Compound Interest Calculator US
See how savings grow with compound interest and regular contributions.
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Year-by-year growth
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How it works
Compound interest earns returns on both your original money and the returns already added. Adding a regular monthly contribution accelerates growth further over time.
A = P(1 + r/n)^(nt) + contributions compounded monthly- Longer time horizons benefit most from compounding
- More frequent compounding gives slightly higher growth
- The donut shows how much of your final balance is interest vs money you put in.
Compound interest guides
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest, so it grows faster over time.
How does compounding frequency affect growth?
More frequent compounding (daily or monthly vs annually) produces slightly more growth, because interest is added to the balance sooner and starts earning its own interest.
What is the Rule of 72?
It is a shortcut for estimating doubling time: divide 72 by the annual percentage rate. At 6% money doubles in about 12 years; at 8% in about 9 years.
Why does starting early matter so much?
Because compounding accelerates over time, the earliest dollars have the most years to grow. Starting a decade earlier can easily double the final balance even with the same contributions.
Does this calculator account for inflation or taxes?
No. It shows nominal growth. Real (inflation-adjusted) returns and any taxes on interest or gains would reduce the effective result.
How do regular contributions change the result?
Adding a fixed amount each period dramatically increases the final balance, because each contribution begins compounding from the moment it is added.
How compound interest works
Compound interest is interest earned on both your original principal and on the interest already added — "interest on interest." It is calculated with A = P(1 + r/n)nt, where P is the starting principal, r is the annual rate, n is how many times interest compounds per year, and t is the number of years. The more frequently interest compounds and the longer it runs, the faster the balance grows.
Worked example
Invest $10,000 at a 7% annual return, compounded monthly, for 30 years. The balance grows to roughly $81,000 — even though you never added another dollar. Of that, $10,000 is your principal and about $71,000 is compounded growth. Stretch it to 40 years and it climbs past $160,000, which shows why time is the single most powerful factor in compounding.
The Rule of 72
For a quick estimate of how long money takes to double, divide 72 by the annual rate. At 7%, money doubles in about 72 ÷ 7 ≈ 10.3 years; at 9% in about 8 years. It is an approximation, but it is accurate enough for mental math and a useful way to sense-check any growth projection.
Regular contributions
Adding money on a schedule changes the picture dramatically. Contributing $200 a month to that same 7% account for 30 years adds roughly $244,000 of contributions-plus-growth on top of the original lump sum. This is the engine behind long-term savings vehicles — small, consistent deposits compounding over decades.
Results are estimates for general guidance in United States and may not reflect the latest local rates, fees or rules. Check official sources before making decisions.