Small contributions. A surprisingly big difference.
What happens when time, consistency, and compound interest work together? Let’s follow the numbers.
Start with the habit, then look at the horizon
A savings plan has three moving parts you can control in a model: what you start with, how regularly you contribute, and how long you leave the money invested. A return assumption adds a fourth input, but it is a scenario rather than a promise.
Consider a $10,000 starting balance and $200 added at the end of each month for 10 years. Even with no return at all, those deposits add up to $34,000: $10,000 + $200 × 120 months.
What interest on interest actually means
Assume a constant 6% nominal annual rate, compounded monthly. The monthly rate is 0.06 ÷ 12 = 0.005, or 0.5%. After the first month, $10,000 earns $50, then the $200 contribution brings the balance to $10,250. The next month earns interest on that larger balance.
Applying the same rule for 120 months gives approximately $50,969.84. Of this, $34,000 is your own money and about $16,969.84 is modeled growth. All dollar amounts here are USD, rounded to cents. Taxes, fees, inflation, and changing returns are excluded.
Three comparisons worth making
First, change the monthly contribution while holding everything else constant. This isolates the effect of saving more. Second, compare shorter and longer timelines. Third, test a range of return assumptions instead of choosing only an optimistic one.
A higher projected balance is not automatically a better plan if it requires contributions you cannot sustain. Use the comparison feature to keep several scenarios side by side.
Put the estimate in context
A constant return model does not describe market volatility or a guaranteed savings product. Investments can lose value; product-specific deposit terms can change accrual and withdrawal rules. Inflation also reduces what a future balance can buy.
The useful output is a clearer relationship between the inputs and the result. Open the Compound Interest Calculator, enter your assumptions, and check the step-by-step calculation below the chart.
Make the numbers your own.
Put this guide into practice with a free calculator.