SIP compounding formula:
how monthly investing grows

What is the SIP compounding formula?

The SIP compounding formula is future value = P x ((1 + r)^n - 1) / r x (1 + r), where P is the monthly investment, r the monthly rate and n the number of months. Rs 5,000 a month at 12% for 10 years grows to about Rs 11,61,695.

It treats each instalment as made at the start of the month and compounds every one separately. The result is an estimate that assumes a steady return; real returns vary and are not guaranteed, and this is not financial advice.

This is the same expression the SIP future value calculator uses, so you can follow every step by hand or check your spreadsheet against it.

The formula and its inputs

Reading the result: of the Rs 11,61,695, only Rs 6,00,000 is money you paid in. The rest, about Rs 5,61,695, is growth from compounding, and most of it arrives in the final years.

Growth over time: Rs 5,000 a month at 12%

Invested money rises in a straight line, but the value curves upward. Between years 15 and 20 the value grows by more than Rs 24 lakh on only Rs 3 lakh of new contributions.

The table is useful for setting goals: if you need roughly Rs 25 lakh in 15 years and your assumed return is 12%, Rs 5,000 a month gets you close, while a lower assumed return would need a larger monthly amount.

How the return assumption changes the result

For Rs 5,000 a month over 10 years, 10% a year gives about Rs 10,32,760, 12% gives about Rs 11,61,695, and 14% gives about Rs 13,10,457. A two-point change in return moves the outcome by over Rs 1.2 lakh, so test several rates.

Time matters more than rate. Rs 10,000 a month at 10% for 5 years reaches about Rs 7,80,824, while Rs 5,000 a month at 12% for 20 years reaches almost Rs 50 lakh.

A shorter plan shows the same pattern. Rs 2,000 a month at 8% for 24 months comes to about Rs 52,212 against Rs 48,000 paid in, a small gain because there is little time to compound.

Common mistakes with the SIP formula

• Entering the annual rate as a decimal. The input is a percentage, so type 12, not 0.12.
• Counting years instead of months. Ten years is 120 instalments.
• Treating the estimate as a promise. Market-linked returns move up and down.
• Forgetting exit charges, taxes and expense ratios, which the formula leaves out.
• Using the end-of-month version of the formula and getting a slightly lower number than this one.

The tip above is worth checking in practice. Investing the whole Rs 6,00,000 on day one at the same 1% a month would reach about Rs 19,80,232 after 120 months, far more than the SIP, because every rupee would have the full ten years to grow. A SIP builds the habit and spreads timing risk; it does not beat a lump sum on paper.