SIP vs Lumpsum: What the Maths Says and What It Misses

Put ₹12 lakh into an equity fund on day one and, at a steady 12% a year, it grows to about ₹39.6 lakh in ten years. Put the same ₹12 lakh in as ₹10,000 a month and you end up with about ₹23.2 lakh. Same money, same return, and the lumpsum is ahead by ₹16.4 lakh. If that were the whole story, nobody would bother with a SIP.

It isn't the whole story, but the arithmetic deserves a fair hearing before the arguments for SIP. Every figure below uses the same rules: monthly compounding at the annual rate divided by 12, each SIP instalment invested at the start of the month, no fund costs, no tax, and a return that stays constant. That last rule never holds in real life. These are illustrations of how the maths behaves, not forecasts of what any fund will deliver.

Why lumpsum wins on paper

The reason is only time. A lumpsum has its entire amount compounding from month one. A SIP puts in ₹10,000 in month one, ₹10,000 in month two, and so on, so the money from the last instalments has barely started working when the period ends. On average the SIP money has been invested for about half the period.

₹10,000 a month at 12% a year	SIP	Same total as lumpsum on day one
10 years (₹12 lakh put in)	₹23.2 lakh	₹39.6 lakh
20 years (₹24 lakh put in)	₹99.9 lakh	₹2.61 crore

So if you believe markets rise more often than they fall, lumpsum comes out ahead more often than not. That is what "on average" means here. It follows from the assumption, and I'm not quoting a measurement. If markets turned out flat or falling over your period, the ranking could flip.

The catch: you don't have ₹12 lakh on day one

The table compares a SIP with money you don't have yet. A salaried person earning, say, ₹80,000 a month doesn't choose between a SIP and a lumpsum. The salary arrives monthly, so monthly is the only way to invest it. The table doesn't show SIP losing. It shows what waiting costs.

The real choice shows up when a bonus, a maturity payout or a property sale puts a large amount in your account at once. My position: if that money is meant for at least seven or eight years out, and you could watch it fall by a third without selling, investing it in one go is defensible. If a 30% drop would make you sell, spread it over six to twelve months. The staggered version gives up some average return in exchange for a plan you can actually stick to.

The behavioural reason is the real one

Here are three hypothetical three-year paths, with returns spread evenly across each year. In each case ₹3.6 lakh goes in, either as ₹10,000 a month or all at the start.

Yearly returns	SIP ends at	Lumpsum ends at
+10%, +10%, +10%	₹4.18 lakh	₹4.79 lakh
-30%, +15%, +30%	₹4.56 lakh	₹3.77 lakh
+30%, +15%, -30%	₹3.02 lakh	₹3.77 lakh

In the steady climb, lumpsum wins. When the fall comes first, the SIP wins, because it keeps buying cheap units while the lumpsum sits through the drop. When the fall comes last, the lumpsum wins again, and the SIP ends below what you put in. None of these paths is a prediction. They show that the winner depends on the order of returns, which nobody knows in advance.

What I think matters more than the table is what a person does during the second path. The lumpsum investor opens the app in month eight, sees ₹3.6 lakh turned into ₹2.5 lakh or so, and is tempted to sell at the bottom. The SIP investor has an auto-debit that quietly keeps going. That's a claim about people, not something I can compute, but a plan you don't abandon beats a better plan you abandon.

12% against 8% is a bigger gap than it looks

Four percentage points sounds small. Over long periods it isn't. Same ₹10,000 a month:

Period	Put in	At 8%	At 12%	12% ends higher by
10 years	₹12 lakh	₹18.4 lakh	₹23.2 lakh	₹4.8 lakh (26% more)
20 years	₹24 lakh	₹59.3 lakh	₹99.9 lakh	₹40.6 lakh (68% more)

Over 20 years, the gain at 12% is ₹75.9 lakh against ₹35.3 lakh at 8%, a little over double, from a return that is only a third higher. Long periods amplify small differences in the rate. This is also why a projection built on 12% is fragile: if the real figure is 9%, the outcome shifts far more than the headline numbers suggest.

Inflation takes a large bite out of the headline

Assume prices rise 6% a year (an assumption, not a forecast). Prices then multiply by about 1.79 in ten years and 3.21 in twenty. Divide the final amounts by those:

Return	10 years, in today's money	20 years, in today's money
12%	₹12.97 lakh	₹31.2 lakh
8%	₹10.3 lakh	₹18.5 lakh

That ₹99.9 lakh after 20 years buys what about ₹31 lakh buys today. At 8%, the ₹59.3 lakh is worth ₹18.5 lakh in today's terms, which is less than the ₹24 lakh you actually put in. The nominal figure grew; the buying power didn't. Anyone setting a goal such as a child's education fund should work in today's rupees and then inflate the target, not the other way round.

There's a second inflation problem hidden in a flat SIP. An instalment of ₹10,000 in year 20 is worth about ₹3,300 in today's money. You are quietly investing less every year in real terms.

Step-up SIP: the lever most people leave alone

A step-up SIP raises the instalment by a fixed percentage each year, usually in line with your salary hike. Start at ₹10,000 and raise it 10% every April, still at 12%:

• 10 years: ₹19.1 lakh put in, ₹33.7 lakh at the end, against ₹23.2 lakh for the flat SIP.
• 20 years: ₹68.7 lakh put in, about ₹1.99 crore at the end, against ₹99.9 lakh for the flat SIP.
• A flat SIP would need to start at ₹19,906 a month to match that 20-year result.

It's the strongest lever here, and it costs very little at the start. The honest limit: by year 20 the instalment has grown to about ₹61,000 a month. That works only if your income rises similarly. With a 5% step-up the 20-year corpus is about ₹1.37 crore, which is still well above the flat SIP. Choose a step-up you can sustain, not the biggest one the calculator allows.

Starting early matters as much. Beginning the flat SIP one year late (19 years instead of 20) leaves ₹87.5 lakh instead of ₹99.9 lakh, a gap of ₹12.4 lakh from one year's delay.

What these projections can't tell you

Returns don't arrive as 1% every month. Fund expenses, exit loads and tax on gains reduce what you keep, and tax rules change, so check the current ones before you plan. Mutual fund ads carry the line "mutual fund investments are subject to market risks" for a reason: nothing here is promised, and I'm not recommending any fund or any category. If you want to test your own numbers, the compound interest calculator lets you change the rate, period and compounding. If the percentages in these tables feel slippery, the guide to percentage mistakes covers the usual traps.

This article is education, not investment advice. For decisions about your own money, a SEBI-registered adviser who knows your full situation is the right person to ask.