See how changing prices alter weights and what restoring a starting mix entails.
Manage Portfolio RiskIntermediate4 min
Hypothetical
Plain English
Weights move even without a trade
A portfolio weight is a holding’s share of the total value being measured. If two holdings change by different amounts, their weights change even when no units are bought or sold. This movement is called drift. It describes a change in composition, not necessarily a mistake or a reason to trade.
Rebalancing changes holdings towards specified weights. Those weights must come from some separate decision or model; the rebalancing arithmetic cannot establish whether they are appropriate. Understanding this separation prevents a mechanical calculation from being presented as an allocation recommendation. A mathematically correct trade can still involve costs and risks that the calculation does not address.
The same idea applies to holdings, asset groups or other defined exposures, but the denominator must be clear. A statement may exclude cash, include liabilities or group assets differently. Comparing weights without consistent definitions can create apparent drift that is actually a reporting difference. The example below uses only two holdings and no debt or external cash flows.
Worked Example
Restore a hypothetical starting split
Begin with £500 in holding A and £500 in holding B. The total is £1,000 and each weight is 50%. These equal starting weights are chosen only to make the example transparent; they are not a recommended mix. Assume A later rises to £750 while B remains at £500, with no distributions, contributions, withdrawals, fees or taxes.
The total is now £1,250. A’s weight is £750 divided by £1,250, or 60%; B’s is 40%. To return to the hypothetical original 50% weights at these values, each holding would need to be worth £625. Transferring £125 from A to B produces £625 in each and leaves the total unchanged before costs.
Assume fractional trading is possible and prices remain fixed while the transfer occurs. The example describes the value exchanged, not a guaranteed execution price or a practical order instruction. Selling and buying are separate transactions in many real portfolios; spreads, commissions and settlement arrangements can prevent the final values from matching this frictionless calculation exactly.
Initial value of each holding
£500
Drifted value of A
£750
Drifted value of B
£500
Drifted total
£1,250
Drifted A / B weights
60% / 40%
Restored value of each holding
£625
Transfer from A to B
£125
Reading the Result
Restoring weights changes the exposure
After the transfer, the portfolio has less exposure to A and more to B than immediately before it. The operation does not undo A’s earlier gain or create an additional gain on its own. It changes how subsequent movements will affect the portfolio. What happens next depends on those movements, not on the fact that weights were restored.
If A continues rising faster, the restored mix can grow less than the drifted mix. If A subsequently falls relative to B, the smaller exposure can reduce that effect. Neither possibility establishes a forecast. Rebalancing is a way of changing composition under a chosen rule, not a promise of higher returns or protection against loss.
Costs and Trade-offs
Costs and changing circumstances remain outside the arithmetic
A real process can review weights at intervals or after specified deviations, but different rules produce different timing and turnover. Frequent trading can increase costs. Tax consequences depend on the account, jurisdiction and realised gains. This article does not select a schedule, threshold or tax treatment; those choices need information absent from the example.
New contributions or withdrawals can also alter weights and may change the transactions required to reach a stated mix. More fundamentally, the original weights might no longer represent the intended exposure. Returning automatically to an obsolete assumption is different from reviewing why it was chosen. The SEC educational source explains drift and rebalancing; its general examples are not personalised instructions for the reader.
Common Mistake
Using the gain instead of the new total
Moving the full £250 gain from A to B would leave £500 and £750, simply reversing the imbalance. The restoration calculation uses half the new £1,250 total, not half the original £1,000. Calculate the desired values first, then compare them with current values. This distinguishes a transfer needed for a hypothetical target from a story about taking profits.
Self-check
Check your understanding
Why does A become 60%?
Its £750 value is divided by the new £1,250 total, giving 60%.
Why transfer £125 rather than £250?
The hypothetical target is £625 in each holding; A is £125 above that value and B £125 below it.
Does restoring weights guarantee a better result?
No. It changes future exposure. Subsequent prices, costs and the validity of the chosen weights determine the consequences.
Continue learning
Connect the ideas
Follow the related articles below to examine these assumptions in another setting.
Disclaimer
Educational Use Only
This article is for informational and educational purposes only. It does not provide personalised investment advice.
