Regular contributions: separate deposits from assumed growth
A large future balance includes money you added month after month. Separating deposits from assumed growth makes comparisons clearer and prevents every increase from being called profit.
Break down the final number
Record the starting amount, monthly addition and number of months, then total the deposits before considering growth. Investor.gov’s model separates these inputs from the rate and compounding frequency.
This calculator divides a nominal annual assumption by 12 for constant monthly growth. That assumption is not a current product rate, an asset’s return history or a description of market volatility.
References: Investor.gov — Compound interest and contributions
Contribution timing is an assumption too
A beginning-of-month addition enters before that month’s growth; an end-of-month addition does not grow during that month. With a nonzero rate, the difference is not a rounding error.
Before comparing calculators, align timing, month count and rate meaning. Do not compare a balance including deposits with a figure labelled growth alone.
Check the no-growth scenario
A 0% calculation shows what the contributions alone accumulate; it does not guarantee preserved purchasing power. If deposits change, withdrawals occur or charges apply, use matching assumptions rather than extending an old result.
This is fixed-assumption arithmetic, not a portfolio or product recommendation. Selecting a higher rate does not make it more probable or realistic.
Two months make timing visible
An initial 1000 plus 100 for two months at 0% gives 1200 with no growth. At an assumed nominal annual 12%, or 1% monthly, end-of-month additions produce 1110 and then 1221.10.
Beginning-of-month additions instead produce 1111 and then 1223.11. Deposits total 1200 in both cases; the difference comes from model timing, not evidence of a guaranteed return.