What changes when you add a little, regularly?

A starting balance is only part of a savings projection. Regular contributions create a second source of growth, because each deposit has its own time left to compound. The first deposit and the last deposit do not receive the same amount of growth.

Separate contributions from growth

If you begin with 100,000 and add 5,000 every month for ten years, you contribute 700,000 in total: the initial 100,000 plus 120 deposits of 5,000. That contribution total does not depend on the interest rate, as long as your contribution schedule stays the same.

At a zero rate, the ending balance is exactly 700,000. At a positive assumed rate it may be higher, while a negative assumed rate can leave it lower. Keeping “total contributed” separate from “estimated growth” makes it easier to see what is doing the work in a projection.

Beginning or end of the month?

A start-of-month deposit has one extra month of exposure compared with an end-of-month deposit. In a positive constant-rate model this increases the projected balance. With a negative rate, earlier exposure can instead reduce the balance.

As a small example, start at zero and deposit 100 per month for one year at a nominal annual rate of 12%, compounded monthly. End-of-month deposits produce approximately 1,268.25. Start-of-month deposits produce approximately 1,280.93. Both schedules contribute the same 1,200.

Increase contributions each year

An annual step-up increases the deposit amount rather than the return rate. Starting at 5,000 per month with a 10% annual increase means 5,500 per month in year two and 6,050 per month in year three. The increase compounds on the contribution amount.

Across those three years you would contribute 60,000 + 66,000 + 72,600 = 198,600, excluding any starting balance. A step-up projection should be tested against what you can realistically contribute; entering an increase does not mean future income will support it.

Work backward from a target

Goal mode asks a different question: given your initial balance, rate, schedule and duration, what starting regular contribution would reach a target? The same annual increases and contribution timing apply. The goal is a future nominal amount, not an inflation-adjusted target.

At zero return with no starting balance, reaching 120,000 in ten years requires 1,000 monthly when there are no annual increases. That is simply 120,000 divided by 120 deposits. Positive or negative rate assumptions change the required contribution.

Keep the comparison useful

Change one input at a time. First compare contribution amounts at the same rate and duration. Then explore a lower assumed return. Finally, try a longer or shorter horizon. Comparisons with different durations can answer planning questions, but the difference between their ending balances is not a measure of investment performance.

Try these examples in the compound interest calculator. Read compounding frequency explained before comparing different deposit schedules, or revisit the basics of compound growth.