A Simple Interest Calculator computes interest as principal times rate times time, without compounding. Short-term loans, some notes, and classroom problems still use simple interest. Enter the principal, annual rate, and time in years or days according to the tool. A Simple Interest Calculator is also useful as a lower bound when comparing with compound interest, because compound interest grows faster when all else is equal. People like it because the formula is transparent. The trap is using simple interest on a product that actually compounds, which understates cost or growth. Matching the convention to the contract keeps the transparency.
Convert time into the units the Simple Interest Calculator expects. Ninety days is not 90 years, and 90/365 is not the same as 90/360 if a bank uses a banker year. Some consumer loans quote add-on simple interest that still produces a payment schedule; read the contract. If interest is discounted up front, proceeds differ from face value. Compare a simple-interest scenario with a compound scenario on the same nominal rate to see the gap. For savings, simple interest is uncommon today except as a teaching contrast. Document the day-count. Then check the result by hand once: principal times rate times time. If that mental check disagrees, you entered a percent as a decimal twice or a decimal as a percent. The calculator should be a convenience, not a fog machine over a formula that is meant to stay visible. The extended discussion of Simple Interest Calculator covers workflow, mistakes.