Compound Interest Calculator
See how principal, monthly contributions and compounding grow over time — with a yearly chart
Examples
20-year index plan
10,000 initial · 200/month · 7% nominal · monthly · end-of-period≈ 142,000 final · ≈ 58,000 contributed · ≈ 84,000 interest · EAR 7.23%More than half the final balance arrives as interest — the case for starting early.
Timing effect
Same plan, contributions moved to beginning of periodFinal balance rises by a low single-digit percentageEvery contribution earns one extra compounding period.
About this tool
Compound growth is simulated one period at a time rather than collapsed into a closed-form formula — so contribution timing, compounding frequency and multi-decade horizons all produce the honest number. The effective annual rate (EAR) sits next to the nominal rate so the cost of infrequent compounding is visible.
The chart overlays cumulative contributions against the balance: the widening gap is interest at work. A yearly schedule (downloadable as CSV) backs every figure on screen.
How to use
Set the plan
Initial amount, monthly contribution, expected annual return and horizon in years.
Choose mechanics
Compounding frequency (annual/quarterly/monthly) and contribution timing (beginning or end of period).
Read the split
Final balance, total contributed, interest earned and the interest-share bar.
Export
Copy the summary or download the yearly schedule as CSV for spreadsheets or an advisor.
Use cases
Retirement projection
Sketch long-horizon growth of regular contributions at a conservative rate.
Frequency comparison
See how EAR and final balance shift between annual and monthly compounding.
Common mistakes
Mistake:Comparing nominal rates across products with different compounding
Fix:Compare EARs — 12% monthly-compounded is ~12.68% effective, 12% annual is 12%.
Mistake:Treating the projection as guaranteed
Fix:Markets vary; sequence risk and fees are outside this model. Use it as a nominal, pre-cost sketch.
Frequently asked questions
Related guides
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References & standards
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