📈 Compound Interest Calculator
See how money grows exponentially — year by year — with compound interest.
Our free Compound Interest Calculator shows exactly how an investment or savings balance grows over time with interest compounding daily, monthly, quarterly, or annually. Enter your principal, annual rate, compounding frequency, and time period to see the full growth breakdown.
Compound Interest Formula
A = final amount | P = principal | r = annual rate (decimal) | n = compounding periods per year | t = years
Example — $10,000 at 8% for 30 Years
Annual compounding: $10,000 × (1 + 0.08/1)^(1×30) = $100,627
Monthly compounding: $10,000 × (1 + 0.08/12)^(12×30) = $109,358
Daily compounding: $10,000 × (1 + 0.08/365)^(365×30) = $109,752
The $9,125 difference between annual and daily compounding over 30 years shows why frequency matters — though time and rate matter far more.
The Rule of 72
The Rule of 72 is the fastest way to estimate how long it takes to double your money: divide 72 by your annual interest rate.
- At 4%: 72 ÷ 4 = 18 years to double
- At 6%: 72 ÷ 6 = 12 years to double
- At 8%: 72 ÷ 8 = 9 years to double
- At 10%: 72 ÷ 10 = 7.2 years to double
- At 12%: 72 ÷ 12 = 6 years to double
Why Time Is More Important Than Rate
This is the most important insight in all of personal finance. Consider two investors:
- Alice invests $5,000/year from age 25 to 35 (10 years, $50,000 total), then stops.
- Bob invests $5,000/year from age 35 to 65 (30 years, $150,000 total).
At age 65 with 8% annual returns: Alice has $602,070. Bob has $566,416. Alice wins by $35,654 despite investing three times less money — because her money had 10 extra years to compound.
Compound Interest Working Against You
The same mathematics works identically on debt. A $5,000 credit card balance at 24% APR, paying only the 2% minimum payment, takes over 10 years to pay off and costs more than $5,000 in interest — doubling the original debt. High-interest debt elimination is the highest guaranteed return available to most people.
📈 See your money grow year by year with our free Compound Interest Calculator.
Open Calculator →Frequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest — interest on interest. Over long periods, this difference is enormous. $10,000 at 8% simple interest for 30 years = $34,000. At 8% compound interest = $100,627.
How often should interest compound for the best results?
More frequent compounding produces slightly higher returns. Daily compounding produces more than monthly, which produces more than annual. However, the difference in compounding frequency has far less impact than the rate and time period. Focus on rate and starting early rather than compounding frequency.
What is a realistic long-term investment return to use?
The S&P 500 has returned approximately 10% annually (7% after inflation) over the long term. High-yield savings accounts currently offer 4–5%. CDs vary by term. Use a conservative rate (6–7%) for long-term retirement planning to avoid overestimating your future balance.