How this calculator works
Simple interest accrues only on the original principal. Elapsed days are divided by 365 under Actual/365 Fixed, including days in a leap year. Supply the day count directly so there is no ambiguity about which date endpoints to include. The calculation does not compound interest.
Formula
t = elapsed days/365; I = P × r × t; final balance = P + I. P is the original principal and r is the simple annual rate as a fraction.
Worked example
For $10,000 at 5% over 90 days, interest is $123.29 and the ending balance is $10,123.29. With zero elapsed days, interest is $0 and principal is unchanged.
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Frequently asked questions
Is a leap year divided by 366?
Not under Actual/365 Fixed. Every elapsed day uses 365 as the denominator. Actual/366 is a different convention.
Will this match my contract?
Only if the agreement uses simple interest with Actual/365 Fixed. Actual/360, 30/360 and compound-interest agreements need different calculations.
Does the interest compound?
No. Interest accrues on the original principal only and is not added to the balance during the term.