Why CAGR Isn’t Enough for Irregular Investments
CAGR (see this project’s own CAGR guide) works cleanly when you have one investment made at one point in time and one final value at a later point in time. But if you’ve invested different amounts on different dates — like a SIP’s monthly instalments, or a few ad-hoc lump-sum top-ups — there is no single clean “beginning value” and “time period” for a standard CAGR calculation to use.
What XIRR Does Differently
XIRR (Extended Internal Rate of Return) solves this by taking every individual cash flow — each investment (as a negative amount) and each withdrawal or final valuation (as a positive amount) — along with its exact date, and calculating the single annualized rate of return that would make all of those cash flows mathematically consistent with each other. It effectively accounts for exactly how long each rupee was invested, not just the first and last dates.
A Conceptual Example
Imagine you invested ₹10,000 in January, another ₹15,000 in June, and your holdings were worth ₹30,000 the following January. A simple return calculation ignoring timing would be misleading, since the June investment had far less time to grow than the January one. XIRR weighs each cash flow by its actual date, producing a return figure that fairly reflects this timing difference — which is exactly why XIRR (not a simple CAGR) is the standard way SIP and other irregular-investment returns are reported.
Why This Matters for a SIP
Every monthly SIP instalment is, by definition, a separate cash flow on a separate date. Reporting a SIP’s "return" meaningfully requires accounting for this — which is why platforms tracking your actual SIP investments typically compute and display XIRR rather than a simple average or CAGR figure, and why the SIP guide on this project uses an assumed annual rate for projection rather than claiming a single historical return figure.
Calculating XIRR Yourself
XIRR requires solving for the rate that makes a sum of discounted cash flows equal zero — mathematically an iterative calculation, not a simple formula you’d compute by hand. In practice, XIRR is calculated with spreadsheet software (which has a built-in XIRR function taking your cash flows and dates as input) or by your investment platform, rather than manually.