How compound interest works
Compound interest is the engine behind almost every long-term wealth-building strategy — from retirement accounts to high-yield savings. Unlike simple interest, which only pays on your original deposit, compound interest pays interest on your interest. Over years and decades, that small difference snowballs into life-changing sums.
This free compound interest calculator lets you model exactly how a starting balance grows when you add a fixed monthly contribution and let it compound at a given rate. Adjust the inputs to compare scenarios — a 1% difference in rate or an extra $100 per month can be worth tens of thousands by retirement.
The compound interest formula
For a lump sum with no contributions, the standard formula is:
Where A is the final amount, P is the principal, r is the annual rate (as a decimal), n is the number of compounding periods per year, and t is the number of years. When you add regular monthly contributions, this calculator extends the formula period-by-period so deposits earn interest from the moment they land.
What the inputs mean
- Principal — the money you start with today.
- Monthly contribution — what you add each month (set to $0 if you only want to model a lump sum).
- Annual rate — the yearly interest or return rate. Historical U.S. stock market average is roughly 7% after inflation; a high-yield savings account today is typically 4–5%.
- Years — your time horizon. The longer, the more dramatic the compounding effect.
- Compounding frequency — how often interest is calculated and added. Daily compounding is slightly more favorable than annual, but the difference is small compared to rate and time.
Why time matters more than rate
If you save $200 a month at 7% for 40 years, you will end up with around half a million dollars — even though you only put in $96,000. Starting ten years earlier can literally double your final balance. That is the magic of compounding: each dollar you invest today gets the maximum number of years to multiply.
This tool is for educational illustration only and does not account for taxes, fees, or inflation. Returns shown are nominal and assume a constant rate of return, which real-world investments do not provide.