Compound Interest Calculator
This compound interest calculator shows how your money grows when interest earns interest. Enter a starting amount, a rate, how long you'll save, and how often it compounds, then add a regular contribution if you like. You'll see the future balance, how much you put in, how much the interest added, and a chart of the growth year by year.
- Any compounding frequency
- Regular contributions
- Future balance
- Growth chart
- Steps shown
Last updated June 16, 2026 A = P(1 + r/n)^(nt) Reviewed by the Calcowa finance team
Enter a starting amount, rate, and years above 0.
Set the contribution to 0 for a lump sum only. Estimates for planning, not a guarantee.
A = P(1 + r/n)^(nt)
How does compound interest work?
Compound interest is interest that earns interest. Each period, your balance grows by the rate, and the next period's interest is figured on that bigger balance, so the growth keeps accelerating. Simple interest only ever pays on the original amount, but compounding lets the gains pile on top of each other. Given enough time, that snowball does most of the work, and that's why money left to grow for decades ends up far larger than the deposits alone.
The compound interest formula
The formula is A = P(1 + r/n)^(nt). Here's what each letter means, using $10,000 at 7% compounded monthly for 10 years:
- 1
P is the starting amountThe principal you begin with, here $10,000.
- 2
r is the yearly rateAs a decimal, so 7% becomes 0.07.
- 3
n is how often it compoundsMonthly means n = 12, so the rate per period is 0.07 ÷ 12.
- 4
t is the years, and you raise itThe exponent is n × t = 120 periods. The lump sum alone grows to about $20,097, and monthly deposits push the total higher.
Daily, monthly, or yearly compounding
Compounding frequency is how often the interest gets added back in. Daily compounding adds it 365 times a year, monthly 12 times, and yearly just once. More frequent compounding earns a bit more, because the interest starts earning its own interest sooner, though the difference is smaller than people expect. The effective annual rate above captures it: at 7%, monthly compounding works out to about 7.23% a year. It's a small edge, but it's real over a long stretch. Switch the frequency above and you'll watch that number, and the final balance, shift.
Why regular contributions matter so much
A starting balance is only half the story. When you add money every month, each deposit gets its own runway to compound, so steady saving often outgrows the initial amount. In the default above, $200 a month does more heavy lifting than the $10,000 you started with, and that's typical. The chart splits the two, so you can see what you contributed against what the interest added. It's the clearest argument for paying yourself first. To work out a savings rate as a percent, the percentage calculator helps, and for a home loan the mortgage calculator shows compounding from the borrower's side.
Frequently asked questions
How long until my money doubles?
Use the rule of 72: divide 72 by the rate. At 7%, money roughly doubles in about 10 years; at 9%, in about 8. It's an estimate, but it's a handy way to picture how a rate translates into growth without running the full formula.
Compound interest earns interest on your interest, not just on the original amount. Each period the balance grows, and the next period's interest is figured on that larger balance, so growth speeds up over time. The longer the money sits and the more often it compounds, the bigger the snowball. That's why starting early matters more than almost anything else in saving.
For a lump sum it's A = P(1 + r/n)^(nt), where P is the starting amount, r is the yearly rate as a decimal, n is how many times a year it compounds, and t is the number of years. With regular contributions you add the future value of those deposits on top. This compound interest calculator handles both, so you don't work the powers by hand.
It's how often interest gets added: daily, monthly, quarterly, or yearly. More frequent compounding earns a little more, because interest starts earning its own interest sooner. The gap is real but modest. At 7%, daily compounding beats yearly by only a fraction of a percent a year, though over decades even that adds up.
A lot, usually more than the starting balance over time. Steady deposits get their own compounding runway, so $200 a month at 7% can grow into tens of thousands over a couple of decades. The chart above splits what you put in from what the interest added, and the contributions line is often the larger share early on.
It's a quick shortcut: divide 72 by the yearly interest rate to estimate how many years it takes money to double. At 8%, that's about 9 years; at 6%, about 12. It isn't exact, but it's close enough for a fast mental check, and it shows why a higher rate shortens the doubling time so sharply.
For savings, yes, since you earn on the growing balance instead of just the original. Simple interest pays the same amount each period and never compounds, so it falls behind. For borrowing it's the reverse, because compound interest on a debt makes it grow faster, which is why paying down high-rate balances quickly helps.
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