To rebalance a UK ISA portfolio: (1) decide your trigger — either calendar (e.g. once per year in April) or threshold (e.g. 5% drift from target); (2) prefer contribution-only rebalancing where possible — direct new money toward underweight assets instead of selling existing holdings; (3) inside an ISA there is no CGT to worry about so selling is tax-free; (4) outside an ISA (GIA) use the £3,000 CGT AEA strategically; (5) keep it simple — over-rebalancing erodes returns through trading costs.
Why rebalance at all
Without rebalancing, a portfolio drifts toward whichever asset has performed best. A 60/40 portfolio that starts 60% equity in 2010 would have been ~80% equity by 2020 — much higher risk than intended. Two reasons to push back:
- Risk control. Drift means you're taking more risk than you intended. The whole point of holding bonds is to limit drawdown — a portfolio that drifts to 90% equity loses that protection.
- Systematic buy-low-sell-high. Rebalancing forces you to sell what is expensive and buy what is cheap. Studies (Vanguard 2024 UK edition) suggest this adds ~0.3-0.5% per year of risk-adjusted return over time.
The three rebalancing approaches
Approach A: Calendar rebalancing
Pick a date and rebalance on that date regardless of drift. Common choice: once per year, either at tax year start (6 April) or at year-end (December).
Pros: Simple, disciplined, removes timing emotion.
Cons: Can rebalance unnecessarily if drift is minimal. Can miss large drifts mid-year.
Approach B: Threshold rebalancing
Rebalance whenever any asset class drifts more than X% from target (typically 5% absolute or 25% relative). Check monthly or quarterly.
Pros: Only acts when drift is meaningful.
Cons: Requires regular monitoring. Can trigger multiple rebalances in volatile years.
Approach C: Contribution-only rebalancing
Direct all new contributions toward the underweight asset class until it returns to target. No selling required.
Pros: Zero trading costs, zero CGT exposure (in GIA), no behavioural drama.
Cons: Slow to correct large drifts. Works best for accumulation phase.
Step-by-step annual rebalance
ISA vs GIA — different rebalancing rules
| Wrapper | Selling generates... | Rebalancing approach |
|---|---|---|
| ISA | No tax | Rebalance freely |
| Pension (SIPP) | No tax | Rebalance freely |
| GIA (taxable account) | CGT above £3,000 AEA | Use contributions; sell strategically against AEA |
| LISA | No tax (within rules) | Rebalance freely (but withdrawal restrictions) |
Worked example: rebalancing a £200,000 ISA after a strong equity year
Start of year: £140k equity (70%) + £50k bonds (25%) + £10k cash (5%)
Target: 70/25/5
End of year after equity grew 18% and bonds grew 2%:
- Equity: £165k (78%)
- Bonds: £51k (24%)
- Cash: £10k (5%)
- Total: £226k
Rebalance to 70/25/5 = £158k equity, £56k bonds, £11k cash. Sell £7k of equity, buy £5k bonds + £1k cash (with £1k of equity sale becoming cash).
Inside ISA: zero tax cost. Total time: 10 minutes including documentation. Rebalancing-bonus benefit: small but compounds over decades.
Costs and frequency — finding the sweet spot
Trading costs on UK ETFs are typically £5-£10 per trade (or 0% on commission-free platforms like InvestEngine, Trading 212, Freetrade). Bid-ask spreads add a further 0.05-0.15% effective cost.
| Rebalance frequency | Trading cost as % per year | Practical assessment |
|---|---|---|
| Monthly | 0.10-0.30% | Too frequent — costs erode rebalancing bonus |
| Quarterly | 0.04-0.10% | Acceptable for active monitoring |
| Annually | 0.01-0.03% | Sweet spot for most UK investors |
| Threshold (5%) | Variable | Acceptable — typically rebalances 0-2 times per year |
Most rigorous studies (Vanguard, MSCI, JP Morgan) conclude that annual rebalancing captures 95%+ of the maximum theoretical rebalancing bonus at low cost. Going more frequent doesn't add proportionate value.
Common rebalancing mistakes
Project compound growth
The compound interest calculator models how systematic contributions and rebalancing compound over decades — the long-term magic of disciplined investing.
Open compound interest calculator →Sources and references
Rebalancing research from Vanguard "Best Practices for Portfolio Rebalancing" (2024 UK edition). Trading cost data from LSE for typical UK ETFs. Active vs passive performance from S&P SPIVA UK Scorecard 2024. CGT AEA from gov.uk Capital Gains Tax.
UK Tax Drag is not authorised by the Financial Conduct Authority and does not provide regulated financial or tax advice — see the content disclaimer for the full position. There are no affiliate links on this page — provider names are mentioned only to illustrate how different providers handle the same procedure.
Other investing how-to guides
- How to open a SIPP
- How to transfer an old pension to a SIPP
- Cash ISA vs Stocks & Shares ISA vs LISA — which to choose
- How to build a 3-fund portfolio in a UK ISA
- How to rebalance an ISA portfolio
- Drip-feed vs lump-sum invest — the UK research
- How to switch from active to passive investing
- How to take income from investments in retirement
- How to claim foreign withholding tax on US ETFs (W-8BEN)
- How to do bed-and-ISA without triggering CGT
- Investing hub
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