Compound Interest Calculator
Free compound interest calculator — see how your investments grow over time with different contribution schedules and compounding frequencies. No affiliate bias.
Future Value
$0
Total Contributions
$0
Interest Earned
$0
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$0$500K
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$0$5K
%
0%20%
years
1 yr40 yrs
Growth Over Time
Formula Used
FV = P × (1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)]
Where:
FV = Future value
P = Initial principal
r = Annual interest rate (decimal)
n = Compounding periods per year
t = Time in years
PMT = Regular contribution per compounding period
For monthly contributions with monthly compounding:
FV = P × (1 + r/12)^(12t) + PMT × [((1 + r/12)^(12t) - 1) / (r/12)]
How This Compound Interest Calculator Works
Compound interest is the most powerful force in finance. Our calculator uses the standard future value formula, accounting for both your initial investment and regular contributions.
Why Compounding Frequency Matters
- More frequent compounding = slightly more growth: $10K at 7% for 20 years: annual = $38,697, daily = $40,552
- The real magic is time: Starting 5 years earlier with the same contributions can mean 40%+ more at retirement
- Consistent contributions win: $500/month for 20 years at 7% = ~$260K, vs only $38K from the initial $10K alone
The Rule of 72
Divide 72 by your rate to estimate doubling time. At 7%: 72/7 ≈ 10.3 years to double. At 10%: 7.2 years. This works for any rate.
Frequently Asked Questions
How does compound interest work?
Compound interest is interest earned on both your original principal and on previously accumulated interest. Unlike simple interest (which only earns on the principal), compounding creates exponential growth over time. For example, $10,000 invested at 7% compounded monthly grows to ~$40,000 in 20 years — far more than the $24,000 you'd have with simple interest. The more frequently interest compounds, the faster your money grows.
What is the compound interest formula?
The future value with compound interest is calculated as: FV = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is the number of compounding periods per year, and t is the number of years. For regular contributions, a more complex future value of an annuity formula is added. Use CalcDeck's free compound interest calculator above to model your exact scenario including monthly contributions.
How often should interest be compounded to maximize returns?
More frequent compounding always yields higher returns — daily compounding beats monthly, which beats quarterly, which beats annually. However, the difference diminishes. On a $10,000 investment at 7% over 20 years, daily compounding yields about $250 more than annual compounding. The bigger factor is your rate of return and how long you stay invested. CalcDeck's calculator lets you toggle between annual, quarterly, monthly, and daily compounding to see the real difference.
What is the Rule of 72 and how do I use it?
The Rule of 72 is a quick formula to estimate how long it takes your money to double: divide 72 by your expected annual return rate. At 7%, money doubles in ~10.3 years (72 ÷ 7). At 10%, it doubles in ~7.2 years. This simple mental math helps you compare investment opportunities without a calculator. For precise projections including regular contributions, use CalcDeck's compound interest calculator.
How much should I invest each month to reach my savings goal?
Your required monthly contribution depends on three factors: your goal amount, your time horizon, and your expected return rate. For example, to reach $1 million in 30 years at 7% annual return, you'd need to invest about $850 per month (starting from $0). Use CalcDeck's compound interest calculator to adjust all three variables and find the monthly contribution that fits your budget.
Why is starting to invest early so important?
Starting early harnesses the exponential nature of compound interest. A 25-year-old investing $500/month at 7% will have ~$1.2 million at 65. Starting at 35 yields only ~$567,000 — less than half, even though you only contributed $60,000 more. The 'cost of waiting' is enormous because you miss years of compounding on your earliest contributions. Use CalcDeck's calculator to compare different starting points and see the staggering difference.