Portfolio Drift: Five Percentage Points Is Not Five Percent

A rebalancing instruction can sound precise while leaving its most important term undefined. Review when an allocation moves by five percent could mean five percentage points of the whole portfolio or a 5% change relative to that allocation’s target. Those definitions produce different review dates and can be dramatically different for small positions.

The examples here compare rules, not recommended thresholds. A review trigger is also different from an automatic order. It can prompt a check of objectives, costs and the reason for the drift without requiring an immediate trade. Keeping those distinctions explicit makes a portfolio policy easier to follow when markets are moving quickly.

Compare two definitions on a major allocation

Take an equity target of 60%. An absolute band of five percentage points creates review boundaries at 55% and 65%. A relative band of 5% of the target creates boundaries at 57% and 63%, because 5% of 60 percentage points is three percentage points.

Suppose the current equity weight is 64%. The absolute deviation is four percentage points. The relative deviation is four divided by sixty, or approximately 6.67%. The portfolio remains inside the first rule but crosses the second. Both calculations can be correct; the rules are simply different.

This becomes easier to audit when expressed in dollars. On a $100,000 portfolio, the 60% equity target is $60,000 and the 64% current weight is $64,000. Returning to the target at that unchanged total would require reducing equity exposure by $4,000. The review trigger determines when to consider that adjustment; it does not change the size of the observed deviation.

A small allocation exposes the ambiguity

Now consider a target of 5% for a fictional satellite holding. An absolute five percentage point band would run from 0% to 10%. At the upper edge, the allocation would have doubled relative to its target. A relative 5% band instead runs from 4.75% to 5.25%, a much narrower interval.

Neither interval is automatically sensible merely because it follows a mathematical rule. The first might tolerate a much larger concentration than intended. The second might create frequent reviews over small dollar changes. On a $20,000 portfolio, moving from 5% to 5.25% is only $50 at a constant total. The administrative and transaction costs could matter more than such a small difference.

Recompute weights after a market move

Suppose a portfolio starts with $60,000 in equities and $40,000 in bonds. Equities rise 10% while bonds are unchanged. The new amounts are $66,000 and $40,000, for a total of $106,000. The equity weight is about 62.26%, not 66%. A 10% gain in an asset does not mean its portfolio weight rises by ten percentage points.

Relative to the original 60% target, that is a drift of about 2.26 percentage points, or about 3.77% of the target. Both the numerator and denominator changed. Calculating the new dollar value but retaining the old portfolio total is a common source of exaggerated drift readings.

Decide what belongs in the denominator

Cash, multiple accounts and assets outside the platform can change the interpretation. If a displayed portfolio excludes cash reserved for a near term payment, its investment weights answer a different question from a balance sheet that includes that reserve. Do not move between those definitions when judging whether a boundary has been crossed.

Similarly, two accounts with the same owner may be managed together for allocation purposes even though their tax treatment or available investments differ. A combined weight does not imply that every account needs the same trades. This case does not estimate taxes, transfer restrictions or account specific consequences.

Use the holdings tool for the next layer

After calculating allocation drift, inspect the overlap analyzer to see whether the holdings inside two funds repeat the same companies. A portfolio can sit exactly on its target fund weights while becoming more concentrated at the underlying company level. Conversely, a small change in fund weights may have little effect on a diversified holding set.

Use holdings from comparable dates and state whether the lists cover the complete funds. A neat percentage is not proof that the data is complete. The target allocation, pairwise overlap and combined company weights each describe a different part of the problem, so keep the labels distinct in your record.

Write a rule that survives an uneasy market day

Specify the target, whether the band is absolute or relative, which assets are included and what a boundary crossing initiates. State whether contributions will be considered before sales and how costs will be checked. The rebalancing guide provides context for turning those definitions into a repeatable review.

The purpose of this precision is not to produce an elaborate policy for its own sake. It is to prevent the same observed portfolio from being described as within limits on one day and outside them on another because the wording changed. A clear rule makes an allocation decision easier to explain, question and revise when circumstances genuinely change.