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Simple Interest Calculator (by Days)

Calculate simple interest and the total amount from a principal, annual rate, and number of days. Uses 365-day daily accrual and can work out the days automatically from a start and end date.

Input

$
%

How to set the number of days

days

Result

Interest (3% annual · 90 days · simple)

$7,397.26

Principal

$1,000,000.00

Principal + interest

$1,007,397.26

Days elapsed

90 days

How it works

  • Interest is calculated as simple interest: principal × annual rate × days ÷ 365. Because interest is not added back to the principal during the term, it is lower than compound interest.
  • Days are counted on a 365-day-year basis (always 365 days, including leap years). In practice, conventions differ, such as a 360-day basis or including one or both endpoints.
  • Enter the number of days directly, or derive it automatically from a start and end date. The days are calculated as end date − start date.
  • The total is calculated as principal + interest, with the annual rate and principal assumed constant throughout the term.
  • Amounts are rounded for display. Actual interest may differ due to rounding rules or withholding tax.
  • Results from this tool are estimates and do not guarantee exact interest or tax amounts. Check the conditions with your financial institution or contract for real transactions.