Breakdown

Interest vs Contributions

A useful compound interest result should show both sides: what you put in and what the growth assumption adds.

Contribution total

startingAmount + monthlyContribution * months

This is the amount you directly add to the balance.

Estimated interest

endingBalance - contributionTotal

This is the growth above direct contributions before taxes, fees, and inflation.

Decision guide

Measure how much of the result came from you

A final balance combines initial principal, later contributions, and modeled interest. Splitting the result into those parts makes a savings plan easier to control. Contributions are the lever you can usually change immediately; the assumed return is uncertain and may be lower than the rate used in a projection.

Read the contribution share

Suppose you start with $10,000 and add $250 each month for ten years. Direct contributions total $40,000, including the initial amount. If a modeled result reaches roughly $53,000, about $13,000 is projected growth. The exact split depends on rate, timing, fees, and whether returns actually arrive as assumed.

Before you decide

  • Count the initial balance as contributed principal.
  • Do not count transfers twice.
  • Use after-fee, after-tax assumptions when relevant.
  • Compare the plan with a zero-growth baseline.

Method and limitations

The calculator performs attribution, not investment analysis. It assumes the entered rate compounds smoothly and contributions occur on schedule. Use low, middle, and high scenarios; for market investments, include the possibility of losses and uneven returns rather than relying on one constant annual percentage.