How the compound interest calculator works
This calculator simulates your balance one compounding period at a time — the same style of
period-by-period simulation used in our
FIRE, Retirement,
and DRIP calculators. Each period, the balance earns interest
at Annual Rate / Compounding Frequency, and then your regular contribution (if any) is
added, repeated for Years × Compounding Frequency total periods.
Total contributed is your principal plus every periodic contribution added along the way. Total interest earned is simply your final balance minus that total contributed figure — the portion of your final balance that came purely from compounding, not from money you put in directly.
Effective annual yield converts your compounding frequency into a single comparable
annual rate: (1 + Periodic Rate)^Frequency − 1. Compounding more frequently at the same
stated annual rate produces a slightly higher effective yield, because interest starts earning its
own interest sooner.
Finally, balance without additional contributions runs the same principal and rate with no recurring contributions, isolating how much of your final balance is attributable to the contribution habit itself versus the starting lump sum compounding on its own.
Worked example
Suppose you start with $10,000, earning 6% annually, compounded monthly, adding $200 every month, for 10 years.
Your final balance reaches $50,969.84, having contributed $34,000.00 in total (the $10,000 principal plus 120 monthly payments of $200). That leaves $16,969.84 in pure interest earned — and because interest compounds monthly rather than annually, your effective annual yield is 6.17%, slightly above the stated 6% nominal rate.
Without the monthly contributions, that same $10,000 principal compounding alone would only reach $18,193.97 — showing that the majority of the final balance in this example comes from the contribution habit, not the starting lump sum.
Common mistakes to avoid
1. Comparing rates without matching compounding frequency
A 6% rate compounded monthly is not the same as 6% compounded annually — always compare effective annual yields, not just stated nominal rates, when weighing two different savings or investment options against each other.
2. Underestimating the effect of regular contributions
As the worked example shows, consistent contributions can end up contributing more to a final balance than the original lump sum, especially over long time horizons — don't focus only on your starting amount when projecting future growth.
3. Forgetting this is a nominal, pre-tax projection
This calculator doesn't model taxes on interest as it's earned, or inflation eroding the real value of your final balance. A large nominal number over a long time horizon buys less in real terms than the same number would today.