How this calculator works
Future value with monthly compounding: each month the balance earns APY ÷ 12 and the deposit is added. The table shows the balance at the end of each year.
- Monthly rate r = APY ÷ 12 ÷ 100 and n = years × 12.
- Future value = start × (1 + r)ⁿ + deposit × ((1 + r)ⁿ − 1) ÷ r.
- Interest earned = future value − (start + deposit × n).
Worked example
With these inputs: starting balance $1,000; monthly deposit $200; annual return 4.5%; years 5.
The result is balance after 5 years $14,680.91, with interest earned $1,680.91 and total deposited $13,000.00.
Time does most of the work
Compound interest earns interest on past interest, so growth speeds up the longer money stays saved. Compare 5, 10 and 20 years with the same deposit: the interest line grows much faster than the deposits line.
The return you enter matters. A savings account's APY is fairly predictable. Investment returns are not, and past averages aren't guarantees. Use a conservative figure for planning.
Comparing accounts? The money market calculator runs the same plan against the FDIC's national average rate, the CD ladder calculator spreads savings across CDs that mature on a schedule, and the APY calculator turns a quoted interest rate into the APY to enter here.
Frequently asked questions
How is compound interest calculated monthly?
Each month, the balance earns APY ÷ 12, and then your deposit is added. The formula above does all the months at once.
How much will $200 a month grow to?
Starting from $1,000 at 4.5% APY, $200 a month grows to roughly $14,000 in five years. Change the inputs to see your own numbers.
What's the difference between APY and APR?
APY includes the effect of compounding within a year, while APR doesn't. For savings, banks quote APY, so enter that.