Precision Utility
Compound Interest
Calculator
S&P 500 Avg
10.0%
Real Return
~7%
See how your savings and investments grow over time with the power of compound interest. Enter your initial deposit, monthly contribution, interest rate and time period — the calculator shows your future value, total contributions, interest earned and a full year-by-year growth schedule. Built for US investors and savers.
Investment Parameters
Future Value
$0
Future Value
$0
Total Deposited
$0
Interest Earned
$0
Rule of 72
— yrs
The Rule of 72 — how fast does your money double?
The Rule of 72 is a simple shortcut to estimate how long it takes for an investment to double in value. Divide 72 by your expected annual rate of return and you get the approximate number of years to double your money.
For example, at a 7% annual return (the inflation-adjusted S&P 500 average), your money doubles in roughly 10.3 years. At 10%, it doubles in about 7.2 years. At a savings account rate of 4%, it takes 18 years.
The rule works best for rates between 6% and 10%. For very low or very high rates, the estimate becomes less precise, but it remains a useful mental shortcut for quick financial planning.
Compound interest and your Roth IRA
A Roth IRA is one of the most powerful tools for tax-free compound growth available to US investors. Contributions are made with after-tax dollars, but all growth and qualified withdrawals in retirement are completely tax-free.
Key Roth IRA facts for 2025/2026:
- Annual contribution limit: $7,000 (or $8,000 if you're 50 or older)
- Income limits apply: single filers earning above $161,000 (MAGI) begin to phase out
- No required minimum distributions (RMDs) during the owner's lifetime
- Contributions (not earnings) can be withdrawn at any time without penalty
- The earlier you start, the more decades of tax-free compounding you benefit from
Use this calculator with $583/month ($7,000 / 12) and a 7% return to see how maxing out a Roth IRA can build significant wealth over 20-30 years.
S&P 500 historical average returns
The S&P 500 — an index of 500 large US companies — is often used as a benchmark for long-term investment growth. Since its inception, it has delivered an average annual return of approximately 10% before inflation and about 7% after inflation.
Important context for these numbers:
- Returns vary dramatically year to year — the index has gained over 30% in some years and lost over 30% in others
- The 10% average assumes dividends are reinvested, which is a form of compounding
- Past performance does not guarantee future results
- Dollar-cost averaging (investing a fixed amount monthly) helps smooth out volatility over time
When using this calculator for retirement projections, 7% is a widely used conservative estimate that accounts for inflation. Use 10% for a nominal (before-inflation) projection.
How the compound interest calculator works
Enter your initial deposit — the lump sum you're starting with. Then set your monthly contribution — the amount you plan to add each month. Choose your expected annual interest rate and investment period.
Select the compounding frequency: monthly (most common for savings accounts and index funds), quarterly, or annually. More frequent compounding produces slightly higher returns.
The calculator uses the standard future value formula with periodic contributions:
FV = PV(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)]
Where PV is present value, PMT is the periodic payment, r is the annual rate, n is the compounding frequency and t is time in years. The year-by-year schedule breaks this down so you can see exactly how your balance grows over time.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, your money grows exponentially because you earn interest on your interest.
How does compounding frequency affect my returns?
More frequent compounding (e.g. monthly vs annually) means interest is calculated and added to your balance more often, resulting in slightly higher returns. Monthly compounding is the most common for US savings accounts and investments.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes to double your money. Divide 72 by your annual interest rate. For example, at 7% annual return, your money doubles in approximately 72 / 7 = 10.3 years.
What is the average return of the S&P 500?
The S&P 500 has returned an average of roughly 10% per year since inception, or about 7% after adjusting for inflation. Past performance does not guarantee future results, but many financial planners use 7% as a reasonable long-term estimate.
How much should I save each month?
A common guideline is the 50/30/20 rule: 50% of income for needs, 30% for wants and 20% for savings and investments. The right amount depends on your income, goals and timeline. Even small monthly contributions compound significantly over decades.
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the stated annual rate without compounding. APY (Annual Percentage Yield) includes the effect of compounding. A 7% APR compounded monthly gives an APY of about 7.23%. APY always equals or exceeds APR.
Can I use this calculator for Roth IRA projections?
Yes. Enter your current Roth IRA balance as the initial deposit, your planned annual contribution divided by 12 as the monthly contribution, and an estimated annual return (e.g. 7%). The result shows your projected balance at the end of the period — tax-free in a Roth IRA.
Does this calculator account for taxes?
This calculator shows gross returns before taxes. In taxable accounts, investment gains may be subject to capital gains tax. Tax-advantaged accounts like Roth IRAs and 401(k)s shelter your gains from annual taxation.