Compound Interest Calculator
See how savings grow with contributions and compounding over time. e.g. “What will $10,000 grow to in 20 years?”
Enter your values
Results update as you type. All calculation happens in your browser.
Methodology
See how savings grow with contributions and compounding over time. This tool uses a standard, documented formula and runs entirely on your device.
Last reviewed January 2026 · Runs client-side
Why compound interest rewards time
Compound interest is interest earning interest. Each period, your balance grows, and the next period's interest is calculated on that larger balance. Over a few years the effect is modest; over decades it becomes the dominant force in your ending balance, which is why starting early matters more than starting big.
Regular contributions accelerate the effect. Every deposit you add starts compounding from the day it lands, so a steady monthly contribution can end up contributing more growth than a larger one-time deposit made later.
How to use this calculator
- Enter your starting amount and any regular monthly contribution.
- Set the annual interest rate and how often it compounds.
- Choose a time horizon in years and read the ending balance, total contributions, and interest earned.
What the inputs mean
- Starting amount
- The lump sum you begin with.
- Monthly contribution
- What you add each month; these compound alongside your balance.
- Compounding frequency
- How often interest is applied, more frequent compounding grows slightly faster.
$10,000 with no further contributions at 7% compounded monthly grows to about $20,100 in ten years, it roughly doubles without you adding a cent.
The formula, in plain terms
FV = P(1+i)ⁿ + PMT·((1+i)ⁿ−1)/i, where P is the starting amount, PMT is the contribution per period, i is the periodic rate, and n is the number of periods.
Good to know
- The rate you enter is an assumption, real returns vary year to year.
- Small differences in rate compound into large differences over long horizons.
Frequently asked questions
What is the rule of 72?
Divide 72 by your annual rate to estimate the years it takes to double. At 7%, that's about 10 years, which matches the example above.
Is this the same as investment return?
The math is the same, but investment returns aren't fixed. Use a conservative rate and treat the result as a projection, not a guarantee.
Last reviewed January 2026. This explainer is general information, not professional advice.