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Compound Interest Calculator
See how your savings or investments grow over time. Enter a starting amount, monthly contribution, rate and time, then press Calculate.
Written by TopicDrill Editorial Team·Updated June 2026
Balance over time
Balance Contributed
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How compound interest works
Compound interest pays you interest on your interest. Each period, the rate is applied to your whole balance, including gains from earlier periods, so the balance grows faster and faster. The longer your money stays invested, the more dramatic the effect.
The standard formula is A = P(1 + r/n)nt, where 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. This calculator extends that by adding your monthly contributions, which is how most people actually invest.
A quick example
Start with $10,000, add $200 a month, and earn 8% a year compounded monthly for 20 years. You contribute $58,000 of your own money, yet the balance grows to well over $160,000. The difference is compound interest doing the heavy lifting, as the chart above shows the gap between what you put in and what you end with widening every year.
Things to keep in mind
Returns are not guaranteed and real investments fluctuate, so treat the rate as a long-term average, not a promise. For broad investor education see Investor.gov from the SEC. You can also explore our other financial calculators to plan savings, retirement and loans.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Over time this snowball effect makes your balance grow faster than simple interest, which only pays on the original amount.
How is compound interest calculated?
The core formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is how many times a year it compounds, and t is the number of years. This calculator also adds your monthly contributions as they are made.
Does compounding frequency matter?
Yes, but less than people expect. More frequent compounding (daily vs annually) raises the total slightly because interest starts earning interest sooner. The rate, contributions and time horizon matter far more than the frequency.
Why do contributions matter so much?
Regular monthly contributions are often the biggest driver of the final balance, especially early on. Each contribution has more years to compound, so investing consistently over a long period usually beats a single large lump sum added later.
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