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, which is common in commercial and government lending. The calculator compares both so you can see the difference.
Start Date: Pick the first day interest begins. 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 Clear to reset all fields and start over.
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. This is the basis of 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. 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 Days
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. This is the standard calendar year method and is widely used for personal loans, savings accounts, and most everyday calculations.
- Actual / 360: Divides the annual rate by 360. This method is common in commercial lending and mortgages, and federal prompt-payment interest is computed on a 360-day year.1 Because you divide by a smaller number, each day's interest is slightly higher, which means you pay more over the same period.
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.