Rebalancing Rule: Drift, Trading, Return Effect

Five rebalancing rules compared: how far the Bitcoin weight drifted, what was traded and what each rule did to returns. Euro-based simulation since 2017.

Weight deviation, purchases and sales, and return effect

A portfolio with three per cent Bitcoin, reset to its target weights once a year in line with the rule, at times held 9.6% Bitcoin, more than three times the intended weight. Therein lies the real challenge for mandate management in practice: a rule can be formally satisfied while the actual structure of the portfolio is far from the agreed one.

The rebalancing rule therefore belongs to the mandate strategy and should be documented before the first investment: target weights, permitted deviations, review frequency and the procedure in the event of a breach, including the deliberate decision not to adjust. It should be oriented towards the structural requirements of the mandate, not towards the historically most profitable simulation. Which rule would have performed best in hindsight says little about which will perform best in future.

The previous newsletter issue, “How to check whether a new strategy diversifies a portfolio”, showed how strongly a historical result can depend on rebalancing and on its rebalancing date. The example again comes from digital assets: because Bitcoin fluctuates so strongly, it makes particularly clear how far weights can drift. The questions behind it arise with any addition to a portfolio.

The example is a portfolio of 60% global equities and 40% currency-hedged global bonds, to which three per cent Bitcoin is added as a model case. The addition is financed pro rata from equities and bonds. This gives starting weights of 58.2% equities, 38.8% bonds and 3.0% Bitcoin, which are also the target weights. The basis is daily closing prices from December 2017 to September 2026, calculated in euros.

Five rebalancing rules are compared. The study does not recommend any of them for a mandate but shows which deviations and which further portfolio adjustments would have arisen from applying them. Three questions serve this purpose:

How far and for how long do the weights deviate from the target weights under the rule?
Which position triggers the adjustment, and which holdings are changed?
What do the adjustments achieve compared with a clearly named alternative, and how dependent is the result on market path and implementation?

Result in brief: Depending on the rule, the highest observed Bitcoin weight was between 4.3% and 12.4%. The largest interim losses, by contrast, were much closer together than the highest Bitcoin weights.

With annual rebalancing, Bitcoin contributed more to profit than with buy and hold, yet the portfolio’s total return was lower. The reason: annual rebalancing resets not only Bitcoin but also equities and bonds to their starting ratio. In a period in which equities rose considerably more than bonds, this cost more return than the adjustment of the Bitcoin weight added.

A position’s profit contribution therefore does not show what the rule achieves. In addition, the result depended heavily on the start year: with a start at the end of 2021, annual rebalancing was marginally ahead of buy and hold; with a start at the end of 2018, it was clearly behind.

A return disadvantage relative to buy and hold does not mean that a rule has failed in its task. Buy and hold keeps the initial holdings, not the agreed allocation; the return comparison alone therefore does not decide which rule suits a mandate.

What the rule controls

Rebalancing rules can be divided into two basic forms. Calendar rules reset the portfolio to the target weights on fixed dates, regardless of how far the weights have deviated. Band rules intervene only when a weight leaves a corridor defined in advance around its target weight.

Two variants of each are examined: quarterly and annual rebalancing, and a narrower band of 2 to 4% and a wider one of 1.5 to 4.5% around the Bitcoin target weight. The fifth rule is the deliberate decision not to adjust at all, buy and hold, which also serves as the benchmark for comparison. When an adjustment is made, all four other rules reset all positions to the target weights, so the bands do not merely reset to the band limit.

An example: on 28 December 2020, the Bitcoin weight under the narrower band rose to 4.3%. It was reset to 3.0% by selling Bitcoin and buying equities and bonds. The return of the band rules therefore does not depend on Bitcoin trading alone.

The following chart shows, for each of the five rules, how the Bitcoin weight developed over time, in each case with the highest weight reached. The range from 2 to 4% serves as a common benchmark for all rules.

The table that follows adds on how many days the weights were outside the corridor, how large the largest interim loss was, and how much the rules reallocated to achieve this.

Target weight and calendar do not determine which interim deviations are accepted. Under annual rebalancing, the Bitcoin weight was outside 2 to 4% on 54.9% of observation days, including underweights; the longest continuous phase covered 204 observation days. With the narrower band, the longest such phase was never more than one observation day.

These values are observed, not upper limits. Checks are made only on days when all three assets trade, at the closing price; in between, and at weekends when only Bitcoin trades, the weight can be higher. The short duration of deviations under the bands is also partly due to the assumption that trading takes place at the same closing price.

If execution takes place only at the next closing price, the highest Bitcoin weight under the narrower band rose from 4.3% to 4.6%, and under annual rebalancing from 9.6% to 11.1%. Even with this execution, the highest Bitcoin weights under the band rules remained clearly lower than under annual rebalancing.

To trigger trading, calendar rules need no ongoing monitoring of weights, whereas band rules do. If the mandate provides for investment limits, however, weights must be monitored under a calendar rule as well, because between two dates the Bitcoin weight can reach a multiple of the target weight.

The differences had far less effect on the largest loss of the overall portfolio: the maximum drawdown lay between −23.2% for buy and hold and −21.6% for the bands.

Profit contribution and rule effect

Profit contribution and rule effect measure different things. The profit contribution shows how much a single asset contributed to the total return. The rule effect shows how much better or worse the whole portfolio performed with the rule than without it.

The two can diverge. If Bitcoin is reset to 3% at year-end, the sale proceeds flow into equities and bonds, or a top-up purchase is financed from them. Their holdings change as a result, and so does everything they earn afterwards. A higher profit contribution from Bitcoin therefore does not mean that the rule raised the total return. This applies to any addition, not only Bitcoin.

A decomposition of annual rebalancing into two steps shows how far the two can diverge. One resets Bitcoin to the target weight of 3% at year-end and offsets this pro rata via equities and bonds; the other restores the ratio of 60 to 40 between equities and bonds. Both steps together make up full annual rebalancing to the original target weights. The decomposition applies only to annual rebalancing; it does not explain the return differences between all five rules. The table compares the two steps individually and together with buy and hold.

With full reset, Bitcoin contributed 6.5 percentage points more to profit than with buy and hold, yet the total return was 6.6 percentage points lower. In a period in which equities rose considerably more than bonds, resetting equities and bonds cost more return than resetting the Bitcoin weight added. Why the two individual effects do not simply add up is explained in the technical article.

What the result depends on

The main calculation begins on 29 December 2017. If annual rebalancing is started at the end of 2021, the total return was 0.3 percentage points above buy and hold; with a start at the end of 2018, it was 50.9 percentage points below. Depending on the start year, the rule was thus sometimes marginally ahead of, sometimes clearly behind buy and hold. The size of the gaps, by contrast, can hardly be compared, because the periods are of different lengths and overlap.

In this study, the return effect of rebalancing depends on market path, start, rebalancing date and implementation, and cannot be generalised from about nine years with few Bitcoin cycles. The technical article shows further start dates and rebalancing dates.

Three questions for your own rebalancing rule

The study yields three questions that can be put to any rebalancing rule.

1. How far does the portfolio deviate? Recalculate the rule on historical prices: how high and for how long were positions outside the intended ranges, and what monitoring, response and escalation does the mandate provide for a breach?

2. What is traded in total? For a typical trigger day, list all purchases and sales and check them against the mandate requirements. Whether only the triggering position or all building blocks are reset is part of the rule.

3. Against which alternative is the return effect measured? In the performance discussion, state the benchmark explicitly, for example buy and hold. Selecting a rule by its backtest return does not hold up: if the sign changes when the start or the implementation shifts, a return advantage or disadvantage cannot be generalised.

Rebalancing controls the allocation; whether it raises or lowers the return depends on the market path and on implementation. What matters for the mandate is that target weight, permitted deviation, trigger and trading scope fit together, and that the return effect of a rule is assessed against a clearly named alternative.

The full calculation with definitions, all start dates, rebalancing dates and the data check is contained in the accompanying technical article.

Context and limitations

The study design and evaluation rules were set down in writing before the calculations and frozen on 15 October 2026; earlier own calculations on rebalancing are not included.

The calculation uses two real UCITS ETFs, an accumulating global equity ETF and a currency-hedged global bond ETF, as well as the Bitcoin price in euros. The share classes are authorised for distribution in Germany; product names are not disclosed.

The calculation is not a model allocation for a client portfolio. It shows trading volume and historical effects, but not the economically best way of implementation. The ETF prices reflect fund costs; the Bitcoin series is calculated without assumed product costs; additional transaction costs and taxes of rebalancing are not included. ETF and Bitcoin closing prices do not represent the same point in time.

Boris Jurczyk is the founder of 21Nexus Capital and is therefore economically connected to the topic of digital assets. 21Nexus Capital acts as adviser to a fund that invests, among other things, in digital assets.