A SIP, or Systematic Investment Plan, is a way of investing a fixed amount in a mutual fund at regular intervals - usually a set sum every month - rather than putting in one lump sum. It is the most popular way Indians invest in equity mutual funds, because it turns investing into a disciplined habit and smooths out the ups and downs of the market. This guide explains how a SIP grows your money, the formula behind the returns, and the two forces that do the heavy lifting: rupee-cost averaging and compounding.
How a SIP actually works
Each month a fixed amount is automatically debited from your bank account and used to buy units of a mutual fund at that day's price (the net asset value, or NAV). When the market is down, your fixed amount buys more units; when it is up, it buys fewer. Over time you accumulate units bought at many different prices, and the value of your holding is the total units multiplied by the current NAV.
The SIP return formula
The future value of a SIP is the future value of a series of regular investments, each compounding for the time it stays invested:
FV = P x [((1 + i)^n - 1) / i] x (1 + i)
Where:
- P is the monthly investment amount.
- i is the monthly rate of return - the expected annual return divided by 12. A 12% annual return is 0.12 / 12 = 0.01 per month.
- n is the total number of monthly instalments. Ten years is 10 x 12 = 120 instalments.
The final (1 + i) reflects investing at the start of each month. You never need to compute this by hand - the SIP Calculator does it instantly - but seeing the formula makes clear why time matters so much.
A worked example
Suppose you invest 10,000 a month for 10 years at an expected annual return of 12%.
- P = 10,000
- i = 0.12 / 12 = 0.01
- n = 10 x 12 = 120
Putting these into the formula gives a maturity value of about 23,23,000. Over those ten years you actually invested 10,000 x 120 = 12,00,000 of your own money - so roughly 11,23,000 of the final amount is growth. More than half the ending value came from returns rather than your contributions, and that gap widens dramatically the longer you stay invested.
Rupee-cost averaging
Because you invest the same amount every month regardless of price, you automatically buy more units when the market is low and fewer when it is high. This is called rupee-cost averaging, and it pulls your average purchase price below the simple average of the prices you saw. It also removes the impossible task of 'timing the market' - you no longer have to guess whether today is a good day to invest, because you are investing on a schedule through every phase of the market.
The power of compounding
The real engine behind a SIP is compounding: your returns earn returns of their own. The units you buy early have the longest time to grow, so they contribute far more to the final value than the units you buy near the end. This is why starting early matters more than investing large amounts.
- 10,000 a month at 12% for 10 years grows to about 23.2 lakh (invested 12 lakh).
- The same 10,000 a month for 20 years grows to about 99.9 lakh (invested 24 lakh).
- For 30 years it grows to about 3.5 crore (invested 36 lakh).
Doubling the time from 10 to 20 years far more than doubles the result, and tripling it to 30 years multiplies the maturity value many times over - even though your monthly outgo never changed. That accelerating curve is compounding at work, and it is the single strongest argument for starting a SIP as early as you can.
Are SIP returns guaranteed?
No. SIPs in equity mutual funds are market-linked, so the actual return varies year to year and can be negative in a bad year. The percentage you enter in a calculator is an expected average, not a promise - long-run equity averages are often used as a guide, but the real path is bumpy. A SIP reduces the risk of investing everything at a market peak, but it does not remove market risk altogether. Treat the projected figure as a reasonable estimate for planning, not a guaranteed outcome.
SIP vs lump sum
A lump-sum investment puts all your money to work immediately, which wins when the market rises steadily from the day you invest. A SIP spreads entry across many months, which protects you when prices are volatile or falling and is far easier to sustain from a monthly salary. For most salaried investors a SIP fits cash flow naturally and removes the pressure to find the perfect entry point - and you can always add an occasional lump sum on top when you have spare funds.
The best way to feel how powerful regular investing is over time is to try different amounts and durations: change the monthly figure, the return and the number of years in the SIP Calculator and watch how much of the maturity value comes from growth rather than your own contributions.
Frequently asked questions
- How is SIP return calculated?
- A SIP uses the future value of a regular investment series: FV = P x [((1 + i)^n - 1) / i] x (1 + i), where P is the monthly amount, i is the monthly return (annual rate divided by 12) and n is the number of instalments. Each instalment compounds for the time it stays invested.
- Is a SIP return guaranteed?
- No. SIPs in equity mutual funds are market-linked, so returns vary year to year and can be negative in a bad year. The rate you enter in a calculator is an expected estimate for planning, not a guaranteed outcome.
- Why does starting a SIP early make such a big difference?
- Because of compounding - your returns earn further returns. Units bought early have the longest time to grow, so a SIP started earlier can end up far larger than one started later, even with the same monthly amount.