Compound Interest Calculator

Enter a starting balance, an annual interest rate, and how often it compounds to see what it grows to over time — updates instantly as you type.

Enter a starting balance, rate, and number of years above to calculate the final balance.

Growth of $10,000 (compounded monthly)

How a lump sum grows at common annual rates over time, assuming no additional deposits or withdrawals.

Annual rate10 years20 years30 years
2%$12,211.99$14,913.28$18,212.09
3%$13,493.54$18,207.55$24,568.42
4%$14,908.33$22,225.82$33,134.98
5%$16,470.09$27,126.40$44,677.44
6%$18,193.97$33,102.04$60,225.75
7%$20,096.61$40,387.39$81,164.97
8%$22,196.40$49,268.03$109,357.30
10%$27,070.41$73,280.74$198,373.99

How is compound interest calculated?

Compound interest earns interest on both the original balance and on interest already added, so the balance grows faster than simple interest (which only ever earns on the original balance):

A=P(1+rn)nt

Where A is the final balance, P is the starting balance (principal), r is the annual interest rate as a decimal, n is the number of times interest compounds per year, and t is the number of years.

Example: $1,000 at 5% annual interest, compounded monthly, for 10 years

A = 1000 × (1 + 0.05 ÷ 12)12 × 10 = $1,647.01, meaning $647.01 of that is interest earned on top of the original $1,000.

Compounding frequency and APY

More frequent compounding earns slightly more, because interest starts earning its own interest sooner. The effective annual yield (APY) captures this: a 6% annual rate compounded monthly works out to an APY of 6.17%, not exactly 6%, because APY=(1+rn)n−1.

Frequently asked questions

What's the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows by the same dollar amount every period. Compound interest is calculated on the principal plus any interest already earned, so the dollar amount it adds grows every period. Over long time spans the difference is large — the same $1,000 at 5% for 30 years earns $1,500.00 with simple interest but about $4,467.74 ($3,467.74 of interest) when compounded monthly.
Does more frequent compounding make a big difference?
Less than most people expect. At a 5% annual rate, compounding once a year yields exactly 5.00% APY, while compounding monthly yields about 5.12% APY — a difference of well under half a percentage point. The compounding frequency matters far less than the interest rate itself or how long the money is left to grow.
What is APY?
APY (annual percentage yield) is the actual rate of return you earn in a year once compounding is accounted for. It's always slightly higher than the stated annual rate whenever interest compounds more than once a year, which is why banks advertise APY rather than a plain interest rate — it's the number that's actually comparable across accounts with different compounding schedules.
Is there a quick way to estimate how long money takes to double?
The "Rule of 72" gives a fast estimate: divide 72 by the annual interest rate to get the approximate number of years to double. At 6% annual, that's 72 ÷ 6 = 12 years. It's an approximation that works best for rates between about 6% and 10% — use the calculator above for an exact figure at other rates.