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Compound Interest Calculator

See how your money grows with compound interest — enter your starting amount, rate and years to get your future value, total interest and a year-by-year breakdown.

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How the Compound Interest Calculator Works

Compound interest means you earn interest on your interest. The formula for a lump sum is: A = P(1 + r/n)^(nt), where P is your starting amount, r is the annual rate, n is how many times interest compounds each year, and t is the number of years. Monthly contributions are added at the end of each month and then compound along with everything else.

Example: $10,000 at 5% a year, compounded monthly, for 10 years. That is 10,000 × (1 + 0.05/12)^120, which comes to about $16,470.09 — $6,470.09 of it is interest you never deposited. Add $100 a month on top and the same 10 years end at about $31,998.32. Time does the heavy lifting: the first years look slow, and the later years do most of the growing.

Compounding frequency matters, but less than time and rate. At 5% over 10 years, $10,000 reaches $16,288.95 compounded annually and $16,486.65 compounded daily — a difference of about $198. Starting earlier, or adding a small monthly contribution, moves the result far more than chasing a slightly higher rate.

Frequently Asked Questions

What is compound interest?
Compound interest is interest earned on both your original money and the interest already added to it. Each compounding period, interest is calculated on a slightly larger balance, so growth speeds up over time — unlike simple interest, which only ever pays on the starting amount.
How does compounding frequency change the result?
The more often interest compounds, the faster your money grows, because each round of interest starts earning its own interest sooner. Monthly compounding beats annual compounding at the same rate, and daily beats monthly — though the difference is small at low rates and short timeframes.
What is the rule of 72?
The rule of 72 is a quick estimate for doubling time: divide 72 by your annual interest rate to get roughly how many years it takes your money to double. At 6%, money doubles in about 12 years; at 8%, in about 9 years. It is an approximation, not an exact calculation.
Does this calculator include taxes, fees or inflation?
No. The result is a before-tax, before-fee estimate in today's dollars, and it assumes a steady interest rate for the whole period. Real accounts pay taxes and fees, rates change, and inflation reduces purchasing power, so treat the figure as a planning estimate, not a promise.
How do you figure compound interest with a calculator?
To figure compound interest, enter your starting amount, annual rate, number of years and how often interest compounds, then add any monthly contribution. The calculator applies A = P(1 + r/n)^(nt) and shows your future value, interest earned and a year-by-year breakdown — the same result as working it out by hand, without the arithmetic.

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