Simple Interest Calculator
Solve any variable in the fundamental interest equation.
Simple Interest Calculator
Simple interest is the most straightforward way to calculate interest on a loan or deposit. No compounding — just multiply principal by rate by time. This calculator handles it instantly.
What Is Simple Interest?
Simple interest is calculated only on the original principal amount, not on previously accumulated interest. It's used for short-term loans, some car loans, and treasury bills. The formula is clean and transparent.
How to Use This Calculator
- Enter the principal amount (original deposit or loan amount).
- Enter the annual interest rate as a percentage.
- Enter the time period in years (or months — select the unit).
- Click Calculate to see total interest and final amount.
Simple Interest Formula
I = P × R × T
- I = Interest | P = Principal | R = Annual rate (as a decimal) | T = Time in years
Total Amount = P + I
Example Calculation
Principal: $5,000 | Rate: 6% annually | Time: 3 years
- I = $5,000 × 0.06 × 3 = $900
- Total amount = $5,000 + $900 = $5,900
Simple vs. Compound Interest
On the same $5,000 at 6% for 3 years with monthly compounding, you'd earn $983 instead of $900. Compound interest earns more because it calculates on accumulated interest. See our Compound Interest Calculator for comparison.
Common Mistakes to Avoid
- Using the rate as a whole number — 6% must be entered as 0.06 in the formula. The calculator handles this for you.
- Mixing time units — If the rate is annual, time must be in years. For 18 months, use T = 1.5.
- Assuming all loans use simple interest — Mortgages, credit cards, and most savings accounts use compound interest.
Frequently Asked Questions
When is simple interest used in real life?
Auto loans often use simple interest. Short-term personal loans, payday loans (unfortunately), and some student loans also use simple interest. US Treasury Bills use a simple interest (discount rate) method.
Is simple interest better for borrowers?
Yes — simple interest borrowers pay less over time than compound interest borrowers at the same rate. But for savers, compound interest earns more.
Can I calculate simple interest for days instead of years?
Yes: T = Days ÷ 365 (or 360 for some bank calculations). For 90 days: T = 90/365 = 0.2466 years.
What is the daily simple interest mortgage?
Some mortgages charge simple interest daily on the outstanding balance. Making payments earlier in the month reduces total interest owed.
Conclusion
Simple interest is transparent, predictable, and easy to verify. Use this calculator for any scenario where interest is calculated only on the original principal — loans, short-term deposits, or quick financial estimates.
Related: Compound Interest Calculator | Interest Calculator | Loan Calculator | Savings Calculator
Expert Tip
Simple interest is standard for short-term personal loans. If you're building long-term wealth, look for compound interest accounts to leverage the power of reinvestment.
