Two investment illustrations can disagree simply because one assumes contributions at the start of each month and the other assumes the end. The SIP Investment Calculator uses beginning-of-month contributions and a constant annual return assumption divided by twelve. Its output is an illustration of those inputs, not a forecast of an investment’s actual returns.
Set the contribution and time horizon
Enter Monthly investment, Assumed annual return (%), and Number of months, then choose Calculate. Contributions must be nonnegative, the annual assumption is limited to 0 through 100 percent, and the horizon must be a whole number from 1 through 1,200 months.
The result separates Total contributed, Estimated value, and Estimated growth. Use one currency or unit for the contribution and interpretation of the output. The tool does not convert currency, select an investment, or retrieve market performance.
Trace three contributions by hand
For monthly contribution 1,000, annual assumption 12%, and 3 months, the monthly rate is 1%. At the end of the third month, the first contribution has grown for three monthly periods, the second for two, and the third for one.
Their modeled values are 1000 × 1.01³ = 1030.301, 1000 × 1.01² = 1020.10, and 1000 × 1.01 = 1010. Together they produce an estimated value of 3,060.40, contributed amount 3,000.00, and estimated growth 60.40 after display rounding.
In general, with contribution C, monthly rate r, and n months, the formula is C × ((1 + r)^n − 1) / r × (1 + r). The final factor accounts for beginning-of-month timing. At a zero assumption, the formula reduces to C × n.
Keep the assumptions visible
The annual percentage is divided by twelve; it is not converted from an effective annual return. Every modeled month receives the same return. The calculator does not represent changing prices, negative-return months, missed contributions, fees, taxes, or withdrawals. A positive assumption is not a guarantee, and the form does not model negative assumed rates.
When comparing another calculator, first match both its rate convention and deposit timing. Differences in those inputs can explain different outputs without an arithmetic error.
Processing runs locally in the browser without account access. Use the Compound Interest Calculator for a single initial balance without new contributions. Use the Savings Goal Calculator when solving for a contribution toward a target, remembering that it uses month-end contributions instead.