Translate execution costs into annual portfolio drag

Keep three different quantities visible

The cost rate is basis points per traded dollar. The dollar cost is that rate divided by 10,000 and multiplied by order notional. Portfolio drag is dollar cost divided by AUM, again expressed in basis points. These are related quantities with different denominators.

The SEC’s fee bulletin distinguishes transaction charges from ongoing expenses. For a cost budget, maintain that separation: execution costs arise from the trading schedule, while an assumed annual management or data expense follows a different charging rule. The workbench does not combine these into an all-in product fee or calculate investor-specific taxes.

A monthly scenario with both sides counted

Assume a hypothetical $25 million portfolio trades $2 million in purchases and $2 million in sales each month. Monthly gross trading is $4 million, or 16% of AUM. At 8 bps per traded dollar, monthly cost is $3,200 and portfolio drag is 1.28 bps.

If the same event repeats twelve times with unchanged AUM and cost assumptions, annual gross trading is $48 million, or 192%. Annual modeled cost is $38,400 and drag is 15.36 bps. The 8 bp order-cost rate must not be reported as the annual portfolio expense, and the 192% gross traded fraction must not be called a regulatory fund turnover rate.

Use a weighted rate when orders differ

Suppose a second hypothetical event trades $3 million at 5 bps and $1 million at 20 bps. The costs are $1,500 and $2,000, totaling $3,500. The traded-dollar-weighted rate is $3,500 / $4 million × 10,000 = 8.75 bps. The simple average of 5 and 20, or 12.5 bps, overweights the smaller expensive order.

The expensive order still deserves attention: it generates more than half the cost while representing one-quarter of trading. A weighted portfolio total is useful for budgeting, but retaining each order’s contribution reveals where a change in assumptions matters most. Do not replace the detailed rows with a single average before reviewing constraints.

Annualization requires a repeatability assumption

Multiplying one month by twelve assumes that month represents future activity and conditions. A rebalance-heavy month, a transition portfolio or a one-off cash withdrawal may not repeat. State the event frequency and separate recurring turnover from exceptional transitions.

If costs are deducted from a changing portfolio, exact return impact may require a path calculation rather than subtracting one annual percentage from an assumed return. This guide’s annual example is a static budget with constant AUM. It does not project compound wealth, strategy performance or the timing of future cash flows.

Use the budget to expose dependencies

A useful sensitivity table varies gross trading and per-dollar cost independently. At a fixed 8 bp cost rate, 100%, 200% and 300% annual gross trading imply 8, 16 and 24 bps of simple portfolio drag. At 200% gross trading, doubling the rate to 16 bps doubles drag to 32 bps.

In reality, more turnover may concentrate orders and raise the cost rate, so the two inputs need not be independent. The workbench can illustrate each event’s participation sensitivity before aggregation. Keep that nonlinear effect visible, and report excluded financing, borrow, taxes, market movement and unfilled-order opportunity costs alongside the execution budget.

Keep in the decision record

• Convert order rates into dollars before aggregating portfolio costs.
• Weight average costs by traded notional.
• State the recurrence and AUM assumptions behind annualization.

Source notes

1. SEC Investor.gov — How Fees and Expenses Affect Your Investment Portfolio
2. SEC — Form N-1A informational version

Sources inform the definitions and limitations. Scenario inputs and arithmetic are publication illustrations, not extracted market observations. Access checks are dated in the source inventory.

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