SIP Explained: How Monthly Investing Works (With Numbers)

A systematic investment plan, or SIP, is one of the simplest ideas in personal finance: invest a fixed amount every month, automatically, for a long time. It is not a product with special powers. It is a habit, and the numbers behind it show why habits beat good intentions. This guide explains how a SIP works, how its future value is calculated, and what to check before you start.

What a SIP is

With a SIP you commit to investing the same amount at regular intervals, most often monthly, into an investment such as a mutual fund. The money is taken automatically from your bank account. You do not choose when to buy. You buy every month whether the market is up or down.

The word SIP is common with mutual funds in India and Pakistan, but the same method is called regular investing or dollar-cost averaging elsewhere. You can do it with index funds, exchange-traded funds or any investment you can buy in small amounts.

Why it works: buying at different prices

When you invest a fixed amount, you automatically buy more units when the price is low and fewer when it is high. Suppose you invest 5,000 a month and the unit price is 100, then 80, then 125:

• Month 1: 5,000 buys 50 units
• Month 2: 5,000 buys 62.5 units
• Month 3: 5,000 buys 40 units

You invested 15,000 and hold 152.5 units, an average cost of 98.36 per unit. The simple average of the three prices is 101.67, so your average cost is lower than the average price. This is sometimes called rupee-cost or dollar-cost averaging. It does not guarantee a profit or protect you from losses, but it smooths out the price you pay and removes the pressure of timing the market.

How the future value is calculated

The calculation treats each monthly payment as growing from the month it is invested. The formula for payments made at the start of each month is M = P × ((1 + i)^n − 1) ÷ i × (1 + i), where P is the monthly amount, i is the monthly return (the yearly return divided by 12) and n is the number of months.

For 5,000 a month at 12 percent a year:

• 10 years: 6,00,000 invested, about 11,61,695 in value
• 15 years: 9,00,000 invested, about 25,22,880 in value
• 20 years: 12,00,000 invested, about 49,95,740 in value

The amount invested grows in a straight line, but the value grows in a curve. The last five years add almost as much as the first fifteen combined.

Time beats amount

Two investors show the point clearly. One invests 10,000 a month for 10 years and ends with about 23,23,391. The other invests 2,500 a month for 20 years and ends with about 24,97,870, having invested only 6,00,000 against 12,00,000. Investing a quarter of the amount for twice as long produced more. Starting early and staying invested matter more than the size of the monthly amount.

Step-up SIP

A step-up SIP raises your monthly amount by a fixed percentage every year, so it grows with your income. Starting at 5,000 and adding 10 percent each year for ten years at 12 percent, you invest about 9,56,245 and end with about 16,87,163. The flat 5,000 SIP ends with 11,61,695. Raising the amount by 10 percent a year is a small change each year but a large difference at the end.

Choose a step-up you can keep. A steady 5 or 10 percent is better than an ambitious number you drop after two years.

Choosing a realistic return

The expected return is the input that changes the result most, and nobody knows it in advance. At 8 percent, 5,000 a month for 10 years grows to about 9,20,828. At 10 percent, about 10,32,760. At 12 percent, about 11,61,695. The gap between 8 and 12 percent is almost 2,40,000.

Plan with a cautious rate and treat higher returns as a bonus. Remember that fund fees and taxes reduce what you keep, and that real returns are uneven: there can be years of losses.

Things to check before you start

1. Your goal and time horizon. Money needed within a few years should not depend on the stock market.
2. Costs. Look at the fund's expense ratio and any exit load. A difference of 1 percent a year adds up over decades.
3. An emergency fund first. Keep some months of expenses in savings so you are not forced to sell during a downturn.
4. The risk you can live with. A fund that falls 30 percent is only a problem if you sell. Choose one you can hold.
5. Automation. Set the SIP date just after your salary arrives.

Try your own numbers

The SIP Calculator shows the invested amount, returns and final value for any monthly amount, return and period, with an optional yearly step-up, a chart and a year-by-year table. For lump sums or a starting balance, use the Compound Interest Calculator. Compare with a guaranteed option using the FD Calculator, check the real growth of what you already hold with the CAGR Calculator, and see what the final amount will buy with the Inflation Calculator. For the ideas behind all of them, read how compound interest works.