Simple Dietz Return Calculator

The Simple Dietz method is a widely used approximation formula for measuring the rate of return on an investment portfolio over a specific period. Developed by Peter Dietz in the 1960s as part of his doctoral research, it was designed to provide a quick and practical way for fund managers and performance analysts to calculate returns without needing the exact dates of every cash flow.

At its core, the Simple Dietz method makes one key simplifying assumption: all external cash flows — deposits, withdrawals, contributions, and distributions — are assumed to have occurred at the midpoint of the measurement period. This assumption dramatically reduces the complexity of the calculation while still producing a reasonably accurate return figure for most practical purposes.

Before modern software could track daily cash flows with ease, the Simple Dietz method was the industry standard for performance attribution. Today it remains relevant for back-of-the-envelope checks, quarterly reporting where precision matters less than speed, and educational contexts where understanding the building blocks of return measurement is essential.

The method sits between two extremes: simple holding-period return (which ignores cash flows entirely) and the full Time-Weighted Return (TWR) or Money-Weighted Return (MWR), which require sub-period valuations or iterative solvers. Simple Dietz gives you most of the accuracy with a fraction of the complexity.

Investment consultants, plan sponsors, and individual investors use this calculator to quickly benchmark how a portfolio performed net of contributions and withdrawals — turning raw dollar figures into a percentage return that can be compared against benchmarks or other managers.

Using this Simple Dietz calculator is straightforward. You need four pieces of information: your portfolio's beginning value, its ending value, the total net cash flows that occurred during the period, and the length of the period in days.

Beginning Value (BV): Enter the portfolio's market value at the very start of the measurement period. This should be the fair market value of all assets held, including accrued income, before any new cash flows for the period are counted. For most brokerage accounts this is the statement value on the first day of the period.

Ending Value (EV): Enter the portfolio's market value at the very end of the measurement period. This is the statement value on the last day, after all cash flows and price changes have been reflected. Make sure the ending value already includes any dividends received, interest credited, or fees deducted.

Total Cash Flows (CF): Enter the net of all external cash flows during the period. Deposits are positive numbers; withdrawals are negative numbers. For example, if you deposited $10,000 and withdrew $3,000 in the same period, enter $7,000. Internal cash flows like dividends reinvested within the portfolio do not count here — only money that moved in or out from an external source.

Period (Days): Enter the number of calendar days in the measurement period. The calculator uses this only to annualize the result so you can compare the return to a 12-month benchmark. The Simple Dietz formula itself does not require day counts.

Once all four inputs are filled, the calculator instantly displays the Simple Dietz return percentage, the annualized return, the net investment gain in dollar terms, and the average capital employed throughout the period.

The numerator of the Simple Dietz formula — EV − BV − CF — is the net gain: the increase in portfolio value that is attributable purely to investment performance after stripping out the effect of cash injected or removed by the investor. If your portfolio grew from $100,000 to $112,000 but you deposited $5,000 along the way, the actual investment gain is only $7,000, not $12,000.

The denominator — BV + CF / 2 — represents the average capital employed. Because the method assumes cash flows arrived halfway through the period, only half of the cash flow is treated as "invested capital" for the full duration. A deposit made at the midpoint has been at work for exactly half the period, so it is fair to weight it at 50% when computing the capital base against which returns are measured.

This mid-period assumption is what distinguishes Simple Dietz from a plain holding-period return. A pure holding-period calculation would use BV + CF as the denominator — implying every dollar was invested from day one — which would understate the return whenever large deposits arrived late in the period. Simple Dietz corrects for this bias by cutting the cash flow contribution in half.

The annualized return is computed by compounding the sub-period return to a 365-day basis:

Annualized Return = (1 + R)^(365 / days) − 1

This is the standard geometric annualization used across the finance industry. A 6.83% return over exactly 365 days annualizes to 6.83%. The same return over 180 days would annualize to roughly 14.1%, reflecting that the investor achieved that gain in half a year.

One important caveat: the Simple Dietz method can produce misleading results when cash flows are very large relative to the beginning portfolio value, or when cash flows are concentrated at the start or end of the period rather than near the middle. In those scenarios, the Modified Dietz method — which weights each cash flow by the fraction of the period it was invested — provides a materially more accurate result.

Portfolio performance measurement offers a spectrum of methods, each trading off accuracy against data requirements. Understanding where Simple Dietz sits in this spectrum helps you choose the right tool for each situation.

The Simple Dietz method is best when you have only a beginning balance, an ending balance, and a summary of net deposits or withdrawals — which is exactly the information available on most quarterly brokerage statements. It requires no sub-period valuation and no knowledge of exactly when each contribution arrived.

The Modified Dietz method is a direct upgrade when you know the exact date of each cash flow. It uses the same net-gain numerator but replaces the CF/2 denominator with a time-weighted sum of each cash flow, producing a more accurate capital base. For portfolios with cash flows heavily skewed toward the beginning or end of the period, Modified Dietz can differ from Simple Dietz by several percentage points.

The Time-Weighted Return is the gold standard for evaluating a portfolio manager's skill because it eliminates the effect of the investor's own timing of contributions and withdrawals. GIPS (Global Investment Performance Standards) requires TWR for composite performance reporting. However, TWR demands a portfolio valuation at every external cash flow date, which requires daily pricing systems.

The Simple Dietz calculator is particularly valuable in several real-world scenarios. Individual investors reviewing annual brokerage statements can quickly calculate how well their portfolio performed net of contributions, giving them a return figure that is comparable to published index returns. Many brokerage statements still report only dollar-change figures, leaving investors unsure whether growth was due to new deposits or actual investment gains.

Financial advisors use Simple Dietz to produce rapid client reports between full performance attribution cycles. When a client calls asking for a quick performance update, the advisor can compute an approximate return in seconds using beginning and ending statement balances plus a tally of net transfers.

Institutional fund administrators still encounter Simple Dietz in legacy systems and historical data archives. Understanding the method is essential for reconciling older performance records or replicating calculations produced before modern portfolio accounting software became ubiquitous.

Despite its convenience, the Simple Dietz method has important limitations. The mid-period assumption introduces error whenever actual cash flows are concentrated at either extreme of the period. A large year-end bonus deposited on December 28 effectively has only three days to generate returns, yet Simple Dietz treats half of it as invested for six months. In such cases the method will overstate the return.

The method also becomes unreliable when cash flows are very large relative to the portfolio — for instance, when starting from a near-zero balance and adding a large deposit. The average-capital denominator breaks down in extreme cases. As a rule of thumb, if net cash flows exceed 10% of the beginning portfolio value, consider using Modified Dietz or a full Time-Weighted Return calculation instead.

Finally, note that the Simple Dietz return is a money-weighted approximation, not a true time-weighted return. It will be influenced by the investor's own timing decisions — a strength if you want to understand the investor's actual experience, but a weakness if you are trying to evaluate the manager's skill in isolation.