Beginning vs End-of-Month Contributions in an Investment Calculator

Two investment calculators can use the same monthly contribution, annual return, and time horizon yet produce slightly different balances. One common reason is whether each contribution is added at the beginning or the end of its period.

Annuity due and ordinary annuity

End-of-period contributions follow the pattern often called an ordinary annuity. Beginning-of-period contributions follow an annuity-due pattern. Each beginning contribution receives one additional period of modeled growth.

For a monthly rate r, contribution P, and number of payments n, the simplified future value of end-of-month contributions is:

P × (((1 + r)^n − 1) ÷ r)

For beginning-of-month contributions, multiply that result by (1 + r).

This formula is useful for explaining the timing relationship. The site’s calculation engine also accounts for the selected contribution and compounding frequencies, so comparisons should use the same settings throughout.

Why the difference grows

One extra month of growth is small for a single deposit. Repeating it for many years creates a larger cumulative gap. The size of that gap depends on the assumed return, payment amount, and number of contributions.

The model does not imply that a beginning-of-month payment receives a positive return in every real month. It simply places the contribution into the selected fixed-return schedule earlier.

Practical interpretation

Use the timing that best represents the cash flow being modeled. A salary-based contribution might arrive after payday. An automated transfer could occur on the first day of a month. Neither label is universally correct; the important point is to make the assumption visible.

When checking another calculator:

• identify the contribution date assumption;
• verify whether its annual rate is divided monthly or converted to an equivalent periodic rate;
• check whether growth compounds monthly or annually;
• compare results only after aligning those choices.

You can isolate this difference in the monthly investment calculator. Keep every input fixed, switch contribution timing, and compare the year-by-year tables. The output is hypothetical and excludes taxes, fees, and actual market volatility unless represented in the inputs.