Compound Interest
Calculator
Small steps. Room to grow—with monthly contributions.
See what steady saving could become. Then explore what a little more time—or a little more each month—could change.
Your projected balance
An estimate, not a promiseEnter valid values to see your projection.
Illustrative constant-rate projection. Excludes taxes, fees and inflation. Investment returns vary and may be negative.
What could make a difference?
Compare one change with your current plan.
Delayed start needs more than 5 years; extending a plan is available up to 95 years.
See the annual breakdown
| Year | Contributed | Growth | Balance | Alternative |
|---|
How compound interest works
You earn interest on your original money and on previous interest. Monthly contributions add more money that can grow over time.
For example, 10,000 at a constant 5% APY becomes 10,500 after one year and 11,025 after two years, without additional deposits. The second year earns 525, compared with 500 in the first year.
The formula behind your projection
For end-of-month deposits: B = P(1+r)ⁿ + C((1+r)ⁿ − 1) / r. P is your initial balance, C is your monthly contribution, n is the number of months and r is the effective monthly rate.
For APY a, r = (1+a)^(1/12) − 1. For nominal annual rate j compounded m times a year, r = (1+j/m)^(m/12) − 1. Rates in these formulas are decimals. Beginning-of-month deposits receive one additional month of growth. At 0%, the result is simply P + C × n.
We model smooth monthly growth, including for yearly compounding. Actual bank crediting schedules, daily balances and rounding may differ. Values are rounded only for display.
A few useful answers
Is APY the same as an annual interest rate?
APY is an effective annual yield that includes compounding. A nominal annual rate does not. Select the type that matches your quoted rate so compounding is not counted twice.
What does “start 5 years later” assume?
Your initial money stays as idle cash earning 0% for five years. Monthly contributions also start five years later. Both plans finish at the same date. The difference includes missed contributions and missed growth; it is not all lost interest.
Why does saving for five more years make a difference?
This scenario has a later finish date, with five additional years of contributions and growth. It is a longer plan, not a comparison over the same duration.
Does this predict stock market returns?
No. It applies your chosen rate consistently every month. Actual returns fluctuate; taxes, fees and inflation can reduce the outcome.
Method & privacy
Calculations run locally. Entered balances and rates are not sent to analytics. No financial advice or guaranteed return is implied. Method updated October 19, 2026.
Learn more from the SEC’s Investor.gov compound interest calculator. Explore our other free calculators.