By the Tooliday Team. Last updated 14 November 2026.
Most SIP articles start with the monthly amount and show what it could grow to. In real life it often works the other way round: you have a goal, such as a down payment or a child’s course fee, and want to know how much to invest each month. This guide shows the reverse calculation. If you want the forward version first, read How a SIP Works, and test the numbers in the SIP Calculator.
The formula, rearranged
The future value of a monthly investment P at the end of each month is FV = P × ((1 + i)^n − 1) ÷ i. To find P for a target, rearrange it:
P = Target × i ÷ ((1 + i)^n − 1)
Here i is the monthly rate (yearly rate ÷ 12 ÷ 100) and n is the number of months.
Example: ₹25 lakh in 15 years
For 15 years, n = 180. The monthly amount needed depends heavily on the return you assume:
| Assumed yearly return | Monthly SIP needed |
|---|---|
| 8% | about ₹7,225 |
| 10% | about ₹6,032 |
| 12% | about ₹5,004 |
At 10%, you would invest about ₹6,032 × 180 = ₹10,85,723 of your own money, and the remaining roughly ₹14,14,277 of the target would come from assumed growth.
How to use this sensibly
- Plan with the cautious number. The gap between 8% and 12% is more than ₹2,000 a month. Because returns are not guaranteed, plan with a lower assumption and treat anything above it as a bonus.
- Adjust for inflation. ₹25 lakh in 15 years will buy less than ₹25 lakh today. If your goal has a cost that rises each year, such as education, set the target higher.
- Match the investment to the time frame. Short goals, a year or two away, usually need safer options than market-linked ones.
- Review every year. If you are behind, you can raise the SIP amount or extend the period. Even a small yearly increase helps.
Try your own target, years and a cautious rate in the SIP Calculator and compare a few scenarios.
This article is educational and not investment advice. Investments are subject to market risk and returns are not guaranteed. Please consult a qualified financial professional before investing.