Simple Interest Calculator by Days
Enter principal, annual rate, and the exact number of days. This simple interest calculator by days supports 360, 365, and 366-day conventions and shows a transparent formula check.
Written and checked by TheToolNet Editorial Team · Last reviewed: September 27, 2026 · Editorial method
Simple interest = principal × annual rate × days ÷ annual day-count basis.
Calculate simple interest daily
How the daily simple-interest calculation works
First convert the percentage rate to a decimal. Next multiply it by the fraction of a year represented by the entered days. Finally multiply that periodic rate by principal. Previously earned interest never enters the principal, which is what makes the method simple rather than compound.
Which day-count basis should you use?
| Basis | Typical interpretation | Effect |
|---|---|---|
| 360 | Commercial convention used by some contracts | Higher daily rate for the same nominal rate |
| 365 | Actual/365 style denominator | Common calendar-year comparison |
| 366 | Leap-year denominator | Slightly lower daily rate |
The contract controls. This calculator lets you compare conventions but cannot determine which one applies to a particular loan, deposit, judgment, or invoice.
Worked example
On 10,000 at 8% for 90 days using 365, interest is 10,000 × 0.08 × 90 ÷ 365. The result panel also divides the total interest by 90 to display the daily amount.
Simple versus compound daily interest
Simple interest always uses the original principal. Compound daily interest periodically adds interest to the balance. Compare the methods with the daily compound interest calculator.
Frequently asked questions
How do I find the annual rate from known interest?
Rearrange the formula: annual rate = interest × basis ÷ (principal × days). Multiply by 100 for a percentage.
Does this include payments or fees?
No. It calculates one principal for one day span. Payments, changing balances, penalties, taxes, and fees require a dated schedule.