Compound Interest Calculator
Use your own numbers after reviewing the formula and worked example.
Quick answer
Simple interest is calculated on original principal. Compound interest adds interest to the balance so later interest can be earned on earlier interest.
Formula or method
Keep units, percentages and time periods consistent. The formula is useful as a transparent check on the calculator result and helps explain why changing one input changes the answer.
Step-by-step
- Identify principal, annual rate and time.
- Use P×r×t for simple interest.
- For compound interest, choose the compounding frequency.
- Compare ending values using the same inputs.
Worked example
At 5% for 10 years, $10,000 grows to $15,000 with simple interest and about $16,288.95 with annual compounding.
How to interpret the result
The compounding effect grows with time, rate and frequency.
When the answer is used for a purchase, loan, payroll decision, construction order, school grade, health estimate or other real-world choice, verify the assumptions that matter in that context. Accurate arithmetic still depends on accurate inputs.
Common mistakes
- Mixing 5% with decimal 5 instead of 0.05.
- Mismatching rate and compounding periods.
- Comparing APY with nominal rate as if they were identical.
Frequently asked questions
Does more frequent compounding always matter a lot?
The difference can be modest at ordinary rates, but it grows with rate and time.
Can compound interest increase debt too?
Yes; compounding can increase savings or debt depending on context.
What is the easiest comparison?
Use identical principal, rate and time in both calculators.