How to Rebalance Your Portfolio: Calendar vs 5% Bands

Here is the uncomfortable truth about the portfolio you built so carefully: it stops being the portfolio you chose the moment you stop looking at it. A tidy 60/40 split between stocks and bonds does not stay 60/40. Stocks usually grow faster, so their share creeps up, and your "balanced" plan slowly turns into an aggressive one you never signed off on.

Learning how to rebalance your portfolio is how you take that decision back. This guide gives you the drift math, the evidence on how often to do it, the calendar-versus-bands debate settled, and the tax-smart way to rebalance without handing the taxman a bonus. If you build your core around index funds, it pairs naturally with our ETF and index-investing course.

What does it mean to rebalance your portfolio?

Rebalancing is the act of returning your investments to their target allocation by selling assets that have grown beyond their intended share and buying those that have fallen below it. If your plan is 60% equities and 40% bonds, and a strong stock year pushes you to 68/32, rebalancing sells enough equities - or adds enough bonds - to get back to 60/40.

In plain terms, it forces you to sell high and buy low on a schedule, without needing a forecast. You are not chasing performance; you are restoring the risk level you originally decided you could live with.

Here is a concrete example. Say you started the year 60/40 with $100,000: $60,000 in a global equity fund and $40,000 in bonds. A strong equity year lifts the stock sleeve to $78,000 while bonds inch up to $42,000. You now hold $120,000, of which 65% is equities. To rebalance back to 60/40 you would trim about $6,000 from equities and move it into bonds - or simply steer your next few months of contributions entirely into bonds until the split self-corrects.

Notice what just happened: rebalancing made you take profit on the asset that ran hot and add to the one that lagged. That is the opposite of what most investors do emotionally, and it is why a written rule matters more than willpower.

Why a 60/40 quietly becomes an 80/20

This is the part most investors underestimate. Because equities out-compound bonds over long stretches, an un-rebalanced portfolio does not stay diversified - it silently concentrates into stocks. That is portfolio drift, and it works against you at the worst possible moment.

Vanguard studied exactly this. A portfolio of 60% global equities and 40% global bonds at the end of 2003, left completely un-rebalanced, would have been about 80% equities by the end of 2022. The chart below shows the same effect using simple compounding assumptions.

Why does an extra 20 points of equity matter so much? Because risk is not linear. An 80/20 portfolio can fall far harder in a downturn than the 60/40 you planned for - and the drift always peaks right before a bad year, since it is a long run-up that inflates the equity share in the first place. You end up maximally exposed to stocks at exactly the moment you can least afford a deep drawdown.

What this means for you: if you have not rebalanced in years, check your weights today. You may be carrying far more stock-market risk than your plan intended - which is fine in a bull run and brutal in a crash. The fix is the same discipline that decides your core index holding, whether that is VTI or VOO: define the target, then hold yourself to it.

How often should you rebalance your portfolio?

Less often than you think. The most useful finding in the research is how little the exact frequency matters. Vanguard compared monthly, quarterly, annual and 5%-threshold rebalancing of a 60/40 over decades and found essentially identical risk-adjusted returns across all of them. Monthly rebalancing did not win - it just generated far more trades and cost for no benefit.

So the practical answer most professionals land on is simple: review your allocation every 6 to 12 months, or whenever it drifts beyond a set band. A widely used planner framework pairs an annual review with a 5% drift trigger, giving you the best of both.

The one wrong answer is "never." Doing it too often wastes money; not doing it at all lets the drift above take over.

There is a real cost to over-rebalancing that beginners miss. Every trade in a taxable account can crystallise a gain, and even in a free-to-trade account, monthly tinkering trims your winners so often that you clip the compounding you were trying to capture. This is the myth of the "rebalancing bonus" - the idea that frequent rebalancing itself adds return. In practice, the extra trades usually cost more than they add. Rebalance when your plan says to, not when the headlines make you nervous.

Calendar vs threshold bands: which rule should you use?

There are two honest ways to decide when to rebalance. Calendar rebalancing acts on a fixed date - say, every January. Threshold-band rebalancing acts on drift - you rebalance only when an asset class moves at least 5 percentage points from its target. Here is how they compare.

Source: Vanguard, "Best practices for portfolio rebalancing," 2015 (equivalence of frequencies and the 5% threshold band).

The verdict for most people: combine them. Look once a year, but only trade if something has drifted past 5 points. You get the discipline of a schedule and the cost savings of ignoring trivial moves.

How to rebalance your portfolio in 5 steps

The theory is simple; the execution is where people slip. Here is the exact sequence.

How to rebalance without a big tax bill

In a tax-sheltered account, rebalancing is free of tax friction - trade as much as your plan needs. The trap is the taxable account, where selling an appreciated fund can trigger a capital-gains bill that quietly eats your return.

Three moves keep the tax cost near zero, in order of preference:

• Rebalance in your sheltered accounts first. An ISA, SIPP, 401(k) or IRA lets you sell and buy with no capital-gains tax at all, so do as much of the work there as possible.
• Redirect new money. Point fresh contributions at whatever is underweight instead of your usual auto-buy. Over a year, steady inflows can correct most drift on their own.
• Turn off dividend reinvestment on the winners. Send dividends and distributions to the lagging asset class rather than back into the one that has already grown too large.

Only when those run out should you sell in a taxable account - and then favour lots with the smallest gains. Because bonds and equities are taxed differently, it also pays to know how UK government bonds are taxed before you decide which sleeve to trim. And if your equity sleeve mixes strategies, understand how equal-weight and market-cap S&P 500 funds differ so you rebalance between genuinely distinct holdings, not near-duplicates.

One honest caveat: do not expect rebalancing to boost your returns. Across long bull markets an un-rebalanced portfolio often ends up ahead on raw return precisely because it let its winners run. What rebalancing buys you is control - a risk level that matches your plan, and no nasty surprise when the market finally turns.

Common rebalancing mistakes to avoid

Most rebalancing errors are not about the maths - they are about behaviour and account choice. Watch for these:

• Rebalancing on emotion, not on rule. Selling stocks after they fall because you are scared is not rebalancing - it is panic. A band or a calendar date takes the fear out of the decision.
• Ignoring the account type. Doing all your selling in a taxable account when the same trade in an ISA, SIPP, 401(k) or IRA would have cost nothing in tax is the most expensive habit here.
• Rebalancing between near-identical funds. Trimming one broad US index fund to buy another barely changes your risk. Rebalance across genuinely different sleeves - equities versus bonds, or home versus overseas.
• Forgetting to update the target itself. As you age or your goals change, your target allocation should shift too. Rebalancing to a mix that no longer suits you is precision aimed at the wrong point.
• Chasing an exact split. You do not need to hit 60.0%. Getting back inside your band is enough; over-precision just adds trades and cost.

Frequently asked questions

This article is educational content, not personal investment advice. Your target allocation and tax treatment depend on your own circumstances, goals and country of residence.