Compound Interest & 401(k) Growth Calculator
Project the future value of savings with regular contributions, and see it in today's dollars.
Monthly compounding with end-of-month contributions. Last reviewed October 4, 2026. Sources · Methodology
How compound growth is calculated
The calculator compounds monthly. Each month the balance grows by the annual rate ÷ 12, then that month's contributions are added:
Balancenext = Balance × (1 + r ÷ 12) + monthly contributions
After the final month, the balance is divided by (1 + inflation)years to show its value in today's dollars.
Worked example: $10,000 to start, $500 a month, 7% for 30 years
- Future balance: $691,150
- Total deposited: $190,000
- Growth earned: $501,150
- Value in today's dollars at 3% inflation: $284,745
Why starting early matters
| Years | Deposited | Growth | Balance |
|---|---|---|---|
| 10 | $70,000 | $36,639 | $106,639 |
| 20 | $130,000 | $170,851 | $300,851 |
| 30 | $190,000 | $501,150 | $691,150 |
| 40 | $250,000 | $1,225,521 | $1,475,521 |
Saving for 20 years instead of 30 means depositing $60,000 less but ending with $390,300 less. The last ten years add more than the first twenty because growth is working on a much larger balance.
Using it for a 401(k)
- Enter your own monthly contribution and your employer's match as separate amounts.
- An employer match is an immediate return on your contribution, so contributing at least enough to receive the full match is usually worthwhile.
- Annual contribution limits are set by the IRS and change most years. Check the IRS page in the sources below for the current limit.
What this estimate leaves out
- Market swings. Real returns vary year to year; a steady rate is a simplification.
- Fees and taxes.
- Raises. Contributions stay flat here, while many people increase them over time.
Frequently asked questions
What is compound interest?
Compound interest is growth earned on both your original money and on the growth already added. Over long periods this snowball effect, not the deposits, produces most of the balance.
What rate of return should I use?
Nobody knows future returns. Many planners test a range, for example 4% to 8% a year for a diversified stock-heavy portfolio before inflation, and lower for bonds or savings accounts. Try several rates rather than relying on one.
How does the employer match work here?
Enter the dollar amount your employer adds each month. It is treated exactly like your own contribution and grows at the same rate.
What does the value in today's dollars mean?
It divides the future balance by expected inflation over the period, showing what that money would buy at today's prices.
Are taxes and fees included?
No. Investment fees reduce your return, and taxes depend on the account type. Traditional 401(k) and IRA withdrawals are taxed as income, while qualified Roth withdrawals are tax-free.
Sources
- U.S. Securities and Exchange Commission, Investor.gov: compound interest calculator
- IRS: 401(k) and profit-sharing plan contribution limits
Last reviewed October 4, 2026. Results are estimates; see our methodology and disclaimer.