How to Calculate Simple Interest by Days: 365 vs 360

 

Simple interest is calculated on the original principal without adding earlier interest to the balance. When a period is expressed in days, the calculation needs a year basis as well as a day count. Dividing by 365, 360 or 366 produces different year fractions and therefore different interest amounts.

The Simple Interest Calculator estimates interest and principal plus interest using one fixed year basis. It assumes the principal and annual rate remain unchanged for the entire period. It does not calculate a repayment schedule or automatically count days between dates.

Use the simple interest formula

The formula is interest = principal x (annual interest rate / 100) x (days / year basis). Divide the annual percentage by 100 before multiplying. The days divided by the chosen basis represent the fraction of a year used in the calculation.

For example, 6 percent becomes 0.06. A period of 90 days on a fixed 365-day basis represents 90 / 365 of a year. The original principal remains unchanged; interest is added separately to show the combined amount.

Calculate 90 days of interest

Enter Principal amount 10000, Annual interest rate (%) 6 and Number of days 90. Leave Year basis at 365 days (fixed), then select Calculate simple interest.

The unrounded calculation is 10000 x 0.06 x 90 / 365 = 147.945205... . Rounded to the nearest cent, Simple interest is 147.95. Adding it to the principal gives Principal plus interest of 10147.95.

Compare a 360-day basis

Keep the same principal, rate and day count, but change Year basis to 360 days (fixed). Calculate again. Interest becomes 10000 x 0.06 x 90 / 360 = 150.00. Principal plus interest is 10150.00.

The smaller denominator makes the year fraction larger. The annual rate has not changed. Use the basis required by the applicable agreement rather than choosing a basis merely because its result is more favorable.

Understand fixed bases and date conventions

A fixed 365-day basis divides every entered day count by 365. A fixed 360-day basis divides by 360. A fixed 366-day basis divides by 366. This calculator applies one denominator to the entire entered period.

A 360-day denominator alone does not implement a 30/360 convention. Those conventions also adjust the day count derived from calendar dates, and their variants have different rules. Enter a day count that has already been determined according to the relevant agreement.

Similarly, choosing 366 does not automatically implement Actual/Actual. Some Actual/Actual methods split a period across calendar years and apply different denominators to different portions. A single fixed basis cannot reproduce every date-based convention.

Check rounding and zero values

The calculator rounds the final interest amount half up to two decimal places. It does not round a daily interest charge before multiplying by days. A lender or account provider using daily rounding may therefore produce a different total.

Zero days or a zero annual rate gives zero interest, leaving principal plus interest equal to the original principal. A very small positive interest amount can also round to zero cents. The calculator flags that rounding case rather than implying the annual rate itself is zero.

Distinguish simple interest from compounding

With simple interest, accrued interest does not earn additional interest. When other inputs remain unchanged, doubling the days doubles unrounded interest. With compounding, interest is added to a balance at specified intervals, changing later interest calculations.

Use the Compound Interest Calculator for compounded growth. For regular monthly interest payments and a final principal balloon, use the Interest-Only Loan Calculator. Its monthly rounding model differs from this final-rounding calculation.

This estimate excludes fees, taxes, principal repayments during the period and variable rates. It is not APR including charges or an effective annual yield. Check those details before comparing the result with a contractual statement.