Weight the capital available during the period
Let B be beginning value, E ending value, and Cᵢ an external flow occurring at elapsed time fraction tᵢ between zero and one. Its weight is wᵢ = 1 − tᵢ. Modified Dietz return equals (E − B − ΣCᵢ)/(B + ΣwᵢCᵢ). Multiply by 100 for a percentage. Contributions are positive, withdrawals negative. The weighted denominator must be positive, because a nonpositive capital base does not support this return interpretation.
A midpoint contribution example
Suppose the account begins at 10,000, receives a contribution of 2,000 halfway through the period, and ends at 12,600. Net investment change is 12,600 − 10,000 − 2,000 = 600. The contribution weight is 0.5, making weighted capital 11,000. Modified Dietz return is 600/11,000, or approximately 5.455%. Moving that same contribution to the period's beginning changes the denominator to 12,000 and the estimate to 5%.
Know when the approximation can weaken
A large cash flow during a sharp market move can make the linear time-weighting approximation materially different from a true return linked at every flow. The time fraction should describe elapsed calendar time under one consistent policy, not the fraction of trades already completed. The calculator does not infer transaction classifications or convert currencies. Where accurate valuations exist around each contribution and withdrawal, use the time-weighted return tool to link those measured subperiods directly.