SIP vs lumpsum — what each actually does to your money

A systematic investment plan is not a kind of mutual fund. It is a way of buying one: a fixed amount, at a fixed interval, into a scheme you have chosen. A lumpsum is the other way: one amount, once. Both buy the same units of the same scheme at the day’s NAV; they differ only in when the money goes in. Everything said for or against either follows from that one fact.

How the units add up

Each SIP instalment buys units at the applicable NAV of that day (SEBI investor education). Suppose ₹5,000 a month. A month when the NAV is ₹50 buys 100 units; a month when it is ₹40 buys 125; a month at ₹62.50 buys 80. The same rupees buy more units when the NAV is low and fewer when it is high, so the average cost per unit over many instalments is a little below the average of the NAVs on those days. This is what “rupee cost averaging” means. It is arithmetic, not a promise: it does not make the return positive, and it does not beat a lumpsum in every market.

A lumpsum buys all its units at one NAV. If the NAV then rises steadily, the lumpsum bought everything at the lowest price and ends up ahead of an SIP that kept buying at higher prices. If the NAV falls and later recovers, the SIP bought cheaply on the way down and ends up ahead. Which of the two happens is not knowable in advance, which is why neither method is “better”; the comparison depends on a path that has not happened yet.

The question that actually decides it

Most people do not choose between an SIP and a lumpsum. They choose according to how their money arrives. A salary arrives monthly, so monthly investing is the natural shape of it, and an SIP is simply the standing instruction that does it without a decision each month. A bonus, a sale or an inheritance arrives at once, so the question becomes whether to invest it at once or spread it — and spreading a lumpsum over months, usually by parking it in a liquid scheme and moving a slice each month (a systematic transfer plan), is an SIP by another route. The method follows the cash flow; the scheme is the real choice.

What the calculators here assume

The SIP calculator and the lumpsum calculator on this site take an amount, a period and an annual rate, and show what the arithmetic gives. The rate is yours: an assumption typed in, compounded monthly, applied evenly. Real NAVs do not move evenly, so the figure is not a forecast of any scheme; it is what a steady rate would produce, useful for comparing amounts and periods against each other. The formulas are printed beside the results. Every mutual-fund page on this site also carries the line that past performance does not indicate future returns, and the same applies to any rate you type.

The mechanics of an SIP

• Mandate. An SIP runs on a bank mandate (an auto-debit instruction) for a date you pick; the fund house draws the instalment and allots units at that day’s applicable NAV, published in AMFI’s file.
• Minimums. Each scheme sets its minimum instalment and the number of instalments.
• Pausing and stopping. An SIP can be paused or stopped by instruction; missing instalments does not penalise the units already held, though the bank may charge for a failed debit.
• Each instalment is a separate purchase. The exit-load clock and the holding period for tax start on each instalment’s own date; when units are redeemed, the earliest bought are treated as sold first.

What neither method changes

Neither an SIP nor a lumpsum changes what the scheme holds, what it costs or how much it swings. Those belong to the scheme, and they are the same whichever way you buy. A direct plan has the lower expense ratio either way (direct vs regular plans), and a NAV means the same thing either way (how to read a NAV). The method decides when you buy; it does not decide what you own.