What it means
Imagine an account that grew from $500,000 to $600,000 over a year. That looks like a 20% gain until you learn the owner paid in $90,000 during the year, at which point the actual investment gain is only $10,000.
External cash flow is the name for that $90,000 and for every other movement driven by the owner rather than the market. The reason it gets its own term is performance measurement.
Fund managers are judged on the returns they generate, not on whether clients happened to add or remove money, so return calculations are designed to neutralise the effect of external flows. Two standard approaches exist.
Time-weighted return breaks the period into sub-periods at each flow and chains them together, which removes the timing effect entirely and is the usual basis for comparing managers. Money-weighted return, including the modified Dietz method, weights each flow by how long it was invested and answers a different question: what return did this particular investor actually experience?
The distinction matters in real decisions. A pension trustee comparing two managers wants time-weighted numbers, while a business owner asking what their own portfolio did for them wants the money-weighted figure, because their contribution timing is part of their result.
Large external flows also create practical problems beyond measurement. A big redemption may force a manager to sell holdings at an awkward moment, and a big contribution can leave cash uninvested, so many funds apply notice periods or dilution levies to protect remaining investors.
In practice
Real-world examples.
Example
A family office adds $2,000,000 to its equity portfolio a week before a market fall. The time-weighted return still shows the manager beating the index, while the money-weighted return the family actually experienced is negative, and both numbers are correct.
Example
A corporate pension scheme pays out $8,000,000 in benefits during the year. The scheme actuary strips those external flows out before assessing whether the investment strategy delivered its target return above liabilities.
Example
A wealth manager onboards a client who transfers in $1,500,000 of existing shares rather than cash. The in-specie transfer is still an external cash flow for performance purposes and must be valued and dated correctly, or the client's reported return will be wrong.
Think of it
“External cash flow is money from outside the business-financing from lenders or investors.
Formula
Calculation
Net External Cash Flow = Contributions - Withdrawals
Modified Dietz Return = (End Value - Start Value - Net External Cash Flow) / (Start Value + Sum of weighted flows)
A charity's investment portfolio starts the year at $5,000,000. Exactly halfway through the year the charity contributes $1,000,000 from a legacy, and there are no withdrawals, so net external cash flow is $1,000,000. The portfolio ends the year at $6,550,000.
Investment gain = $6,550,000 - $5,000,000 - $1,000,000 = $550,000
The contribution was invested for half the year, so its weight is 0.5. The average capital employed is $5,000,000 + ($1,000,000 x 0.5) = $5,500,000.
Modified Dietz Return = $550,000 / $5,500,000 = 10%. Ignoring the external flow would have produced a misleading headline of $1,550,000 growth on $5,000,000, or 31%.Case study
Seen in the real world.
The following is an illustrative, fictional scenario. Ashgrove Foundation, an invented charitable endowment, told its board that its portfolio had returned 18% for the year. The finance committee chair questioned the figure because the foundation had also received a $3,000,000 bequest in February.
Recalculating properly showed the picture clearly. Starting value was $20,000,000, the bequest arrived with about ten months of the year remaining, and the ending value was $25,600,000. Investment gain was $25,600,000 - $20,000,000 - $3,000,000 = $2,600,000, against average capital of $20,000,000 + ($3,000,000 x 0.833), or roughly $22,500,000, giving a return near 11.6% rather than 18%.
The fictional foundation adopted a policy of reporting both a time-weighted return for judging its manager and a money-weighted return for judging its own funding progress. It also began dating and valuing every external flow on the day it settled rather than the month end.
Watch out
Common mistakes.
- Treating the change in account value as the investment return, which double counts every contribution the owner made.
- Recording all flows at month end for convenience, which distorts weighting when a large contribution actually arrived on the first day of the month.
- Counting internal movements, such as selling a bond and buying a share inside the same portfolio, as external cash flow when no money crossed the portfolio boundary.
Questions
People also ask.
Are dividends external cash flow?
Only if they are paid out of the portfolio; dividends reinvested inside it are internal and form part of the return.
Which return measure should a client see?
Ideally both, with the time-weighted figure for manager performance and the money-weighted figure for the client's own outcome.
Do fees count as external cash flow?
Fees deducted from the portfolio are usually treated as a reduction in return rather than as an owner-driven flow, though conventions differ and should be stated clearly.
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