Interest Calculator
Calculate simple interest on any amount using an annual rate and a time period in years, months or days.
Interest Calculator — input your values
What does this financial calculator do?
The interest calculator computes simple interest using the classic formula I = P × r × t, where P is the principal, r is the annual rate and t is the time. You can express the time in years, months or days, and the calculator converts it for you.
The result shows the interest earned or owed, the total balance, and the interest as a percentage of the principal. Simple interest is the right model for many short-term loans, bonds and basic savings comparisons, where interest is not added to the balance.
Formula and variables
Total balance = P + I. Simple interest does not compound.
| Symbol | Meaning |
|---|---|
| I | Interest |
| P | Principal amount |
| r | Annual interest rate (as a decimal) |
| t | Time in years (months ÷ 12, days ÷ 365) |
Worked example
You deposit $10,000 in a bond paying 5% simple interest per year for 2 years.
- I = P × r × t = $10,000 × 0.05 × 2.
- I = $1,000.
- Total = $10,000 + $1,000 = $11,000.
Answer: You earn $1,000 in interest, leaving a balance of $11,000.
Tips and common mistakes
Tips
- Banks quote most savings rates as annual, but pay monthly — divide the annual rate by 12 to see what you earn each month.
- For short periods, using days (t = days ÷ 365) gives the most accurate simple interest figure.
- Simple interest suits short-term bonds and promissory notes; long-term savings earn more with compound interest.
- Check whether a loan uses simple interest or adds unpaid interest to the balance — the two cost very different amounts.
Common mistakes to avoid
- Using the rate as a whole number (5) instead of a decimal (0.05) in the formula.
- Entering months or days as if they were years, which inflates the interest dramatically.
- Assuming interest is paid on the growing balance — simple interest ignores prior interest entirely.
Interest Calculator — frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it never grows. Compound interest adds earned interest back to the balance, where it earns more interest — a difference that becomes huge over long periods.
How do I calculate simple interest in days?
Divide the number of days by 365 to convert to years, then apply I = P × r × t. For example, $5,000 at 6% for 90 days is $5,000 × 0.06 × (90 ÷ 365) ≈ $73.97.
When is simple interest used?
Commonly for short-term loans, some bonds, car loans in certain countries, and any situation where interest is not added to the principal during the term.
Is simple interest better for borrowers or savers?
For borrowers it usually means lower total interest than compounding; for savers it is usually worse, because the balance never snowballs. On equal terms, compound interest grows money faster.
What does the daily rate tell me?
The daily rate (annual rate ÷ 365) is the interest charged or earned each day. Lenders often use it to compute interest on partial months.
Does the interest calculator handle negative rates?
No — a rate below 0% is treated as an error, since negative interest is rare and usually reflects a typo. Enter 0 for an interest-free situation.