A Systematic Investment Plan is a scheduled contribution, usually monthly, into the same investment. The behavioural argument for it is strong and well understood: automating the decision removes the need to feel confident about any particular month. The mathematical argument is more nuanced than most summaries admit, and that nuance is what these guides are for.
Contributions do not all compound for the same length of time. Your first instalment gets the full horizon, your last gets almost none, and the effective average is roughly half your stated period. That single fact explains why a SIP total looks disappointing next to a lump sum projection at the same rate over the same years, and why the comparison is unfair unless you also account for the fact that most people do not have the lump sum. The SIP calculator models the instalment schedule directly, so you can see where the money actually is rather than assuming an average.
Cost averaging is the other claim worth examining honestly. Buying at a fixed rupee or dollar amount does mean you acquire more units when prices are low, which lowers your average cost relative to buying a fixed number of units. It does not protect you from a market that falls and stays down, and it is not a reason to prefer a SIP when you genuinely have a lump sum available and a long horizon. Knowing which of those situations you are in decides the answer.
Sizing is the last piece. Working backwards from a target is a different calculation from projecting forwards from a contribution, and it is the one that tells you whether your plan is actually adequate. Expect the honest answer to involve either a larger contribution, a longer horizon, or a revised target.
