Hypothetical worked example and primary educational references; not observed market data.
Relative performance changes portfolio weights
A portfolio can move away from its intended allocation even when the investor makes no trades. If one holding appreciates faster than another, it becomes a larger fraction of the total. Investor.gov describes rebalancing as bringing holdings back toward their original asset allocation. This is a risk-management action tied to a defined policy, rather than proof that the recently successful asset is about to decline. A target allocation should first be appropriate for the investor's goals and capacity. Mechanically restoring an unsuitable target would preserve the wrong risk mix with impressive procedural consistency.
Supporting references: 1. Investor.gov: Asset Allocation and Diversification
Calculate drift using the new total
Start a hypothetical portfolio with USD 12,000 in stocks and 8,000 in bonds, a 60/40 mix. Assume stocks rise 20% and bonds remain unchanged. The new values are 14,400 and 8,000, totaling 22,400. Stock weight is now approximately 64.29%. Restoring 60% requires stock value of 13,440, so the investor shifts 960 from stocks to bonds, before costs and taxes. These invented outcomes show why the denominator must be recalculated after market changes. Selling 20% of the original stock balance would not answer the target-weight question.
Hypothetical worked example
Restore a hypothetical 60/40 portfolio
Model setup
Read these assumptions before comparing the scenarios.
- All monetary amounts in this example are hypothetical U.S. dollars (USD), not current prices, quoted account terms or observed market data. Percentages, counts, time periods and index levels retain their stated units.
- Initial stock/bond values are 12,000/8,000.
- Stocks gain 20%, bonds are flat; total is unchanged by the cost-free transfer.
Stock weight (%)
- After price change64.29
- After target restoration60.00
Expanded scale, not zero-based; dots show small differences.
- Lowest displayed value
- 60.00
- Highest displayed value
- 64.29
Hypothetical cases from the table below, in the stated units. Not market observations, a forecast or a recommended choice. Read the scenario assumptions before comparing.
| Stage | Stocks (USD) | Bonds (USD) | Stock weight (%) |
|---|---|---|---|
| After price change | 14,400 | 8,000 | 64.29 |
| After target restoration | 13,440 | 8,960 | 60.00 |
Calculation details
- Drifted stock weight
- 64.29 %
((14400 / 22400) * 100) - Target stocks
- 13,440.00 USD
(22400 * 0.59999999999999998) - Transfer
- 960.00 USD
(14400 - 13440) - New bonds
- 8,960.00 USD
(8000 + 960)
Separate portfolio rules from market opinions
A calendar rule checks allocation at scheduled intervals; a threshold rule responds when a weight moves beyond a predefined range. Both can discipline decision-making, but neither guarantees superior return. A narrow threshold may lead to more trading, while a wide threshold permits greater drift. A calendar rule can miss a large interim deviation, while a threshold rule requires monitoring and a clear response process. The choice should reflect implementation costs, account constraints, and the importance of maintaining exposures. These tradeoffs are operational and risk-related rather than evidence that one schedule can reliably anticipate market turning points.
Rebalancing can underperform a continued trend
If the appreciating asset keeps rising, reducing it can lower subsequent gains relative to leaving the portfolio untouched. If the same asset then falls, reducing it beforehand can lessen exposure to that decline. Both paths are possible. Presenting rebalancing as an automatic way to buy low and sell high can obscure the dependence on future outcomes and transaction costs. Its primary defensible purpose is to keep risk closer to a chosen policy. The investor should be prepared for the rule to look disappointing during some periods while still serving the original allocation objective.
Implement across the actual household accounts
Selling in one account and buying in another can have different tax, cost, and access consequences. Some holdings cannot be traded freely, and withdrawals can alter the allocation of remaining assets. FINRA's concentration guidance helps identify drift at the issuer or sector level as well as across broad asset classes. A household-wide review should recognize these overlapping exposures while preserving the intended role of each account. Avoid treating each account's target as independent if the accounts jointly fund one objective. The relevant risk mix is the exposure supporting that objective, not necessarily the mix displayed in every container.
Supporting references: 2. FINRA: Concentrate on Concentration Risk
Document triggers and exceptions
A written rule should specify target weights, review frequency or bands, valuation date, and how contributions or costs affect action. Exceptions can be reasonable when documented, but repeated discretionary overrides can turn a rule into market timing. Revisit the target when goals, liabilities, or capacity change instead of assuming it remains correct forever. The hypothetical transfer calculation is a starting point for understanding mechanics, not an instruction to sell a particular asset. A defensible process connects each action to the chosen risk policy and records why the household can tolerate the resulting exposures under unfavorable conditions.
Test another situationTransaction charges alter the amount that restores a target
A separate hypothetical case. Its inputs differ from the main worked example above.
Transaction charges alter the amount that restores a target
Assume a hypothetical portfolio currently contains USD 13,000 in stocks and USD 7,000 in bonds. Its target stock weight is 60%. A rebalance incurs a fixed USD 100 charge paid from the portfolio's bond-side cash. Holding prices unchanged, the stock sale required to reach the target after that charge is exactly USD 1,060. The fee reduces the final portfolio denominator, so using a cost-free transfer would leave a small allocation discrepancy. More frequent corrections might keep weights closer while increasing friction. The appropriate rule should consider the practical significance of drift against the cost of correcting it.
Specify how the tolerance band is measured
Investor.gov describes both scheduled reviews and deviations from preset allocations. Make a written threshold unambiguous about whether it refers to percentage points of portfolio weight or a relative change from the target. Also define the valuation moment and treatment of incoming cash. Otherwise two people can apply the same written rule and place different trades. A threshold should trigger a review of target suitability and implementation conditions, not an assumption that trading is always feasible. If a documented exception postpones action, record what exposure remains. The rule's purpose is maintaining a chosen risk mix, not guaranteeing a return benefit.
Supporting references: 1. Investor.gov: Asset Allocation and Diversification
Check this case’s calculations
- Stock sale with portfolio-paid fixed charge
- 1,060.00 USD
(13000 - (((13000 + 7000) - 100) * 0.59999999999999998))
Limits of this additional case
- The charge is fixed and assigned to bond-side cash; proportional transaction costs would require a different equation.
Limits of this analysis
- Transfers can incur taxes and transaction costs.
- Rebalancing may reduce gains during a continuing trend.
- A target needs reassessment when household objectives or capacity change.
Sources and scope
- Investor.gov: Asset Allocation and Diversification
Allocation, diversification and rebalancing address different portfolio decisions.
Scope: U.S. investor education; supports the stated mechanism, not the hypothetical scenario or a forecast.. Reference accessed 2026-10-06.
- FINRA: Concentrate on Concentration Risk
Large exposures and overlapping holdings can amplify portfolio losses.
Scope: U.S. investor education; supports the stated mechanism, not the hypothetical scenario or a forecast.. Reference accessed 2026-10-06.
Illustrative calculations are not forecasts or personal recommendations. Editorial standards
