Start with the balance, not the headline rate
Compound interest means later interest is calculated on a balance that includes earlier interest. With $1,000 at 5% compounded annually, the first year adds $50. The next year starts at $1,050, so it adds $52.50. After two years the balance is $1,102.50. The extra $2.50 comes from interest on the first year’s interest.
Keep deposits separate from growth
A rising balance does not all represent investment returns. If you begin with $1,000 and add $100 every month for a year, you contribute $2,200 in total. Compare the final balance with that amount to see how much of the change comes from growth. FinMathly shows contributions separately for this reason.
Make the rate assumption explicit
The calculator uses a constant nominal annual return and converts the chosen compounding frequency to an equivalent monthly growth rate. Contributions arrive at the end of each month. Moving deposits to the start of a period would give them longer to grow. Investment returns fluctuate, so a smooth projection is a scenario rather than a prediction. Taxes, fees, and inflation can also change the outcome.
Try one change at a time
Keep your initial balance fixed, then adjust the contribution or number of years. This isolates the effect of each input and makes comparisons easier to understand.
Further reading
Investor.gov: Compound Interest Calculator
Try the Compound Interest Calculator