Compound Interest Calculator

The calculator grows a single deposit with A = P × (1 + r/n)^(n × t) and shows the final amount and the interest earned (final amount minus principal).

• Principal: the amount you start with. Nothing is added or taken out later.
• Annual interest rate: the nominal yearly rate, before compounding.
• Years: accepts quarter-year steps, so 1.5 means 18 months and 0.25 means 3 months.
• Compounding: how often interest is added to the balance, from yearly to daily.

With the defaults ($10,000 at 8% for 10 years, compounded monthly) the balance reaches $22,196.40, of which $12,196.40 is interest. Switch to yearly and it is $21,589.25; switch to daily and it is $22,253.46. Choose the frequency your account or loan terms state rather than the one that looks best.

The calculator answers "what will I have?". Two other questions come up just as often, and the same formula answers them:

rate needed = n × ((A ÷ P)^(1 ÷ (n × t)) − 1)

years needed = ln(A ÷ P) ÷ (n × ln(1 + r ÷ n))

To turn $10,000 into $15,000 in 5 years with monthly compounding, you need about 8.14% a year. Enter 8.14% and 5 years to check: the calculator shows $15,002.41.

To double $10,000 at 8% compounded monthly takes about 8.69 years. In the calculator, 8.5 years gives $19,694.28 and 8.75 years gives $20,090.80, so the target falls between the two. Testing quarter-year steps like this is a quick way to find when a balance crosses a goal.

The same arithmetic applies to debt that nobody pays down. Say a $3,000 balance carries a 22% rate compounded daily and you make no payments for 2 years. Enter it as principal, rate and years with daily compounding, and the balance grows to $4,657.50, with $1,657.50 of interest.

Real credit card balances also attract fees and require minimum payments, so treat this as a rough picture of what waiting costs. To see how long a fixed monthly payment takes to clear a balance, use the loan payoff calculator instead.

• Regular deposits. If you add money every month, use the investment calculator, which compounds a starting balance plus monthly contributions.
• Tax on interest. The result is pre-tax. If, say, a quarter of each year's interest goes to tax, enter three-quarters of the rate as a rough approximation. At 6% the default deposit grows to $18,193.97 over 10 years; at 4.5% it grows to $15,669.93.
• Rate changes. The rate is fixed for the whole period. For a savings account whose rate moves, run a cautious and an optimistic rate.
• Inflation. The final amount is in future dollars. The inflation calculator shows what that sum would buy in today's money.