In this guide
This guide covers what a SIP is, what happens to each instalment, how money accumulates over time, a worked example, the assumptions behind any SIP estimate, and the mistakes beginners most often make.
- What SIP means
- How a SIP works, step by step
- How monthly investments accumulate
- The role of time and compounding
- A simple numerical example
- Assumptions and common mistakes
What SIP means
SIP stands for Systematic Investment Plan. It is not a product in itself — it is a method of investing in a mutual fund. Instead of investing one large sum, you instruct the fund house to take a fixed amount from your bank account on a chosen date every month (or week, or quarter) and invest it in the scheme you picked.
Because the amount is fixed and automatic, a SIP turns investing into a habit rather than a decision you have to make every month. You can usually start with a small amount, pause, increase or stop the SIP, subject to the scheme's terms.
How a SIP works, step by step
Every instalment buys units of the fund at that day's NAV (net asset value — the price of one unit). When the NAV is low, your fixed amount buys more units; when it is high, it buys fewer. This is often called rupee cost averaging: you do not need to guess the right day to invest, because you invest on every scheduled day regardless.
The value of your investment on any day is simply the number of units you hold multiplied by the current NAV. If the fund's holdings rise in value, the NAV rises and so does your total. If they fall, your total falls too — including below what you put in.
- Month 1: your instalment buys units at that month's NAV.
- Month 2 onwards: each new instalment adds more units at the new NAV.
- Your units accumulate; their value moves with the NAV.
- Any gains in the fund stay invested and can grow further.
How monthly investments accumulate — the role of time
A SIP grows from two sources: the money you keep adding, and the growth on money already invested. In the early years, almost all of your balance is your own contributions. Over longer periods, growth earned on earlier growth — compounding — can become the larger part.
That is why the length of time invested tends to matter more than the size of any single instalment. Each instalment has a different amount of time to grow: your first instalment compounds for the full period, your last for only a month.
A simple numerical example
Suppose you invest ₹10,000 every month and, purely for illustration, assume a constant 12% annual return. This is a hypothetical assumption, not a prediction — real market returns vary from year to year and can be negative.
| Period | Total invested | Estimated value | Estimated gain |
|---|---|---|---|
| 5 years | ₹6,00,000 | ₹8,24,864 | ₹2,24,864 |
| 10 years | ₹12,00,000 | ₹23,23,391 | ₹11,23,391 |
| 20 years | ₹24,00,000 | ₹99,91,479 | ₹75,91,479 |
What the example shows
Doubling the period from 10 to 20 years doubles what you invest, but the estimated value more than quadruples. That gap is compounding at work, under the assumption that returns stay constant. It is the main reason long holding periods are emphasised for SIP investing.
If you expect your income to rise, a step-up SIP — increasing the instalment by a fixed percentage every year — lets the plan grow with you. The Step-Up SIP Calculator shows that effect. To judge how a fund has actually grown in the past, the CAGR Calculator converts a start and end value into an annual growth rate.
Important assumptions behind any SIP estimate
SIP calculators, including CalPaisa's, use a fixed assumed rate to keep the maths readable. Real investing differs in ways worth knowing before relying on any figure.
- Returns are assumed constant; real equity returns move up and down.
- Expense ratio, exit loads and taxes are not deducted.
- Inflation is ignored, so the future value is in future rupees.
- Instalments are assumed to be paid on time every month.
Common beginner mistakes
Most SIP problems come from behaviour rather than arithmetic.
- Treating an estimated return as a guaranteed one.
- Stopping the SIP when markets fall — the period when instalments buy the most units.
- Choosing a fund only on its most recent returns.
- Starting too many SIPs in overlapping funds.
- Not reviewing the plan as income and goals change.
Conclusion
A SIP is a disciplined way to invest a fixed amount regularly, average out your purchase price and give compounding time to work. Use estimates to plan, keep the assumptions in view, and remember that market-linked returns can be higher or lower than any projection.