Many waterfall models use IRR hurdles to determine when distributions move to the next tier. But Excel's IRR function and XIRR function can return materially different results on the same cash flows. If the LPA specifies an IRR-based hurdle, the formula you choose affects which tier is active.
IRR Assumes Equal Periods. XIRR Doesn't.
Excel's IRR function assumes cash flows occur at equal intervals. XIRR uses actual dates and computes a daily rate. For quarterly distributions that happen on exact dates (which they do in practice), IRR can overstate or understate the return depending on whether periods are slightly longer or shorter than the assumed interval. On irregularly timed cash flows, the difference grows.
How Much Does It Matter.
Consider a $1,000,000 investment with 5 cash flows over 3 years. If distributions happen on actual dates rather than exact quarterly intervals, IRR might return 12.8% while XIRR returns 14.1%. When the waterfall hurdle is 12%, both formulas agree the hurdle is met. But when the hurdle is 13%, IRR says no and XIRR says yes. That determines whether the GP earns promote on that distribution.
Note: The 12.8% and 14.1% figures above are illustrative examples to show the magnitude of potential differences.
Which One Should You Use.
Check the LPA. Some specify "IRR" loosely, others reference annualized internal rate of return based on actual dates (which implies XIRR). If the LPA is ambiguous, XIRR is generally considered more precise because it accounts for actual timing. The key is consistency: once you choose, use the same function everywhere. Switching mid-fund creates discrepancies that are hard to explain to LPs or auditors.
The choice between IRR and XIRR is not a preference. It's a calculation decision that can shift tier boundaries and change GP economics. Match the formula to the LPA, document the choice, and apply it consistently.
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