Compound Interest Calculator
Enter a starting amount, an annual interest rate, a time period and how often interest compounds. Add a monthly deposit to see how regular saving adds up.
Compound interest
The formula
A = P × (1 + r/n)^(n × t)
- A
- final amount
- P
- starting amount (principal)
- r
- annual interest rate as a decimal (5% = 0.05)
- n
- compounding periods per year
- t
- time in years
For continuous compounding the formula becomes A = P × e^(r × t). With regular monthly deposits D, each deposit grows for the months remaining, which sums to D × ((1 + i)^m − 1) ÷ i, where i is the monthly rate and m the number of months.
Worked example
10,000 at 5% a year, compounded monthly, for 10 years
- r/n = 0.05 ÷ 12 = 0.0041667
- n × t = 12 × 10 = 120 periods
- A = 10,000 × 1.0041667^120 = 16,470.09
Final balance 16,470.09, of which 6,470.09 is interest.
How compounding frequency changes the result
More frequent compounding earns slightly more, because interest starts earning interest sooner — but the gains shrink quickly. At 6%, going from annual to monthly compounding raises the effective rate from 6% to about 6.17%; going from monthly to continuous adds only about 0.02 percentage points more. The APR to APY calculator shows the exact effective rate.
The rule of 72
A quick estimate: money doubles in roughly 72 ÷ (rate in %) years. At 6% that is about 12 years (exact with annual compounding: 11.9 years). The rule works best for rates between about 4% and 12%.