Investing
How SIP returns are actually calculated
Every SIP calculator shows a single, smooth number at the end. Real mutual fund investing is never smooth. Here is the actual formula behind that number, why the assumed return does almost all the work, and what happens to the math when you start earlier or invest more.
By Aman Multani, Founding Partner & Editor · Published 4 September 2026 · Last verified 4 September 2026
The formula
A SIP calculator projects the future value of a fixed monthly investment, compounded at an assumed annual rate:
where P is the monthly investment, i is the assumed monthly return (annual rate ÷ 12), and n is the number of months. The final (1 + i) accounts for each instalment earning a return from the start of its month.
This is a standard future-value-of-an-annuity calculation, the same category of formula used for EMIs run in reverse — instead of paying down a lump sum, you are building one up. Run it with your own numbers on the SIP calculator.
A worked example
₹5,000 invested every month, at an assumed 12% annual return, for 20 years:
| Amount | |
|---|---|
| Total invested | ₹12,00,000 |
| Estimated growth | ₹37,95,740 |
| Maturity value | ₹49,95,740 |
Notice that the estimated growth is over three times the amount actually invested. That is not a quirk of this example — it is what compounding does over long periods, and it is also exactly why the assumed return matters so much, which is the next section.
The assumed return is doing almost all the work
12% is a commonly used illustrative figure for long-term equity mutual fund SIPs in India — not a promise, not an average anyone can guarantee, and not a figure this calculator or any other can verify in advance. Small changes in that one assumption move the final number dramatically over a long horizon:
| Assumed return | Maturity value (₹5,000/mo, 20yr) |
|---|---|
| 8% | ₹29,64,736 |
| 10% | ₹38,28,485 |
| 12% | ₹49,95,740 |
| 14% | ₹65,81,731 |
What time actually buys you
Holding the monthly amount and assumed return fixed at ₹5,000 and 12%, look at what changes as the horizon extends:
| Years | Invested | Maturity value | Estimated growth |
|---|---|---|---|
| 5 | ₹3,00,000 | ₹4,12,432 | ₹1,12,432 |
| 10 | ₹6,00,000 | ₹11,61,695 | ₹5,61,695 |
| 15 | ₹9,00,000 | ₹25,22,880 | ₹16,22,880 |
| 20 | ₹12,00,000 | ₹49,95,740 | ₹37,95,740 |
| 25 | ₹15,00,000 | ₹94,88,175 | ₹79,88,175 |
| 30 | ₹18,00,000 | ₹1,76,49,569 | ₹1,58,49,569 |
The last five years in this table (25 to 30) add roughly ₹81.6 lakh to the maturity value — more than the entire 20-year figure above it. That is compounding acting on an already-large base, and it is the mathematical reason "start earlier" is the single most repeated piece of SIP advice: the earliest contributions have the most months left to compound, so delaying the start costs disproportionately more than delaying any other single year.
Step-up SIPs: contributing more as you earn more
A step-up (or top-up) SIP increases your monthly contribution by a fixed percentage each year, typically matching expected income growth. Same ₹5,000 starting point and 12% assumed return, stepped up 10% every year for 20 years:
| Flat ₹5,000/mo | Step-up, +10%/year | |
|---|---|---|
| Total invested | ₹12,00,000 | ₹34,36,500 |
| Maturity value | ₹49,95,740 | ₹99,44,358 |
The step-up version invests roughly 2.9 times as much in total and ends with roughly double the maturity value — a larger outcome, but from genuinely committing more money over time, not a mathematical trick. Whether that rising contribution is realistic depends entirely on whether your income actually grows to support it.
Common questions
Is 12% a guaranteed or typical SIP return?
Neither, strictly. It is a commonly used illustrative assumption for long-term equity mutual fund SIPs in India, based on broad historical index performance over long periods. No mutual fund can guarantee a return, past performance does not predict future results, and actual returns depend on the specific fund, market conditions and timing.
Why does a SIP calculator show a smooth curve when markets are volatile?
Because it assumes a fixed annual return compounded evenly every month, which is a simplification for estimation purposes. Real markets move up and down unevenly, and two SIPs with the same average return over the same period can end with different maturity values depending on the sequence of returns.
Does starting a SIP 5 years earlier really make that much difference?
Yes, because compounding is exponential, not linear. At the same ₹5,000 a month and 12% assumed return, the last 5 years alone (from year 25 to year 30) add more to the maturity value than the entire first 15 years of contributions, purely from compounding on the already-accumulated amount.
Is a step-up SIP always better than a flat SIP?
It produces a larger corpus for a larger total amount invested, since you are contributing more in later years. Whether it is "better" depends on whether your income can genuinely support the rising contribution — it is a trade-off, not a free upgrade.
Should I stop my SIP when the market falls?
Stopping contributions during a fall means missing units bought at a lower price, which is when rupee-cost averaging works most in your favour. This is general market behaviour, not personalised advice — consider your own situation and consult a qualified adviser for a decision specific to you.
If you are weighing a SIP against guaranteed-return options, run the same monthly amount through the RD calculator, and a lump sum through the FD calculator, to see how much of the difference is the assumed return doing the work.
Educational information only, not financial advice. Found an error? Tell us and we will correct it with a dated note.