Calculate simple interest on loans or investments with principal, rate, and time inputs showing total interest and final amount.
Simple interest is rent paid on money: the lender is compensated on the original principal only, never on interest already earned. This calculator takes three inputs - principal, annual interest rate, and time - applies the formula I = P x r x t, and returns both the total interest and the final amount. The thing most users get wrong is units: the rate is annual, so time must be measured in years too. Nine months is 0.75 years, not 9, and a rate quoted as 6% means 6% per year even when payments are made monthly.
A concrete input and expected output from the current implementation.
Input
Principal: $1,200 | Annual rate: 4% | Time: 9 months
Expected output
Total interest: $36.00 | Final amount: $1,236.00
The rate is annual, so 9 months converts to 0.75 years before multiplying: 1200 x 0.04 x 0.75 = 36. The final amount is the principal plus the interest: 1200 + 36 = 1236.
The tool computes I = P x r x t, where P is the principal, r is the annual rate written as a decimal (6% becomes 0.06), and t is the time in years, then adds the interest to the principal to get the final amount. Time entered in months or days is converted to years before the multiplication. Because the relationship is linear, doubling the rate or the term doubles the interest.
Simple interest does not compound, and that is what makes it predictable: every period earns the same amount because interest is never added back to the principal. One multiplication covers the whole term instead of a period-by-period recalculation. This matches loans where interest is paid as it accrues, but it understates any arrangement where unpaid interest joins the balance and earns interest of its own.
Interest rates are quoted per year, so the formula requires time in years. Nine months is 0.75 years and 18 months is 1.5 years - convert before entering, or the result is off by a factor of 12. The same rule applies the other way: a monthly rate must be multiplied by 12 to become annual before it fits the formula.
Because most banks compound. Credit cards, mortgages, and many auto loans accrue interest daily or monthly, and unpaid interest joins the balance and earns interest itself. Simple interest ignores that accumulation, so the figure here is the nominal interest only - fees and compounding move the real cost, usually upward. The APR a lender must disclose is the better comparison number.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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