How a ₹15,000 SIP is calculated
A monthly SIP's value is FV = P × [((1 + i)^n − 1) ÷ i] × (1 + i). P is the instalment, n the number of months and i the monthly rate: the expected annual return divided by 12, then by 100. The final (1 + i) is there because each instalment goes in at the start of its month and earns that month's return. The SIP calculator on this site uses the same convention.
For ₹15,000 at 12% for 10 years, i = 0.01 and n = 120. (1 + i)^120 is about 3.300387, so FV = 15,000 × (2.300387 ÷ 0.01) × 1.01 ≈ ₹34,85,086. You put in ₹18,00,000, so the estimated gain is ₹16,85,086.
Time against the rate of return
Doubling the period from 10 to 20 years doubles what you put in, from ₹18,00,000 to ₹36,00,000. At 12% the value rises from ₹34,85,086 to ₹1,49,87,219, about 4.3 times as much, because the early instalments compound for longer.
The assumed return matters too. Over 20 years, ₹15,000 a month reaches ₹88,94,208 at 8%, ₹1,49,87,219 at 12% and ₹2,27,39,325 at 15%. A few percentage points a year make a large difference by the end.
These returns are illustrative
The return columns are assumptions, not forecasts. Mutual fund returns are not guaranteed and are subject to market risk. Equity funds can fall for long stretches, and a real SIP earns a different return each year rather than a steady rate, so the actual value can end up well above or below these figures.
All figures are before tax. Gains are taxed when you redeem units: equity funds attract capital gains tax, with long-term gains taxed differently from short-term ones, and debt funds follow other rules. What you keep will be lower than shown.
Inflation also cuts what the money buys. At 6% inflation, the ₹34,85,086 from 10 years at 12% would buy roughly what ₹19,46,054 buys today. The calculator above shows this figure for your own inputs.