Simple Interest Calculator
Enter the principal, interest rate, and holding period to see the simple interest earned or owed, the maturity balance, and how that translates into monthly and daily amounts.
How simple interest is computed
Simple interest grows linearly with time and does not compound. The accumulated balance is , where is the principal, is the annual interest rate expressed as a decimal, and is the time in years. The interest earned is simply . Because the formula is linear, each additional day of holding contributes the same dollar amount as every other day.
This tool converts months into fractional years by dividing by 12 and days by 365 so you can mix billing cycles and due dates. The optional fee field subtracts upfront costs from the net payout, highlighting how origination charges reduce the effective return for investors or raise the cost for borrowers.
Example schedules
| Scenario | Principal | Rate | Time | Interest | Maturity total |
|---|---|---|---|---|---|
| Short-term equipment loan | $4,500 | 6.5% | 18 months | $438.75 | $4,938.75 |
| 90-day treasury bill | $10,000 | 4.2% | 90 days | $103.29 | $10,103.29 |
| One-year bridge loan with fee | $250,000 | 9.0% | 12 months | $22,500.00 | $272,500.00 |
Compare with other money tools
After running the simple interest projection, explore how compounding changes the outcome using the Compound Interest Calculator, evaluate structured payments with the Loan Payment Calculator, and set long-term targets via the Savings Goal Calculator.