Simple Interest vs Compound Interest
Simple interest is calculated only on the original principal you invested or borrowed. Because the base never changes, your interest grows in a straight line: $1,000 at 5% simple interest earns $50 every year, no matter how long you wait.
Compound interest is calculated on the principal plus any interest already earned. Each period the balance grows a little larger, so the next period earns interest on that larger amount. Over time this creates exponential growth — and that is why compounding is often called the eighth wonder of the world.
How to Use the Interest Calculator
- Enter the principal amount. The starting money you invest or the loan you take out.
- Enter the annual interest rate. Use the yearly rate as a percentage, e.g. 5 for 5%.
- Enter the time period in years. The longer the period, the bigger the compound advantage.
- Choose the compounding frequency. How often interest is credited each year — annually, semi-annually, quarterly, monthly or daily.
- Add a monthly contribution (optional). A fixed amount added each month, like a regular investment.
- Click “Calculate Interest”. Compare both final amounts, the interest earned, a growth chart and a year-by-year table.
How the Math Works
Compound: A = P × (1 + r/n)n×t
Where P is the principal, r is the annual rate divided by 100, t is the time in years, and n is the number of compounding periods per year. With monthly contributions, the tool adds them at the end of each compounding period so you can see the effect of a regular savings habit.
Why Compounding Wins Over Time
The longer the time horizon, the more dramatic the gap between simple and compound growth. After 30 years at 6% interest, a $10,000 principal grows to $28,000 with simple interest but over $57,000 with monthly compounding — the extra amount comes entirely from interest earning interest.
Frequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows linearly. Compound interest is calculated on the principal plus previously earned interest, so the balance grows faster over time — especially as the period gets longer.
How do I use the simple and compound interest calculator?
Enter your principal amount, annual interest rate, time period in years, and choose a compounding frequency. Optionally add a monthly contribution. Click Calculate Interest to compare the simple and compound final amounts side by side.
What does compounding frequency mean?
Compounding frequency is how often interest is added to your balance each year — annually, semi-annually, quarterly, monthly or daily. The more frequently interest compounds, the more interest you earn on interest, so your final amount grows slightly faster.
Is this calculator 100% free?
Yes. The simple and compound interest calculator is completely free for unlimited use with no sign-up, no hidden fees and no premium limits.
Does the tool store my data?
No. All calculations run locally in your browser. Your principal, rate, time and contribution amounts are never uploaded or stored on any server.