Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this simple interest calculator
Choose the unknown quantity, then enter the other values. Simple interest uses the original principal without compounding.
How the calculation works
I = P × r × t, with r as an annual decimal rate and t in years. Months divide by 12; days divide by the chosen 365 or 360 basis.
Reverse forms: P = I/(r×t), r = I/(P×t), t = I/(P×r). A zero denominator can make the question impossible or indeterminate; the calculator requests different inputs instead of inventing an answer.
Ending amount = P + I. No interest is earned on earlier interest.
Worked example
$10,000 at 5% simple annual interest for 2 years earns $1,000, giving $11,000 total. Solving backward from $1,000 interest, 5% and 2 years gives $10,000 principal. The same 2 years is 24 months or 730 model days on a 365-day basis.
Limits and assumptions
One principal and one nonnegative simple rate. No changing balance, payment schedule, compounding, fees, actual-date convention, taxes or jurisdiction-specific disclosure rules. A simple-interest amortizing loan with changing principal requires a repayment model instead.
Frequently asked questions
Can I work backward from the interest?
Yes. Choose Principal, Annual simple rate or Time needed. Enter interest alone, not the final total.
Why offer both 360 and 365 days?
Different scenarios use different fixed-year bases. This tool makes the choice explicit and does not infer your contract’s rule.
Does this reproduce a loan statement?
Not when principal changes after repayments or the lender uses another timing/fee rule. Use an amortization tool for that workflow.