Why I Rebalance My Portfolio on My Birthday

A Date I Won't Forget

Today's my birthday, and for a few years now it's also been the day I rebalance my own portfolio. Not because there's anything financially significant about the date, there isn't, but because it's a date I'll never accidentally skip. That's the whole point of doing it this way.

What Rebalancing Actually Is

If you started with a target of, say, 70% equities and 30% bonds, a good year for shares can quietly turn that into 78/22 without you doing anything at all. Nothing's gone wrong. The portfolio has just drifted, because different assets grow at different rates. Rebalancing is the act of selling some of what's grown faster and buying more of what's lagged, to bring the mix back to the original target.

It sounds like it should be intuitive. In practice it's one of the harder disciplines in investing, because it asks you to sell the thing that's been going up and buy the thing that hasn't. Every instinct runs the other way.

Two Ways to Decide When

There are broadly two approaches to deciding when to rebalance.

Threshold-based rebalancing acts whenever an asset class drifts a set amount from its target, often 5 percentage points. It reacts quickly to large market moves, which is useful, but it means checking your allocation regularly enough to notice, and making a judgement call each time about whether a given drift is "enough."

Calendar-based rebalancing happens on a fixed date, once or twice a year, regardless of what the market has done. It's less responsive to sudden swings, but it removes the judgement call entirely. There's no decision about whether now is the right moment, because the date already decided that for you.

I use the calendar approach, tied to my birthday, for the same reason I'd recommend fewer moving parts in a plan generally: it turns rebalancing from a decision I might make into a habit I just do. A fixed annual date someone will always remember, a birthday, an anniversary, the first of January, does more for consistency than a theoretically more responsive rule that quietly never gets checked.

Where the Wrapper Changes the Maths

This is the part that's easy to miss, and it's UK-specific. Rebalancing inside an ISA or a SIPP is free of tax consequences: you can sell and buy within the wrapper as often as the rebalancing calls for, with no Capital Gains Tax to think about.

Rebalancing inside a General Investment Account is different. Selling an investment that's gained value inside a GIA can trigger a Capital Gains Tax liability, and the annual CGT allowance is small enough now that a meaningful rebalance can use it up. That doesn't mean don't rebalance a GIA, but it does mean the decision isn't purely about target allocation any more; it's also about cost basis, what's already been realised in the same tax year, and whether the drift is large enough to be worth the tax bill it creates.

Checking It Rather Than Guessing

I don't rebalance by feel. I look at where my actual holdings sit against my target allocation in Scenarios's portfolio deep-dive, across every account, and decide from there whether the drift is worth acting on, and in a GIA, whether the tax cost is worth it this year or better left until next. The seven-asset-class correlation matrix behind the simulation is the same one used to project the plan forward, so the allocation I'm checking is the one the projection is actually built on.

This is methodology and information, not financial advice, and Scenarios isn't authorised to provide regulated advice. If a rebalancing decision would trigger a significant Capital Gains Tax bill, that's worth checking with a regulated financial adviser or accountant before acting.

Further Reading

• HMRC. "Capital Gains Tax: what you pay it on, rates and allowances." gov.uk
• Vanguard UK. "The pros and cons of rebalancing your portfolio."