Daily Interest Calculator

Updated Jun 26, 2026 By Jehan Wadia

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. Instead of doing the math by hand, this tool does it for you in seconds.

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. If you need to figure out the rate on an existing loan, try our interest rate calculator.

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. If you need help finding the exact number of days between two dates, our date duration calculator can help.

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, mortgages, and government prompt-payment rules. 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. To see how interest grows when it is reinvested on itself rather than calculated on the principal alone, check out our daily compound interest calculator. You can also use our compound interest calculator for a broader look at how compounding affects your balance over months or years. If you carry a balance on a credit card, our credit card interest calculator can show you how daily accrual impacts what you owe each billing cycle. And for a general overview of interest on any loan or investment, our interest calculator is a helpful starting point.

Formulas used

Number of Days from Date Range

d = \text{End Date} - \text{Start Date} \quad (\text{in days})

Daily Interest Rate

r_{\text{daily}} = \frac{r}{D}

Accrued Interest

I = P \times \frac{r}{D} \times d

Total Payment

\text{Total} = P + I

Frequently asked questions

What is the difference between Actual/360 and Actual/365?

Actual/360 divides the annual rate by 360 days. Actual/365 divides it by 365 days. Since 360 is smaller, each day's interest is a little higher with Actual/360. Over the same number of days, you will pay more interest with Actual/360 than with Actual/365. The calculator shows both results side by side so you can compare.

Can I use this calculator for credit card interest?

You can use it to estimate how much simple interest builds up each day on a balance. However, most credit cards use compound interest, where unpaid interest gets added to the balance and earns more interest. This calculator uses simple interest only. For credit card balances, the result here will be close for short periods but may understate interest over longer ones.

Does this calculator account for compound interest?

No. This calculator uses simple interest only. It multiplies the principal by the daily rate and the number of days. It does not add earned interest back into the principal. If you need compound interest, use a compound interest calculator instead.

How do I know which day-count basis my loan uses?

Check your loan agreement or promissory note. It usually states the day-count method. Most personal loans and savings accounts use Actual/365. Many commercial loans, government contracts, and some mortgages use Actual/360. If you are not sure, ask your lender.

Why does the calculator show two interest amounts?

It shows interest for both the Actual/360 and Actual/365 methods so you can compare them. The one that matches your selected day-count basis is highlighted. The difference between the two shows how much the choice of basis affects your total interest.

Does the start date count as an interest day?

No. The calculator counts the days between the start date and the end date. The start date is day zero and does not earn interest. For example, if you pick June 1 to June 2, that is 1 day of interest.

What happens if I enter days and dates at the same time?

If you type a number in the days field, the start and end date fields are cleared. If you pick dates, the days field updates to match. The calculator uses whichever one you changed last. A badge at the top of the duration section shows which mode is active.

Is daily interest the same as an annual rate divided by 365?

Yes, when you use the Actual/365 basis. The daily interest rate equals the annual rate divided by 365. If you use Actual/360, the daily rate equals the annual rate divided by 360.

Can I use this for a savings account?

Yes. Enter your deposit as the principal and your account's annual interest rate. Choose the right day-count basis and the number of days. The result shows how much simple interest you earn over that period. Keep in mind that many savings accounts compound interest daily or monthly, so actual earnings may be slightly higher.

Why is Actual/360 used in commercial lending?

Actual/360 gives lenders a slightly higher daily rate because the annual rate is spread over fewer days. This is a long-standing convention in commercial and government lending. It means the borrower pays a bit more interest than they would under Actual/365 for the same period.

How accurate are the results?

The calculator rounds results to two decimal places for dollar amounts and five decimal places for the daily rate. It uses standard floating-point math. For most everyday purposes, the results are accurate. For legal or accounting use, confirm with your lender or accountant.

Can I calculate interest for more than one year?

Yes. Enter the total number of days or pick start and end dates that span more than a year. The calculator will compute simple interest for the entire period. Remember, it does not compound, so the result assumes the principal stays the same the whole time.

What is a daily interest rate?

A daily interest rate is the annual interest rate divided by the number of days in the year basis. For example, a 4.250% annual rate on an Actual/365 basis gives a daily rate of about 0.01164%. You multiply this daily rate by the principal to find one day's interest.

How do I reduce the interest I pay on a loan?

Pay off the loan faster. Since daily interest is based on how many days the balance is outstanding, fewer days mean less total interest. Making extra payments or paying early reduces the number of days interest builds up.