Introduction
A daily interest calculator helps you find out how much interest builds up on a sum of money over a set number of days. Banks, lenders, and investors use daily interest to figure out what is owed or earned between payment dates. This tool calculates the interest for you, so you do not have to work it out by hand.
To use it, enter your principal amount, your annual interest rate, and the number of days. You can also pick a start date and end date, and the calculator will count the days for you. It supports both the Actual/360 and Actual/365 day-count methods, so you can match the basis your lender or contract uses. The results show your total interest, your daily rate, and a full step-by-step breakdown of the formula so you can see exactly how the number was reached.
How to Use Our Daily Interest Calculator
Enter your loan or investment details below to find out how much interest builds up each day. The calculator will show you the total interest earned, the daily rate, and a full step-by-step breakdown.
Principal Amount ($): Type in the starting amount of money. This is the total you borrowed or invested before any interest is added.
Annual Interest Rate (%): Enter the yearly interest rate as a percentage. For example, type 4.250 for a rate of 4.250% per year.
Day-Count Basis: Choose how the year is divided. Pick Actual/365 to use a standard calendar year or Actual/360 to use a 360-day year, which some banks use.2 The calculator compares both so you can see the difference.
Start Date: Pick the date interest starts from. The start date itself is not counted, so the first day of interest is the day after it. This works with the end date to count the exact number of days for you.
End Date: Pick the last day of your interest period. It must be a date after the start date.
Number of Days: If you already know the number of days, type it here instead of using the date fields. Entering a number here will clear the start and end dates.
Press Calculate to see your results. Press Reset to start over. It clears the principal, dates and days and puts the rate and basis back to their starting values.
What Is Daily Interest?
Daily interest is the amount of interest that builds up on a loan or investment each day. Banks, credit card companies, and lenders use daily interest to figure out how much you owe or earn over time. The longer you hold a balance, the more interest adds up. Some card issuers add each day's interest to the balance, so the interest compounds daily.4 This calculator works out simple interest, where interest is calculated only on the original principal rather than on accumulated interest.
How Daily Interest Is Calculated
To find daily interest, you take three numbers: your principal (the starting amount of money), your annual interest rate, and the number of days. First, you divide the annual rate by the number of days in the year to get a daily rate. Then you multiply the principal by the daily rate and by the number of days.2 The result is the total interest earned or owed.
The Formula
The daily interest formula is:
Interest = Principal × (Annual Rate ÷ Days in Year) × Number of Days2
What Is a Day-Count Basis?
Not everyone agrees on how many days are in a year for interest purposes. There are two common methods:
- Actual / 365: Divides the annual rate by 365, the number of days in most years.2 US banks must pay interest on savings accounts and other deposit accounts using a daily rate of at least 1/365 of the interest rate, or 1/366 in a leap year.3
- Actual / 360: Divides the annual rate by 360. Some banks use 360 as the number of days in a year.2 A bank may pay interest on a savings account at 1/360 of the interest rate a day, as long as it applies that rate 365 days a year.3 Some Fannie Mae multifamily mortgage loans can accrue interest on an Actual/360 basis.5 Interest that federal agencies owe on late payments is computed on a 360-day year, and interest left unpaid at the end of each 30-day period is added to the principal, for up to one year.1 This calculator does not add interest to the principal, so for those late payments its result matches only periods of 30 days or less. Because you divide by a smaller number, each day's interest is slightly higher, so over the same period you pay more on a loan and earn more on savings.2
Why Daily Interest Matters
Understanding daily interest helps you see the real cost of borrowing money or the true return on your savings. Even a small difference in the interest rate or the day-count method can change the total amount you pay or earn, especially over many days. Paying off a loan sooner means fewer days of interest, which saves you money.