Compound Interest Calculator
A compound interest calculator projects an end balance from a starting amount, the annual rate you type, and how often that rate compounds. An optional monthly deposit is included. Nothing here is a tax calculation.
You type the rate. This page does not look one up.
End balance
$31,998.32
Total deposited
$22,000.00
Interest earned
$9,998.32
Formula
The starting amount grows by the rate, divided by the number of compounds per year. You can compound yearly, quarterly, or monthly. A 5% annual rate compounded monthly uses one-twelfth of 5% each month. The rate is whatever you type. This page does not download a savings-account rate.
A monthly deposit is treated separately, using the monthly rate, and is assumed to land at the end of each month. The deposit formula does not depend on whether the starting amount compounds yearly or monthly. At a 0% rate, the end balance is the start plus every deposit, and interest earned is zero.
The end balance is the grown starting amount plus the grown deposits. Total deposited is the starting amount plus the deposits you put in, with no growth. Interest earned is the difference. Losses, fees, and taxes on interest are not subtracted.
Use the result to compare a rate or a monthly contribution. It is a planning estimate for a constant rate. A real account can change its rate, and tax on the interest depends on the account type, which this page does not model. Try the same annual rate with yearly and with monthly compounding. Monthly compounding produces a slightly higher balance because interest starts earning interest sooner.
A = P × (1 + r/n)^(n×t) + d × (((1 + m)^M − 1) / m)
- P Starting amount.
- r Annual rate as a decimal.
- n Compounds per year: 1, 4, or 12.
- t Years.
- d Amount deposited at the end of each month.
- m Monthly rate, r divided by 12. The deposit uses this rate.
- M Number of monthly deposits, 12 times the years.
Worked example
Start with $10,000, add $100 at the end of each month, and compound 5% monthly for 10 years.
The end balance is $31,998.32. You deposited $22,000.00 in total, so $9,998.32 is interest.
No tax is taken out of the interest.
FAQ
Does this compound interest calculator deduct tax?
No. The balance is before any tax on interest. What you keep in a taxable account can be lower.
What does compounding monthly change?
The same annual rate is applied twelve times in smaller steps, so the starting amount grows slightly faster than with yearly compounding.
When is the monthly deposit added?
At the end of each month. It then earns interest at the monthly rate for the months that remain.
Can the rate be zero?
Yes. At 0% the end balance is your starting amount plus the deposits, and interest earned is zero.