SIP Calculator Guide: How Monthly Investing and Compounding Work

Start with the contribution and the time period

A monthly contribution of ₹1,000 for 12 months means total contributions of ₹12,000. For 24 months it means ₹24,000. Those totals do not need a return forecast: they follow from the schedule, provided every contribution is made. A missed payment, extra deposit or later increase in the amount changes that schedule.

The investment duration tells the model how many contribution periods to include. Twelve years and twelve months are not interchangeable. If the input is in years and the payments are monthly, multiply years by 12. A model with a constant contribution does not automatically include a yearly step-up in that contribution.

Real contributions purchase an investment at the prices available on the transaction dates. A fund’s units, prices and charges affect the eventual result. The AMFI description of SIPs identifies regular investing as a method and notes that cost averaging does not assure a profit or protect against losses.

An expected annual return is an assumption

For the worked model, use an assumed annual nominal return of 12%. Dividing by 12 gives a monthly rate of 1%, or 0.01 as a decimal. This is the convention used by ToolMyth’s SIP Calculator. It is not the same as converting a 12% effective annual return into an equivalent monthly rate; that alternative would use (1.12)^(1/12) − 1.

At a constant 1% monthly rate, a balance that remains invested for 12 months grows by a factor of 1.01^12, about 1.1268. This distinction explains why a calculator’s “annual return” label needs a stated convention. Two calculators can produce different answers simply because their input rates have different meanings.

Compounding applies to each contribution separately

Compounding means later growth is calculated on a balance that already includes earlier growth. If ₹1,000 stays invested for two monthly periods at 1%, it becomes ₹1,010 after one period and ₹1,020.10 after two. The second period adds ₹10.10 because its starting balance is larger.

A monthly plan does not put every contribution to work for the same length of time. The first payment has more periods to grow than the last. Treating all 12 contributions as one ₹12,000 deposit made on day one would overstate the modeled growth of a plan that actually deposits ₹1,000 each month.

The monthly future-value formula

FV = M × [((1 + r)^n − 1) / r] × (1 + r)
M = monthly contribution
r = annual nominal return ÷ 12 ÷ 100
n = number of monthly contributions

This expression assumes contributions at the beginning of each month, followed by that month’s growth. The extra (1 + r) gives every payment that additional period. For end-of-month contributions, omit that final factor. Timing is therefore part of the model, not a cosmetic setting.

At a zero return, use FV = M × n. Dividing by r in the general expression would otherwise divide by zero. A zero-return scenario is also a useful check: the projected final value should equal total contributions if no fees or other changes are included.

Worked example and the effect of duration

For ₹1,000 contributed at the beginning of each month, n = 12 and r = 0.01, the formula gives approximately ₹12,809.33. Contributions total ₹12,000, so the modeled growth is ₹809.33. The result is not 12% of ₹12,000 because most of the money has been invested for less than a full year.

A longer duration adds more contributions and gives early contributions more compounding periods. In the long examples, the difference is large because the same positive monthly rate is repeated throughout. That repetition is exactly the assumption you must question when interpreting a real investment projection. It is not evidence that a market will follow this smooth path.

Read the estimate as a scenario, not a forecast

Market returns vary and may be negative. The order of good and bad periods also matters when money is being added along the way. A fixed monthly-growth formula does not reproduce that changing price path. It also omits fund-specific costs, taxes and changes in contribution timing unless those features are explicitly modeled.

The projected value is nominal: it does not tell you what the amount will buy in the future. Inflation can reduce purchasing power even when the balance rises. Compare a few stated assumptions rather than treating the most attractive output as the likely outcome. AMFI’s risk explanation makes clear that mutual fund returns are not guaranteed.

If you have just a beginning and ending value for an investment with no intervening contributions, CAGR answers a different question: the equivalent constant annual growth rate between those endpoints. It should not be substituted for a cash-flow-aware return measure on a monthly investment history.