Compound Interest Calculator With Monthly Deposits \

How Monthly Contributions Accelerate Compounding

A lump-sum investment grows by the formula FV = P(1 + r/n)^nt. Monthly deposits add a second engine: the future value of an annuity, FV = D × [((1 + r/n)^nt − 1) / (r/n)]. Together, the two produce a total balance that grows faster than either component alone.

At an illustrative 5% APY, depositing $500 per month with no starting balance produces approximately $6,139 after one year — $139 of which is pure interest. Over ten years, the same $500 monthly becomes roughly $77,641, where nearly $17,641 is interest. The non-linear acceleration is what makes long time horizons so powerful: most of the interest is earned in the later years as the compounding base grows.

The calculator charts this progression so you can see exactly when the compounding curve begins to steepen for your inputs.

Adjusting Your Contribution to Fit a Goal

If you have a target balance in mind — a house down payment, a new vehicle, or a cushion for self-employment — work backward. Enter your goal in the calculator and adjust the monthly deposit until the projected balance matches your target at the right date. This reverse approach often reveals that the required contribution is more manageable than expected, especially when compound interest does part of the work for you.

For a weekly contribution cadence instead, try the weekly deposits calculator, which aligns better with biweekly paychecks and puts money to work a few days sooner each cycle. If you already have a lump sum to start with, the compound interest calculator models the growth of that initial deposit alongside recurring contributions. Starting with even a modest lump sum gives compound interest a larger base to work from immediately, which can meaningfully shorten the time needed to reach your target balance.

The rate is held constant for the entire projection. Actual returns may vary if your account rate changes during the period.