Compound Interest Calculator
Estimate how steady contributions and time could grow savings or investments under a chosen return assumption.
Calculator
Compound Interest Calculator
Add the balances you already have saved or invested instead of combining them first.
Starting balance total: $10,000.00
Add one or more recurring monthly contributions instead of combining them before you calculate.
Total monthly contribution: $400.00
Use a conservative long-term estimate rather than a best-case number.
Longer timelines amplify the effect of compounding.
Example values are loaded.
Result
Your result
Saving $400.00 per month for 20 years could grow to about $248,758.05.
Future value
$248,758.05
Total deposits
$106,000.00
Interest earned
$142,758.05
Growth assumptions
Next steps
Compare before you move on
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What this calculator shows
Compounding works because you earn returns on both the money you contribute and the gains that build up over time.
It works well for comparing contribution levels and seeing how time affects the final outcome, especially when the money comes from more than one recurring contribution.
How to use it
- 1. Add your current savings or investment balances.
- 2. Add one or more monthly contributions you expect to keep making.
- 3. Choose an annual return estimate and time horizon to see the future value and interest earned.
Formula and assumptions
This calculator compounds monthly by applying the monthly rate to the current balance, then adding the combined monthly contribution each cycle.
That makes the result practical for recurring savings plans where deposits happen every month.
How to read this result
The biggest signal here is usually the interaction between time and contribution consistency. Small recurring deposits can matter more than a larger one-time change made much later.
Interest earned becomes more meaningful as the horizon gets longer. Early on, deposits drive the balance; later, compounding starts carrying more of the weight.
Use this result to compare saving pace and timeline before you compare products. If the goal is short-term, savings-goal math may matter more than long-horizon compounding.
Common mistakes
Using a best-case return assumption and then treating the future value like a dependable forecast.
Ignoring the time horizon and focusing only on the ending number. A strong result over 30 years says something very different from the same number over 8 years.
Using compound-interest math for a goal that is really a payoff, affordability, or short-term cash-planning decision.
Limits of this estimate
This model assumes a constant annual return, monthly compounding, and a steady monthly contribution pattern over the full time horizon.
It does not account for taxes, investment fees, contribution timing that changes materially, or the year-to-year volatility of real markets.
Use it to compare savings pace and time horizons, not as a forecast of what a portfolio will definitely earn.
Notes
Investment returns vary, so treat the result as an estimate rather than a guaranteed forecast.
Source notes
Monthly compounding model
The projection compounds the balance monthly using the annual return divided by 12, then adds the combined monthly contribution each cycle.
Contribution timing simplification
Recurring contributions are treated as steady monthly deposits. Real deposit timing and market movement can shift the final outcome.
Worked example
Starting with $10,000 and contributing $400 per month shows how much time matters when returns compound consistently.
This example uses the default sample inputs loaded on reset. It does not update with the live calculator entries above.
Future value
$248,758.05
Total deposits
$106,000.00
Interest earned
$142,758.05
Feedback
Found a problem on this page?
Report confusing fields, broken math, or missing assumptions with the exact inputs you used so the issue can be reproduced.
FAQ
FAQ
Why does time matter more than a slightly higher contribution later?
Earlier contributions have more months to compound, so they usually contribute more to long-term growth than equivalent amounts added much later.
FAQ
Can I add more than one monthly contribution?
Yes. Add each recurring deposit on its own row and the calculator will total them before projecting growth.