A systematic investment plan is the easiest way to invest a fixed amount every month without timing the market, yet most people judge it by the wrong number. A fund's projected rate is not what lands in your account. The amount you invested, the years you stayed in, and the tax you pay on the gain are all part of the real figure.
The SIP formula
Every SIP calculator, including the mutual fund industry's own worksheets, uses the same compound annuity:
Future value = P × [((1 + r)n − 1) / r] × (1 + r)
- P = monthly instalment
- r = monthly rate of return (annual rate divided by 12)
- n = number of months
The final (1 + r) factor matters. It reflects that the first instalment is invested for the full n months while the last one earns interest for a single month. Get that term wrong and your projection drifts low.
A worked example: Rs 5,000 a month for 10 years
At a 12% annual return, Rs 5,000 every month for 10 years grows to about Rs 11,61,695. You put in Rs 6,00,000, so the gain is Rs 5,61,695. Nearly half the final amount is not your money, which is the whole argument for staying invested.
Now compare that with putting the same total in on day one. Rs 6,00,000 as a lump sum at 12% for 10 years becomes Rs 18,63,509. The lump sum wins by about Rs 7 lakh, and the reason is timing, not cleverness: a lump sum spends ten years in the market, while the average SIP instalment spends five.
This is why lump sum beats SIP on paper and SIP still wins in practice. Nobody can reliably invest a large sum on the right day. Averaging in removes that risk, so the gap narrows considerably once you account for the anxiety-driven timing mistakes people actually make.
Why starting early beats investing more
The table below holds the return at 12% and changes only the number of years you keep investing Rs 5,000 a month.
| Years | Total invested | Estimated value | Growth |
|---|---|---|---|
| 5 | Rs 3,00,000 | Rs 4,12,432 | Rs 1,12,432 |
| 10 | Rs 6,00,000 | Rs 11,61,695 | Rs 5,61,695 |
| 15 | Rs 9,00,000 | Rs 25,22,880 | Rs 16,22,880 |
| 20 | Rs 12,00,000 | Rs 49,95,740 | Rs 37,95,740 |
| 25 | Rs 15,00,000 | Rs 94,88,175 | Rs 79,88,175 |
| 30 | Rs 18,00,000 | Rs 1,76,49,569 | Rs 1,58,49,569 |
Going from 10 to 15 years at the same instalment more than doubles the outcome, and the 30-year row is where compounding stops being a detail and starts being the whole story. Ten extra years is worth far more than the extra contributions you could make by waiting to save a bigger amount.
The same effect shows up across instalments. At 12% for 10 years:
| Monthly SIP | Total invested | Estimated value | Growth |
|---|---|---|---|
| Rs 1,000 | Rs 1,20,000 | Rs 2,32,339 | Rs 1,12,339 |
| Rs 2,000 | Rs 2,40,000 | Rs 4,64,678 | Rs 2,24,678 |
| Rs 3,000 | Rs 3,60,000 | Rs 6,97,017 | Rs 3,37,017 |
| Rs 5,000 | Rs 6,00,000 | Rs 11,61,695 | Rs 5,61,695 |
| Rs 7,500 | Rs 9,00,000 | Rs 17,42,543 | Rs 8,42,543 |
| Rs 10,000 | Rs 12,00,000 | Rs 23,23,391 | Rs 11,23,391 |
| Rs 15,000 | Rs 18,00,000 | Rs 34,85,086 | Rs 16,85,086 |
| Rs 25,000 | Rs 30,00,000 | Rs 58,08,477 | Rs 28,08,477 |
At 12% your money roughly doubles every six years, so an instalment you can sustain beats a larger one you abandon. Rs 3,000 held for 30 years ends up further ahead than Rs 10,000 held for three.
How a step-up SIP changes the outcome
A step-up SIP raises your instalment by a fixed percentage every year, usually 5% or 10%. The extra contributions are small in year one and large by year ten, and the effect on the final number is substantial.
- No step-up: final value about Rs 11,61,695 on Rs 6,00,000 invested
- 5% annual step-up: about Rs 14,63,145 on Rs 7,92,407 invested
- 10% annual step-up: about Rs 18,55,879 on Rs 10,51,870 invested
A 10% step-up roughly adds Rs 6.9 lakh to the outcome. It also requires a real income rise, so it works best for someone whose salary is growing faster than that and who would otherwise spend the difference.
Adjust for inflation and tax before you decide
A 12% projected return is a nominal number, and prices are not standing still. With 6% inflation, the real annual return is 5.66%, not 12%, because the two compound together: (1.12 / 1.06) − 1.
In today's money, that Rs 11,61,695 projection is worth roughly Rs 8,08,263. The nominal figure looks like nearly twelve lakh; the amount that will actually buy is closer to eight.
On the tax side, equity mutual fund gains above Rs 1.25 lakh a year are taxed at 12.5% under the current rules, with no indexation benefit. On the example above, that Rs 1.25 lakh annual exemption leaves Rs 4,36,695 taxable, and 12.5% of it is about Rs 54,587 - taking the corpus from Rs 11,61,695 to roughly Rs 11,07,108. Debt fund gains are added to your income instead, and are taxed at your slab rate. Rules change, so confirm the current position before acting on it.
Note that the Rs 1.5 lakh ELSS limit under section 80C applies to the amount you put in, not the market value. Rs 1,50,000 invested for 15 years at 12% would grow to roughly Rs 8,21,035, but only the deposits count toward that limit each year.
What return should you assume?
Be conservative. Long-term equity mutual funds in India have delivered something in the region of 11-13% annualised over long horizons, but no individual 10-year period is guaranteed to land there. Using 10-11% gives a projection you are less likely to be disappointed by, and a calculator makes it easy to test several rates side by side.
Run your own figures with the free SIP Calculator - it shows the year-by-year invested-versus-value table so you can see exactly when gains start outrunning contributions. If you have a lump sum already, compare it against monthly investing using the Compound Interest Calculator, and check a debt alternative such as an FD before you commit.