Growth period by period
Compound growth applies the periodic rate to the balance already on hand. With monthly compounding, the monthly rate is the annual rate divided by twelve; each month's interest becomes part of next month's base. The projection also treats each $250 deposit as arriving at the end of its month, so a deposit made sooner would earn for one additional period.
Worked ten-year projection
The default starts with $10,000 and adds $250 at the end of each month for ten years at 5%. Deposits total $40,000. The projected balance is $55,290.66, leaving $15,290.66 of interest. A reader can separate the two sources of the final number: $10,000 arrived on day one, while the later deposits had progressively less time to compound.
What the projection holds fixed
This model holds the rate, deposit schedule, and compounding frequency constant. A savings account may change its yield, and an investment return can vary rather than arrive as a fixed percentage. Monthly deposits also are not the same as an annual lump sum; timing changes the number of compounding periods available to each dollar.
Timing changes the outcome
Try a contribution interruption before treating the ten-year result as a plan. Missing twelve $250 deposits removes $3,000 of principal and the later growth on it. Conversely, a bonus deposited early receives more periods than the same amount spread through the year. The useful comparison is often the effect of a realistic deposit calendar, not the largest possible balance.
Where compound growth stops describing reality
Tax, account charges, withdrawals, inflation, and deposit limits are outside this arithmetic. A taxable account can have a lower after-tax return than the stated rate, while a retirement account can have rules about access. If the rate is a market return rather than a guaranteed account yield, use several return cases instead of reading one projection as a promise.
Make the projection auditable
Keep the starting statement balance, contribution dates, and rate source beside any goal estimate. Those records show whether a later difference came from saving less, a changed yield, or a different timing convention. This page projects accumulation; it does not decide how much risk is appropriate or whether a particular account is available.
One more useful scenario
Compare an annual $3,000 deposit with twelve monthly $250 deposits when cash flow permits either. They contain the same yearly contribution, but the earlier annual deposit has more time in the account. This makes deposit timing a concrete choice rather than an unnoticed detail in a ten-year chart.
End-of-period deposits versus deposits made first
The displayed annuity component treats the $250 contribution as an end-of-month deposit. If payroll puts the same $250 into the account on the first day of each month, every contribution gets one more monthly crediting period. That convention does not change the stated annual rate, but it does change the balance; match the model to the transfer date before comparing a bank statement with the projection.
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It is an educational estimate, not financial, tax, or legal advice.