SIP and Compounding: What Regular Investing Looks Like Over Time

In a SIP you invest a fixed amount every month (or week, or quarter), no matter what the market is doing. The term is widely used for mutual-fund plans, but the maths applies to any regular contribution to any account that earns a return. Because you buy at many different prices, you do not have to pick a single entry date.

It is important to separate the plan from the return. The plan is the discipline; the return is whatever the investments actually earn, which is uncertain and can be negative in some years. Calculators therefore take an assumed constant annual rate. That assumption is a modelling choice, not a forecast.

With a monthly contribution M, an annual rate converted to a monthly rate r (annual ÷ 12), and n monthly contributions, the future value when each deposit is made at the start of the month is:

FV = M × ((1 + r)^n − 1) ÷ r × (1 + r)

If deposits are made at the end of each month, drop the final × (1 + r). The difference is small — about one month of growth — but the two conventions explain why calculators sometimes disagree by a fraction of a percent. Check which one a tool uses.

Take r = 0.07 ÷ 12 and deposits at the start of each month.

Doubling the time from 10 to 20 years multiplies the contributions by 2 but the final value by about 3.0. Going from 20 to 30 years multiplies contributions by 1.5 but the final value by about 2.3. Most of the final balance in the 30-year case is growth, not deposits, and most of that growth arrives in the last decade.

Compare three plans that each end after a different period but invest similar totals of cash early or late:

• $200 a month for 30 years: about $245,417.
• $400 a month for 20 years (twice the deposit, 10 fewer years): about $209,586.
• $600 a month for 10 years (three times the deposit, 20 fewer years): about $104,457.

The person who invests the least per month but starts earliest ends up with the most. This is the practical meaning of compounding: returns earn returns, so the earliest contributions have the longest time to grow. It does not mean starting late is pointless — it means the required monthly amount rises sharply the later you begin.

• Fees. An annual expense ratio or platform fee reduces the effective return. A 1% fee turns a 7% gross return into roughly 6% net; the calculator should be run with the net rate.
• Taxes. Tax treatment varies by country and account type.
• Inflation. Future dollars buy less than today’s dollars; see the inflation guide for real returns.
• Volatility. A constant 7% is a smooth line. Real returns swing, and the order of good and bad years matters for the final balance, especially when you are withdrawing.
• Contribution changes. Many people raise their SIP as income grows; model that as separate runs.

Enter a monthly amount, an annual return and a number of years. Try three rates — a cautious one, a middle one and an optimistic one — to see the range. Then reduce the rate by your expected fees. The invested-versus-gains chart makes it easy to see how much of the end value is your own money.

If you invest a lump sum on top of a SIP, use the Compound Interest Calculator for the lump sum and add the two results. For a quick horizon check, the Rule of 72 estimates doubling time: at 7% roughly 72 ÷ 7 ≈ 10.3 years.

• Treating the assumed return as guaranteed.
• Using a gross return and forgetting fees.
• Mixing monthly and annual rates: enter 7 for 7% a year, not 0.58.
• Comparing a SIP’s future value to today’s prices without adjusting for inflation.
• Pausing contributions in down markets, which removes the part of the plan that buys more units when prices are lower.