Compound Interest Calculator
Small contributions can become serious wealth. See what compounding can do.
Model your starting balance, monthly contributions, annual return, and time horizon to see how your investments could grow.
Compound Interest CalculatorCalculate your investment growth
Project your long-term investment growth
Calculate how compound interest can grow your investments over time.
Growth projection
Projected value over time
Compare your cumulative contributions against the interest that may compound over the selected time span.
Compound growth guide
Turn a growth estimate into a better savings plan
Compound interest is easiest to understand when you separate what you control from what you assume. Contributions and time are plan inputs; returns, fees, taxes, and inflation are variables to stress test.
How compound interest is calculated
The standard formula grows an initial principal by applying interest repeatedly. This calculator also adds monthly contributions, then compounds the growing balance each month.
Compound interest formula
A is the future value.
P is the starting principal.
r is the annual rate.
n is compounding frequency.
Included in this estimate
- Initial investment and monthly contributions
- Monthly compounding at the annual return you enter
- Total contributed, interest earned, and projected value
- Annual chart comparing contributions against interest
Adjust before you rely on it
- Investment fees, fund expenses, and advisory costs
- Taxes on interest, dividends, or realized gains
- Inflation and changes in purchasing power
- Market volatility, sequence risk, or variable returns
Growth drivers
Inputs that shape your future value
Use the calculator to isolate one variable at a time. That makes the tradeoff between saving more, investing longer, and assuming higher returns easier to see.
Starting balance
Your initial deposit begins compounding right away and gives future returns a larger base to build on.
Monthly contribution
Recurring deposits can matter as much as the starting amount, especially when they continue for many years.
Annual return
The calculator assumes a steady annual return, so test conservative and optimistic scenarios instead of relying on one number.
Time horizon
Time is the compounding engine. Longer periods give interest more chances to earn interest of its own.
Planning strategy
How to make compounding work harder
The most useful compound interest estimate is not a single final number. It is a comparison of scenarios that shows how behavior, time, costs, and risk can change the outcome.
Start earlier
Adding time can be more powerful than chasing a higher return assumption.
Automate deposits
Monthly contributions make the plan repeatable and reduce the chance of skipping investment periods.
Watch fees
Small annual fees can compound in the wrong direction and reduce the ending balance.
Match risk to goals
Higher expected returns usually come with more uncertainty, so align the assumption with the account and time horizon.
Before you invest
Use projections as planning ranges
Real portfolios rarely compound at the same rate every month. Run low, middle, and high scenarios so your plan can survive lower returns or higher costs.
Planning checklist
- Run a baseline using the amount you can invest without straining your budget.
- Compare several annual return assumptions instead of treating one estimate as a forecast.
- Adjust for fees, taxes, and inflation before using the number for retirement or education planning.
- Revisit your inputs after major income, expense, or market changes.
Compound interest FAQ
Common questions about compounding
What is compound interest?
Compound interest is interest earned on your original money and on interest that has already been added to the balance. Over time, that reinvested interest can become a larger share of the total value.
How is compound interest different from simple interest?
Simple interest is calculated only on the original principal. Compound interest adds prior interest to the balance, so future interest is calculated on a larger amount.
Does this calculator guarantee an investment return?
No. It is a planning estimate based on a constant annual return and monthly compounding. Actual investments can rise, fall, and produce uneven returns.
How much should I invest each month?
Use an amount you can sustain after essential expenses, emergency savings, and high-interest debt. Consistency usually matters more than an aggressive starting target.
What is the Rule of 72?
The Rule of 72 is a quick estimate for how long money may take to double: divide 72 by the annual return percentage. It is only a rough shortcut.
Does compounding frequency matter?
Yes, but the impact is usually smaller than contribution size, time horizon, and return assumptions. This calculator compounds monthly.
Keep exploring
Turn your numbers into next steps
Dig into the guides behind the math or run a different calculator to round out your plan.
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