Simple Compound Interest Calculator
Enter initial capital, annual return percentage, and investment time in years. The calculator shows final balance, accumulated interest, total growth, and capital doubling time.
Final balance
Interest earned
Total growth
Doubling time (rule of 72)
Compound Interest Calculator – See How Your Money Grows Over Time
Compound interest is often called the eighth wonder of the world — and for good reason. When your investment earns returns on its returns, growth accelerates exponentially. This calculator shows you exactly how much your money will grow over any time period at any interest rate.
The Compound Interest Formula
A = P × (1 + r)ⁿ
Where:
- A = final balance
- P = initial principal (starting amount)
- r = annual interest rate as a decimal (e.g., 7% = 0.07)
- n = number of years
For monthly compounding, the more precise formula is: A = P × (1 + r/12)^(12×n)
Worked Example
You invest $10,000 at a 7% annual return (the approximate historical average of the S&P 500 after inflation):
| Years | Balance | Total Growth | Interest Earned |
|---|---|---|---|
| 5 | $14,026 | +40% | $4,026 |
| 10 | $19,672 | +97% | $9,672 |
| 15 | $27,590 | +176% | $17,590 |
| 20 | $38,697 | +287% | $28,697 |
| 25 | $54,274 | +443% | $44,274 |
| 30 | $76,123 | +661% | $66,123 |
Notice how growth nearly doubles with each new decade — that is the exponential power of compounding. After 30 years, over 86% of your balance is pure interest, not your original investment.
The Rule of 72
A quick mental shortcut: divide 72 by your annual return rate to estimate how many years it takes for your money to double.
| Annual Return | Doubling Time |
|---|---|
| 2% | 36 years |
| 3% | 24 years |
| 5% | 14.4 years |
| 7% | 10.3 years |
| 10% | 7.2 years |
| 12% | 6 years |
Simple Interest vs. Compound Interest
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Returns calculated on | Original principal only | Principal + accumulated interest |
| Growth pattern | Linear (steady) | Exponential (accelerating) |
| $10,000 at 7% for 20 years | $24,000 | $38,697 |
| Best for | Short-term, simple loans | Long-term investments and savings |
The difference between simple and compound interest grows dramatically over time. After 20 years at 7%, compound interest earns $14,697 more than simple interest on the same $10,000 principal.
How Compounding Frequency Affects Returns
The more frequently interest compounds, the more you earn:
| Compounding | $10,000 at 5% for 10 years | Effective Annual Yield |
|---|---|---|
| Annually | $16,289 | 5.000% |
| Quarterly | $16,386 | 5.094% |
| Monthly | $16,470 | 5.116% |
| Daily | $16,487 | 5.127% |
The difference between annual and daily compounding on $10,000 over 10 years is about $198 — meaningful but not dramatic. Over 30 years, the gap widens significantly.
Why Starting Early Matters Most
The most powerful variable in compound interest is time, not the amount you invest. Consider two scenarios:
- Investor A starts at age 25, invests $200/month for 10 years (total: $24,000), then stops contributing. At 7%, by age 65 they have $353,000
- Investor B starts at age 35, invests $200/month for 30 years (total: $72,000). At 7%, by age 65 they have $227,000
Investor A invested one-third the amount but ended up with 56% more money — purely because of 10 extra years of compounding.
APR vs. APY
- APR (Annual Percentage Rate) — The stated nominal interest rate, not accounting for compounding frequency
- APY (Annual Percentage Yield) — The actual effective rate after compounding. APY ≥ APR
When comparing savings accounts or investments, always compare APY for an apples-to-apples comparison.
Frequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal each period. Compound interest is calculated on the principal plus all previously accumulated interest, causing exponential growth. Over long periods, compound interest dramatically outperforms simple interest.
How often does compound interest compound?
It can compound daily, monthly, quarterly, semi-annually, or annually. More frequent compounding produces slightly more interest. Most savings accounts compound daily, while many bonds compound semi-annually.
What is the compound interest formula?
A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual rate as a decimal, n is compounding periods per year, and t is time in years. For annual compounding, this simplifies to A = P × (1 + r)^t.
How much does $10,000 grow at 7% compound interest for 10 years?
Using annual compounding: $10,000 × (1.07)^10 = approximately $19,672 — nearly doubling without any additional contributions. With monthly compounding, it would be about $20,097.
What is APY vs APR?
APR (Annual Percentage Rate) is the stated nominal rate. APY (Annual Percentage Yield) is the effective rate after compounding is factored in. A 5% APR compounded monthly produces a 5.116% APY. Always compare APY when evaluating savings products.
Related Tools
- Investment Return Calculator — Calculate ROI and annualized returns
- Compound Interest Calculator — Advanced version with monthly contributions
- Savings Goal Calculator — Plan how much to save monthly
- CD Calculator — Compare certificate of deposit earnings
- Retirement Calculator — Project long-term retirement savings