Compound Interest Calculator
See how your money grows over time, or flip it around: how much do you need to save monthly to hit a target by a certain age?
How this calculator works
Compound interest means your money earns returns, and then those returns earn their own returns. Both modes here use the standard future-value-of-annuity formula for monthly deposits with monthly compounding: FV = deposit × ((1 + monthly rate)^months − 1) ÷ monthly rate. The "how much per month" mode just solves that same formula for the deposit instead of the future value.
Worked example
Want $1,000,000 by age 65 at a 7% average annual return? Starting at age 20 takes just $264/month. Wait until 30 and it jumps to $555/month (more than double, for the exact same $1,000,000 goal, just from waiting 10 years).
Frequently asked questions
Is 7% a guaranteed return?
No. 7% is a commonly used long-run historical average for a diversified stock-heavy portfolio (before inflation), not a promise. Real returns vary year to year and can be negative.
Why does starting earlier matter so much?
Because growth compounds on growth. Money deposited at age 20 has 10 more years to compound than money deposited at age 30, so each early dollar does far more work than a later dollar.
Does this account for inflation?
No. This shows nominal dollars. $1,000,000 in 40 years will not buy what $1,000,000 buys today. Plan your real target with that in mind.
What accounts should I actually use for this?
Common tax-advantaged options include a 401(k) (see our 401(k) calculator), an IRA, or a taxable brokerage account. This tool models the math only, not account rules or tax treatment.
Assumptions & sources
- End-of-month deposits, monthly compounding, a single fixed average annual return for the whole period.
- 7% is a common, round, long-run historical assumption for a stock-heavy portfolio and is not guaranteed; past performance does not predict future returns.
- Closed-form future-value-of-annuity formula (standard financial math, see any introductory finance textbook or the SEC's investor.gov compound interest calculator for the same formula).
Educational estimates, not financial advice.