perfect alignment. Now six months pass.

Calculating Current Allocation and Drift

The US stock market rises 12%, international markets fall 5%, and bonds rise 3%. Here’s the impact:

Your portfolio is now worth $105,000. US stock has grown to 44.8%—above its 40% target by 4.8 percentage points. International stock fell to 19.0%, below its 20% target by 1.0 point. Bonds are only slightly above at 41.2%.

The question: do you rebalance now?

The Rebalancing Decision

Most individual investors rebalance when drift exceeds 5% in absolute terms, or on a fixed schedule (e.g., annually, quarterly). Here, US stock is drifted +4.8%—borderline. International stock is drifted only −1.0%. Some investors would wait for US stock to hit +5%; others rebalance quarterly or annually regardless.

For this example, assume you decide to rebalance—perhaps because your policy is to rebalance whenever any asset class drifts 5% or more, and you’re close.

Computing Trade Sizes

Your target allocation on a $105,000 portfolio is:

• US stock: $105,000 × 40% = $42,000
• Int’l stock: $105,000 × 20% = $21,000
• Bonds: $105,000 × 40% = $42,000

But you currently hold:

• US stock: $44,800
• Int’l stock: $19,000
• Bonds: $41,200

So you must:

• Sell US stock: $44,800 − $42,000 = sell $2,800
• Buy Int’l stock: $21,000 − $19,000 = buy $2,000
• Buy Bonds: $42,000 − $41,200 = buy $800

Check: $2,800 out = $2,000 + $800 in. ✓

If you’re buying and selling funds through a brokerage, you’d place an order to sell $2,800 of the US stock fund, then use that cash (plus any additional cash on hand) to buy $2,000 of the international fund and $800 of the bond fund.

Practical Considerations: Commissions and Spreads

In the real world, you’ll face frictions:

• Trading commissions: Many brokers offer commission-free mutual fund trades, but some funds or platforms charge flat fees (e.g., $20–$50 per trade).
• Bid-ask spreads: ETFs have spreads (typically 0.01%–0.05% for popular broad-market funds), meaning you’ll pay slightly more to buy and receive slightly less to sell.
• Timing: If you execute the sell and buy at different times, market moves between orders can change the final allocation.

For a $105,000 portfolio rebalancing $2,800, a $50 trading fee is 0.18% of the traded value—material enough to justify only larger drifts. This is why many investors rebalance only annually or on a calendar schedule rather than continuously.

Tax Consequences (Taxable Accounts)

In a tax-advantaged account (401(k), IRA), rebalancing is tax-free. In a taxable brokerage account, selling an appreciated fund triggers capital gains taxes if you’ve held it more than one year (long-term rate, usually lower) or less (short-term, taxed as ordinary income).

In the example, you’re selling US stock that appreciated from $40,000 to $44,800—a $4,800 gain. If the realized gain is $2,800 (proportional to your sale), and your long-term capital-gains rate is 15%, you’d owe $420 in taxes. For that, your rebalancing has become much less attractive.

Many investors in taxable accounts use a threshold of 7–10% drift before rebalancing to justify the tax cost. Others rebalance only in December using tax-loss harvesting opportunities to offset gains.

Variations: Threshold vs. Calendar Rebalancing

Threshold-based rebalancing (also called “bands” or “corridor” rebalancing) requires action only when drift exceeds a preset limit (e.g., 5%). This minimizes unnecessary trading but can let drift widen in long trends.

Calendar rebalancing locks in a date—annual, quarterly, or monthly—and rebalances on that date regardless of drift. It’s simpler psychologically and easier to automate but may force you to trade when drift is minimal, incurring unnecessary costs.

A hybrid approach: rebalance annually if drift is less than 5%, but rebalance sooner if any asset class drifts more than 7%. This balances cost control with drift management.

The Three-Fund Advantage

The appeal of the three-fund portfolio—total US market, total international market, bonds—is simplicity and low cost. Rebalancing is straightforward because you’re moving money between just three liquid, commission-free funds at most brokers. A 20-fund portfolio would be a bookkeeping nightmare.

The mathematical benefit of rebalancing (forcing you to “buy low, sell high”) compounds over decades, though in modest amounts. Rebalancing adds 0.1–0.3% per year of additional return on average—less than the cost of active management, but real. The main benefit is behavioral: rebalancing enforces discipline and keeps risk in check as allocations drift.

See also