You set up a portfolio with 80% equities and 20% bonds. A year later, after a strong stock market, it’s drifted to 90% equities and 10% bonds. You now carry more risk than you intended. Rebalancing is the simple act of bringing your portfolio back to its target allocation — and it’s one of the few disciplined habits that systematically improves long-term results.

Why portfolios drift

Asset classes grow at different rates. Over time, whatever performed best becomes a larger portion of your portfolio. After years of strong equity returns, a balanced portfolio gradually becomes an aggressive one — not because you chose more risk, but because the market moved you there.

This drift is problematic because:

  • Your risk level no longer matches your plan.
  • You’re increasingly concentrated in whatever recently performed best (which often means it’s now expensive).
  • A future correction will hit harder because you have more exposure than intended.

How rebalancing works

Rebalancing means selling what’s grown beyond its target and buying what’s fallen below its target. In practice:

Your target: 80% equities / 20% bonds. Current allocation after drift: 90% equities / 10% bonds. Rebalancing action: Sell enough equities to bring back to 80%, use proceeds to buy bonds up to 20%.

This is counterintuitive — you’re selling your winners and buying your underperformers. But it’s precisely this counterintuitive nature that makes it effective: you’re systematically selling high and buying low.

Over long periods, this disciplined selling-high-buying-low adds a small but meaningful return bonus compared to a portfolio that’s never rebalanced. More importantly, it keeps your risk level consistent with your plan.

When to rebalance

Calendar-based: Rebalance once or twice per year on fixed dates regardless of what’s happened. Simple, predictable, removes emotion from the decision.

Threshold-based: Rebalance whenever any asset class drifts more than 5-10% from its target. More responsive but requires monitoring.

Either works. The specific timing matters far less than doing it at all. Annual rebalancing captures most of the benefit with minimal effort.

What to avoid: rebalancing too frequently. Monthly rebalancing generates unnecessary transaction costs and taxes with minimal additional benefit. Once or twice per year is sufficient.

Rebalancing with contributions

The easiest way to rebalance, especially during the accumulation phase: direct new contributions to whichever asset class is furthest below its target.

If equities have risen and now represent 85% instead of your target 80%, direct your next few monthly contributions entirely to bonds until the allocation is back in line. You achieve rebalancing without selling anything — avoiding transaction costs and potential tax events.

This “contribution-based rebalancing” is the ideal approach for investors still building their portfolio. It’s free, tax-efficient, and automatic if you simply check your allocation quarterly and adjust where your next contributions go.

Rebalancing is portfolio maintenance. Like changing the oil in a car, it’s not exciting, but skipping it leads to problems. The discipline of periodically restoring your target allocation keeps risk in check, forces systematic buying low and selling high, and ensures your portfolio continues to reflect your actual plan rather than whatever the market has done recently.