Restore target portfolio weights after market drift, distinguish targets from trading triggers, and reconcile trades, turnover, cash flows and return trade-offs.
Rebalancing
Rebalancing is adjusting a portfolio’s holdings to bring its current portfolio weights back toward specified target weights. It keeps market movements from silently rewriting the allocation: a portfolio chosen to hold 60% stocks can become substantially more stock-heavy even when its owner makes no trades.
The target answers what allocation should the portfolio hold? The rebalancing rule answers when should holdings be adjusted, and how far? Choosing a different target is an asset-allocation decision; restoring an existing target is its implementation.
A 60/40 portfolio becomes 65.22/34.78
Consider a synthetic USD10,000 portfolio holding USD6,000 in a stock sleeve and USD4,000 in a bond sleeve. Over 1 January–1 February 2026, suppose their simple total returns are +25% and 0%. Distributions are reinvested within each sleeve, and there are no external flows or costs.
The stock sleeve grows to USD7,500, the bond sleeve stays at USD4,000, and total portfolio value becomes USD11,500. Stocks now represent
wstock−=11,5007,500=2315≈65.22%.
The superscript minus denotes the moment before the rebalance. The starting 60% is no longer the current weight. In general, if starting weights are wi0, sleeve returns are ri, and there are no flows,
wi−=1+Rpwi0(1+ri),Rp=i∑wi0ri.
Here the portfolio return is 15%. Weights use the new USD11,500 denominator, rather than the original USD10,000.
The reset ledger
To restore 60/40 immediately, assume unchanged execution prices, fractional holdings and no trading costs. For current values Vi−, total value V− and target weights wi∗, the signed monetary trade is
xi=wi∗V−−Vi−.
Positive means buy; negative means sell.
Sleeve
Before rebalance
Target value
Signed trade
After rebalance
Stocks
USD7,500
USD6,900
−USD600
60%
Bonds
USD4,000
USD4,600
+USD600
40%
Total
USD11,500
USD11,500
USD0
100%
Selling USD600 of stocks funds the USD600 bond purchase. The rebalance transfers capital between sleeves; it creates no immediate profit. The new stock amount is 60% of current wealth, so returning stocks to their original USD6,000 would overshoot the intended sale.
Stock allocation starts at 60%, drifts to 65.22%, and returns to 60%. The illustrative upper trigger is 65%. Selling USD600 of stocks and buying USD600 of bonds restores USD6,900 and USD4,600 holdings.
Original synthetic USD ledger, 1 January–1 February 2026. The dashed 60% line is the target; the 55–65% band is an illustrative trigger policy. The reset assumes unchanged execution prices, fractional holdings and no costs.
The gross amount traded is USD1,200, or approximately 10.43% of pre-trade value. Turnover describes trading activity; it is not a transaction-cost rate.
A target is not a trigger
Suppose the policy permits stock weights between 55% and 65%, and triggers a full reset to 60% when the weight moves outside that band. The observed 65.22% stock weight crosses the upper boundary. 65% tells us when to act; 60% tells us where to reset. Trading only to the band boundary would be a different policy.
Common timing rules include:
Rule
Decision
Calendar
Reset on specified dates.
Threshold
Reset when an allocation breaches a specified band.
Calendar plus threshold
Check on specified dates and trade only if a band is breached.
A band stated in percentage points should be explicit: this example allows five percentage points on either side of 60%. There is no universal band or frequency suitable for every mandate. Wider bands tolerate more allocation drift; tighter bands can demand more monitoring and trading. Both the observation schedule and the reset destination belong in the policy. Vanguard’s rebalancing guide distinguishes these timing approaches.
Costs and cash flows change the trade
A real trade can incur commissions, the bid–ask spread, market impact and account-dependent tax consequences. If costs are deducted from the portfolio, post-trade wealth is smaller than USD11,500; the costless targets above cannot simply be treated as exact execution instructions. Minimum lots and restricted holdings can also prevent an exact reset. The SEC’s allocation guide explains alternatives to selling and flags fees and taxes.
New money can do some of the work. In a separate variation, contributing USD1,000 entirely to bonds changes the drifted holdings to USD7,500 stocks and USD5,000 bonds. The new USD12,500 total is exactly 60/40, with no stock sale. That contribution increases wealth; it is not investment return. Withdrawals and distributions paid into cash likewise need their own ledger treatment before a target trade is calculated.
For the mathematical extension to passive drift and trading demand, see Turnover and the Information Horizon. Its opening sections derive the weight and turnover equations; the public page is a preview, and the full lesson requires membership.
Risk discipline does not guarantee a return premium
Resetting reduces the stock exposure in this example. It does not establish that stocks will underperform bonds next. Starting from the same USD11,500, consider just one further period, with bonds returning 0% and no costs or flows:
Next stock return
Leave USD7,500 in stocks
Reset to USD6,900 in stocks
+10%
USD12,250
USD12,190
−10%
USD10,750
USD10,810
The reset gives up USD60 in the rising-stock scenario and preserves USD60 in the falling-stock scenario. Repeated rebalancing can benefit some return paths and hurt others, while costs reduce its net result. A claimed “rebalancing premium” therefore needs a stated comparator, return path or model, timing rule and cost basis. It is not an automatic reward for selling winners. The practical purpose is to keep exposure aligned with the chosen allocation, as Vanguard explains.
