A SIP that grows with you

A regular SIP — systematic investment plan — invests the same amount every month. The SIP post ran ₹10,000 a month through twenty years of a Nifty 50 index fund.

But salaries rarely stay flat for twenty years. A step-up SIP (fund houses also call it a top-up SIP) raises the instalment automatically at a fixed interval — usually once a year — by a fixed percentage or a fixed rupee amount. Many fund houses and platforms offer it as an option when you set up the SIP.

It’s often sold with a striking chart of how much bigger the final corpus gets. That chart is true. It’s also mostly showing you that you invested more money. This post separates the two.

The formula

Instalment in year k  =  A × (1 + g)^(k − 1)

  A = starting monthly instalment
  g = annual step-up (e.g. 0.10 for 10%)
  k = 1, 2, 3, … (the same amount every month within a year)

Total invested over Y years  =  12 × A × [(1 + g)^Y − 1] / g       (flat SIP: 12 × A × Y)

The final value is each instalment compounded for however long it was invested, added up — which is why XIRR (extended internal rate of return, the single annual rate that fits all those cash flows) is the right way to measure it.

One consequence of that formula matters more than any other: a step-up SIP puts more of its money in late. In a ten-year, 10% step-up, the last year’s instalments are ₹23,579 a month against ₹10,000 in the first year. Late money has less time to compound, so its share of the final value depends heavily on the last few years’ returns.

First, with a steady return

If every month earned exactly the same return, how would the two compare? Take a constant 12% a year, ₹10,000 a month to start, 10 years:

Invested Final value Return (XIRR)
Flat SIP ₹12,00,000 ₹22,40,359 12%
10% step-up ₹19,12,488 ₹32,68,896 12%

The step-up ends with a bigger corpus because it invested more rupees. Its return is identical — with a constant rate, every rupee earns the same 12% however long it’s invested. So any difference in XIRR between the two on real data comes from when the market did well, not from stepping up.

Worked example: real NAVs

UTI Nifty 50 Index Fund, regular plan, growth option — the fund used throughout this series (scheme code 100822 with AMFI, the Association of Mutual Funds in India; source AMFI NAV history via mfapi.in). ₹10,000 a month on the first of every month, raised each April by 0%, 5% or 10% (rounded to the rupee), all ending 31 March 2026. Historical data, for illustration only.

Period Step-up Invested Final value XIRR Last monthly instalment
10 years (Apr 2016 to Mar 2026) None ₹12,00,000 ₹20,75,891 10.57% ₹10,000
  5% a year ₹15,09,348 ₹24,79,722 10.34% ₹15,513
  10% a year ₹19,12,488 ₹29,88,833 10.07% ₹23,579
15 years (Apr 2011 to Mar 2026) None ₹18,00,000 ₹44,17,002 11.13% ₹10,000
  5% a year ₹25,89,420 ₹57,21,882 10.99% ₹19,799
  10% a year ₹38,12,688 ₹76,07,039 10.8% ₹37,975
20 years (Apr 2006 to Mar 2026) None ₹23,90,000 ₹77,80,190 10.69% ₹10,000
  5% a year ₹39,57,908 ₹1,09,99,020 10.69% ₹25,270
  10% a year ₹68,62,988 ₹1,63,12,780 10.64% ₹61,159

(The twenty-year run has 239 instalments, not 240, because the fund’s first NAV (net asset value) in the data is 3 April 2006 — the same reason as in the SIP post. Its flat row matches that post’s ₹77,80,190.)

Reading the table:

  1. The corpus grows a lot. Over twenty years a 10% step-up ended at ₹1,63,12,780 against ₹77,80,190 for the flat SIP.
  2. So did the money put in. It invested ₹68,62,988 against ₹23,90,000. The bigger corpus is the bigger contributions plus the growth on them.
  3. The return didn’t improve. In all three periods, the step-up’s XIRR was equal to or slightly below the flat SIP’s — 10.07% against 10.57% over ten years. These windows all end on 31 March 2026, just after a quarter in which the index fund fell, and a step-up SIP’s largest instalments are its most recent ones, so it felt that fall more.

That last point is about the end date, not about step-ups in general, so it needs a base rate. Across all 120 ten-year windows starting on the first of each month from May 2006 to April 2016 (overlapping, so not independent), the 10% step-up’s XIRR was higher than the flat SIP’s in 92 and lower in 26. So it came out ahead more often than not — but by a median of only 0.15 percentage points, and the range ran from −1.09 to +0.53 points. A step-up tilts the return towards whatever the later years of the plan do; when those years were good it helped a little, and when they were bad, as in the window ending March 2026, it hurt. It mainly changes the amount invested and its timing, and the market decides the rest.

The fair comparison: same money, different timing

If a step-up SIP’s advantage is “more money”, compare it with a flat SIP that invests the same total. Over the ten years to March 2026, the 10% step-up put in ₹19,12,488; a flat SIP of ₹15,937 a month invests almost exactly the same (₹19,12,440).

Same total invested, ten years Final value XIRR
10% step-up, from ₹10,000 ₹29,88,833 10.07%
Flat, ₹15,937 a month ₹33,08,348 10.57%

The flat SIP ended with more — and across all 120 ten-year windows above, the same-total flat SIP finished ahead in 120 of them. It’s no mystery: the same rupees invested earlier had longer to compound, in a market that rose over every ten-year window in this fund’s history (the rolling returns post has the distribution).

That isn’t an argument against step-ups. Most people can’t invest ₹15,937 a month in year one if their income only allows ₹10,000. A step-up is a contribution schedule that tracks a rising income; it’s the right tool for that job. It just isn’t a return strategy, and a chart showing its bigger corpus without the bigger contributions beside it is telling you half the story.

🧒 Explain it like I'm 10 (optional — skip if this is already clear)

You plant one mango sapling every month. Your friend plants one too — but every year, she plants a few more a month than the year before.

After ten years she has a much bigger orchard. Is she a better farmer? No — she planted more trees. Her newest trees are also still small, because they were planted recently.

If you’d planted all her extra saplings in the first year instead, they’d have had ten years to grow and your orchard would be bigger still. She couldn’t, because she only earned the money for them later. That’s fine — it just means her bigger orchard is about planting more, not growing faster.

The SIP and XIRR calculator’s second tab works out the XIRR of any list of dated cash flows. Enter a step-up plan’s instalments as outflows and its final value as the last inflow, and it gives you that plan’s XIRR.

The thread through these posts

A step-up SIP is one more product whose name describes the feature and hides the arithmetic — the same pattern as the other posts around this one. A “balanced” fund is a mix, not a promise; an arbitrage fund’s equity is hedged; an international fund carries a currency and a limit; a ₹10 NFO (new fund offer) is a unit size; the riskometer scores ingredients; a fund can drift without changing its name; a side pocket decides who owns a loss. In every case the label is the marketing, and the mechanism is what happens to your money.

Common mistakes

  • Comparing a step-up SIP’s corpus with a flat SIP’s without showing the money invested. The corpus gap is mostly a contribution gap. Put the “invested” column next to the “value” column, always.
  • Expecting a step-up to raise your return. On a constant return it changes nothing; on real data it moved XIRR up in some windows and down in others.
  • Setting a step-up your income can’t keep up with. With a 10% step-up, the instalment in year ten is about 2.4 times the first year’s, and in year twenty about 6.1 times. If you have to cancel it in year eight, the plan was the wrong size.
  • Forgetting that the late, large instalments carry the most timing risk. A fall near the end of the plan hits the biggest sums invested for the shortest time — the same sequence problem withdrawals face, in reverse.

Sources

Takeaway: A step-up SIP builds a bigger corpus because it invests more money, not because it earns a better return — on real index-fund data its XIRR was sometimes higher and sometimes lower than a flat SIP’s. It’s the right tool for matching savings to a rising income, as long as you judge it by what you put in as well as what you got out.