What Is a SIP? How Systematic Investment Plans Work

A SIP (systematic investment plan) is a standing instruction that moves a fixed amount from your bank account into a mutual fund scheme on a fixed date, usually every month. Each instalment buys as many units as that day's NAV (net asset value per unit) allows, so you build a holding gradually instead of making one large purchase. A SIP is a way of buying a fund, not a type of fund, and it neither produces nor guarantees any return by itself.

How a SIP works, step by step

1. Choose a scheme and complete the investor verification (KYC) that the fund house requires.
2. Set the instalment amount, the debit date and how long the SIP should run. Minimum instalments differ by scheme, so check the scheme's offer document.
3. Sign a bank mandate so the instalment can be debited automatically.
4. On each debit date, units purchased = instalment divided by that day's NAV. A Rs. 5,000 instalment at a NAV of Rs. 50 buys 100 units.
5. When you redeem, you receive the NAV on the redemption date multiplied by the units you hold, less any exit load or taxes that apply to that scheme.

Worked example: what Rs. 5,000 a month can become

The table below assumes 12% a year, applied as 1% per month, with each instalment invested at the start of the month. This is the same method the SIP calculator uses: the future value is the instalment times ((1.01 to the power n, minus 1) divided by 0.01) times 1.01, where n is the number of months. For 10 years (n = 120) that gives Rs. 11,61,695, and a month-by-month loop that adds Rs. 5,000 and then grows the balance by 1% reproduces the same figure. The 12% is an assumption chosen for illustration, not a forecast.

Notice the shape. In the first year the gain is under Rs. 5,000, because little money has been invested for long. By year 20 the projected gain is more than three times everything you paid in. A plan stopped at year 10 would hold Rs. 11,61,695, while the same instalments continued to year 20 add far more in the second decade than in the first. Time in the market matters more than the size of any single instalment.

Worked example: how units are bought

To see why a SIP is described as averaging your cost, take six monthly instalments of Rs. 5,000 into a fund whose NAV moves through hypothetical values and ends where it started. These NAVs are invented to show the arithmetic, not data from any fund.

You invested Rs. 30,000 and hold 703.97 units. The simple average of the six NAVs is Rs. 43.33, but your average cost per unit is Rs. 42.62, because the fixed amount bought more units when the NAV was low. With the NAV back at Rs. 50, the holding is worth Rs. 35,198, a gain of Rs. 5,198 even though the price only returned to its starting point. Mathematically, a fixed rupee amount always gives an average cost at or below the simple average of the NAVs. The benefit is real but modest, and it works only if you keep investing through the dips.

Common mistakes and limits of a SIP

• Treating a SIP as safe. If the underlying fund falls and stays down, a SIP into it falls too. Averaging reduces timing risk, not market risk.
• Stopping when markets fall. Falling NAVs are when each instalment buys the most units; pausing then removes the very effect that averaging relies on.
• Picking a fund only on last year's return. One strong year says little about the next ten.
• Starting late because the amount feels too small. In the example, the same Rs. 5,000 for 15 years reaches Rs. 25,22,880, while investing for only 10 years reaches Rs. 11,61,695, a gap of about Rs. 13.6 lakh.
• Ignoring scheme terms. Exit loads, lock-ins and tax treatment differ by scheme type and change over time, so read the current scheme document and tax rules before you commit.

Where to go next

Decide the amount you can keep paying through a bad year, not the amount you could pay in a good one. Then run it through the calculator at several return assumptions (for example 8%, 10% and 12%) so you see a range instead of a single hopeful number.