Rebalancing is the quiet discipline that most investors know they should do and almost nobody enjoys doing. It sounds bureaucratic — the financial equivalent of tidying a desk — yet it is one of the few actions that genuinely earns its keep over a lifetime. The idea is simple: you set a target mix of investments, and every so often you bring the portfolio back to that mix. The reason it matters is less exciting than the latest stock tip, but considerably more reliable.
Left to its own devices, a portfolio drifts. Shares tend to rise faster than bonds over the long run, so a portfolio you started at 60% stocks and 40% bonds slowly becomes 70/30, then 80/20, without you doing a thing. That sounds harmless, but it quietly makes you much riskier than you planned to be. Rebalancing is the mechanism that stops that drift — and along the way it rewards you for doing exactly what markets make emotionally hard: selling what is going up and buying what is going down.
Why “set and forget” doesn’t work
Consider a simple 60/40 portfolio. If stocks have a strong run, that proportion no longer holds. Over five good years, a 60/40 starting point can morph into something closer to 82/18, with stocks doing almost all the work. The figures here are illustrative, but the shape is entirely typical. The danger is not that you made more money than expected — it’s that your portfolio now carries far more risk than the day you built it. A retirement portfolio designed to be calm can, in a downturn, fall as hard as a much more aggressive one.
The fix is to remember that the target allocation is not a suggestion. It was chosen to match your goals and your appetite for volatility. Rebalancing is how you make sure the portfolio you actually own, rather than the one you designed years ago, is the one running your money.
The three real benefits
The most important benefit of rebalancing is controlling risk. You decided that bonds should cushion 40% of your portfolio, and rebalancing makes sure that cushion is actually there when you need it.
Beyond that, it imposes a discipline most of us lack. Left to feelings, investors buy what is rising and abandon what is falling. Rebalancing inverts that instinct: it trims the winners and adds to the losers. Over time that pattern — buy low, sell high, automatically — can add a modest amount to returns, though nobody should treat it as a get-rich-quick trick. Its true job is keeping risk in line, and the return benefit is a welcome bonus, not the point.
Finally, it removes emotion from decision-making. A rebalancing rule agreed in calm times is far easier to follow than a decision made in a panic. It turns a painful moment into a simple checklist task.
When to rebalance
There are two broad approaches, and most people sensibly combine them.
The first is calendar rebalancing: you pick a fixed date — say once a year — and bring everything back to target regardless of what the market has done. It is simple, low-effort, and for most investors it is the sweet spot. Doing it quarterly creates extra trades, extra spreads and extra tax events for little benefit. Leaving it two years or more lets drift build up further than you’d like.
The second is threshold rebalancing: you only act when an asset strays beyond a set amount from its target, commonly around 5%. It is more precise and catches extremes — such as a sudden crash or boom — but it’s less predictable. The practical answer for most people is a combined approach: an annual review, plus a rule of thumb that you rebalance if anything moves more than 5% off its target.
How to rebalance in practice
There are three main ways to make the adjustment, and they differ in tax consequences.
Sell and buy. You sell the overweight asset and use the proceeds to buy the underweight one. A simple example: if stocks are at 70% when your target is 60%, you sell 10% of the portfolio and put it into bonds, which were at 30% and need to reach 40%. If the portfolio is worth £100,000, that’s selling £10,000 of stocks and buying £10,000 of bonds. It’s the most direct method, but outside an ISA or SIPP it can trigger Capital Gains Tax, so it’s best done inside tax-sheltered accounts.
New contributions. Instead of selling anything, you simply steer your new money into the underweight asset. If stocks are at 65% and you want them back at 60%, all new contributions go to bonds until the balance is restored. This has the enormous advantage of triggering no tax and no trading fees — you’re building the portfolio toward target rather than dismantling it.
Dividend redirect. If your funds pay out income, you can direct the dividends from the overweight asset toward the underweight one. It’s a gentler, slower version of the same idea, and again causes no sales.
A worked example
Imagine a target portfolio of UK stocks 25%, global stocks 25%, bonds 30% and cash 20%. After a year, UK stocks have risen to 30%, global stocks to 28%, while bonds have fallen to 22% and cash sits at 20%. Only the UK allocation is beyond the 5% threshold at +5%, so the cleanest move is to sell £5,000 of UK stocks and buy £5,000 of bonds. The remaining bond shortfall can be closed with new contributions or a dip into cash, letting you avoid any further selling.
Notice that the logic rarely says “sell everything back to exact target.” It says fix what has meaningfully drifted, and use the tax-friendly tools — new money and dividends — for the rest.
The real-world costs
Rebalancing is not free, so it pays to be sensible. Trading fees in the UK typically run to roughly £5–£10 per trade on many platforms, and ETFs carry a bid-ask spread. Outside an ISA or SIPP, selling can also create a Capital Gains Tax bill. Inside an ISA or SIPP, however, none of these problems really bite: the trading is sheltered and there is no CGT on the gains, so the sensible UK habit is to do the bulk of your rebalancing in a tax-wrapped account and let your taxable investing simply drift a little wider.
None of this is a reason to avoid rebalancing — the costs are usually small and annual — but it is a reason to rebalance rather than overtrade. One careful review a year, a 5% trigger, and a preference for using new money first will keep the whole exercise cheap.
The bottom line
Rebalancing is boring, and that is precisely its virtue. It keeps your risk level where you chose it, forces you to buy low and sell high, and freezes out the panic that wrecks otherwise sensible plans. Set your target allocation, review it annually, act when something strays more than about 5%, and prefer new contributions to sales wherever you can. Inside an ISA or SIPP, don’t overthink it — a few clicks once a year is all it takes to keep the portfolio doing what you actually asked it to do.
This article is educational and not personalised financial advice. Consider speaking to a regulated adviser about your own circumstances.