What does rebalancing actually cost?
Rebalancing is not free, and the largest cost is usually the one that never appears on a trade confirmation. Working through the three layers in order of size is what turns a textbook rule into a workable one.
Layer 1 — explicit trading costs (usually small)
Take the €4,320 rebalancing trade from earlier. With commission-free ETF trading and a bid-ask spread of roughly 0.03% on a large, liquid fund, the round-trip cost is on the order of €1–3. On a €112,800 portfolio, that is a rounding error.
Explicit costs only become material for illiquid holdings, small markets, or portfolios that rebalance very frequently across many sleeves.
Layer 2 — implicit costs (sometimes material)
- Market impact: large orders move the price against the trader. Irrelevant at household scale; a genuine engineering problem for a pension fund shifting hundreds of millions.
- Spread on the less liquid leg: rebalancing into a thinly traded sleeve can cost several times what the equity leg costs.
- Time out of market: a settlement gap between sale and purchase leaves the money uninvested. Small, but real.
Layer 3 — tax friction (usually the big one)
In a taxable account, selling an appreciated holding realises a gain, and a realised gain is generally a taxable event. Put numbers on it.
The €4,320 equity sale trims a position bought years earlier. Suppose the cost basis on those shares was €2,320, so the realised gain is €2,000. At an illustrative capital-gains rate of 20%, the tax bill is €400.
Compare the layers: €1–3 of trading cost against €400 of tax. The tax is more than a hundred times larger. Any analysis of rebalancing frequency that stops at commissions has missed the entire question.
Two important qualifiers. First, tax treatment varies enormously by country, account type and holding period — rates, exemptions, loss-offset rules and wrapper structures all differ, and none of this is tax advice. Second, tax-sheltered accounts (pensions, ISAs, IRAs and their equivalents) generally have no such friction at all, which is why the same investor may rationally run a tight rebalancing rule in one account and a loose one in another.
The low-friction techniques
Practitioners reduce the cost of staying near target without triggering sales:
- Cash-flow rebalancing. Direct new contributions to whichever sleeve is underweight. No sale, no realised gain, no tax event. For a portfolio still receiving contributions, this alone can absorb most ordinary drift.
- Dividend and coupon redirection. Instead of reinvesting income back into the sleeve that produced it, send it to the underweight sleeve. Same principle, no extra transaction.
- Rebalance inside the sheltered account first. If the overall target is what matters, the adjustment can be made where it is free, leaving the taxable account untouched.
- Wider bands in taxable accounts. Accepting more drift to trigger fewer taxable events is an explicit, quantifiable trade — and one worth quantifying rather than assuming.
The trade-off, stated cleanly
Every rebalancing policy sits on one line: tighter tracking of the target costs more in trades and taxes; looser tracking is cheaper but allows more risk drift. There is no setting that is free of both. Naming which side of that line an account sits on, and why, is the whole skill.
In the data
A fund runs the same exercise internally and reports how much of its portfolio it replaced in a year. An index fund and an actively managed one:
The trades behind the first line of each table are paid out of the fund's assets before its price is struck, so they never show up as a charge, and they are not part of the expense ratio on the second line either. A holder pays them and never sees them quoted; turnover is the only clue to their size.
Try it now
- Take the ten-year drift calculation from earlier in this unit — the same two sleeves are below — and compute the rebalancing trade size. Estimate the trading cost at a 0.03% spread.
- Assume a cost basis of half the current value on the shares sold, and apply a 20% illustrative tax rate. Compare the two numbers. Which dominates?
- Recompute assuming a year of contributions equal to 5% of the portfolio, all directed to the underweight sleeve. How much of the required trade disappears? Note the mechanism, and remember that tax rules differ by jurisdiction — check yours, or ask someone qualified.