Daily Interest Calculator

May 3, 2026

If you carry a credit card balance or hold money in a high‑yield savings account, interest is calculated every single day. A daily interest calculator shows exactly how much interest accrues on any given day–and how fast it can compound over time.

The calculator above instantly computes daily interest using either simple or compound methods. Enter your principal balance, annual interest rate, and the number of days to see the per‑day charge or earnings, the total interest, and the final balance. It even accounts for different day‑count conventions (365 or 360 days a year) that lenders and banks commonly use.

What Is Daily Interest?

Daily interest (often called per diem interest) is the amount of interest that accumulates each day on a loan or deposit. Instead of waiting until the end of the month or year, the lender or bank applies a daily periodic rate to the outstanding balance. That daily rate is simply the annual rate divided by the number of days in a year–typically 365, though some commercial loans and credit cards use a 360‑day year.

The key distinction is between daily simple interest and daily compound interest. With simple interest, the daily charge is always based on the original principal. With compound interest, each day’s interest is added to the principal, and the next day’s calculation uses the new, slightly higher balance. Over long periods, daily compounding can significantly increase the total cost of a loan or the growth of a savings account.

Daily Simple Interest Formula

Daily simple interest is common for some personal loans, auto loans, and certain mortgages. The formula is:

Daily Interest = Principal × (Annual Rate ÷ Days in Year)

For example, a $1,000 loan with an 18% APR and a 365‑day year: Daily interest = $1,000 × (0.18 ÷ 365) = $0.49315 per day.

Over 30 days, that adds up to about $14.79. Since the interest does not compound, the principal stays constant, and the daily charge remains the same every day.

Daily Compound Interest Formula

Savings accounts, credit cards, and many investments calculate interest using daily compounding. The formula for final balance after $n$ days is:

A = P × (1 + r/days)^(days)

where P is the principal, r is the annual rate, and days is the number of compounding periods (each day). The daily periodic rate is r/days.

If you deposit $5,000 in a high‑yield savings account with a 4.50% APY (daily compounding), after 365 days: Daily rate = 0.045 ÷ 365 = 0.00012329 Balance = $5,000 × (1 + 0.00012329)^365 ≈ $5,229.27

The total interest earned is about $229.27. The calculator above performs this exact math instantly, adjusting for the exact number of days you enter.

How to Estimate Your Daily Credit Card Interest Cost

Credit card issuers typically calculate interest daily using the daily balance method. The formula starts with the average daily balance over the billing cycle, multiplied by the daily periodic rate and the number of days in the cycle.

For an 18.0% APR card with a $2,000 average daily balance, the daily periodic rate is 0.0493% (18% ÷ 365). The daily interest charge is roughly $0.986. You can multiply that by the number of days in the billing cycle (say 30) to get about $29.58 in interest for that month. The daily interest calculator helps you see exactly how much each additional day of carrying a balance costs.

Real‑World Uses for a Daily Interest Calculator

A per diem interest tool is useful well beyond credit cards:

Savings growth projections: See how daily compounding lifts your balance, even over short periods.
Loan pay‑off planning: For daily‑simple interest loans, paying even one day early saves a tiny bit of interest–the calculator quantifies it.
Bridge loans and hard money lending: These often accrue per diem interest; you can verify the daily cost before signing.
Tax and accounting: Calculate interest on late payments or underpayments where the IRS and many state agencies use a daily rate (based on the federal short‑term rate).

This article provides general estimates and does not constitute financial advice. Check with your financial institution for the exact daily rate and compounding method used on your account.

Frequently Asked Questions

How is daily interest calculated on a loan?

Daily interest is calculated by dividing the annual interest rate by 365 (or 360, depending on the lender) to get the daily periodic rate. That rate is then multiplied by the current principal balance each day. For simple interest loans, the daily charge does not compound; for credit cards and some lines of credit, the interest may be added to the balance daily.

Does daily interest accrue on weekends and holidays?

Yes, for loans and credit cards interest typically accrues every calendar day, including weekends and holidays. The annual percentage rate (APR) is divided by 365 to reflect all days. For some financial instruments, like certain bonds or commercial loans, a 360-day year may be used, but interest still accrues on all actual days.

Can I use a daily interest calculator for credit card debt?

Absolutely. A daily interest calculator helps you see how much interest is added to your credit card balance each day and how quickly it compounds. You can estimate the daily cost of carrying a balance and the effect of different APRs or payment amounts over any number of days.

What is the difference between APR and daily periodic rate?

The annual percentage rate (APR) is the yearly cost of borrowing expressed as a percentage. The daily periodic rate is the APR divided by the number of days in a year (usually 365). For example, an 18.0% APR yields a daily periodic rate of about 0.0493%. Lenders use the daily rate to calculate daily interest charges on your outstanding balance.

Is daily interest always compound interest?

No. Daily simple interest means interest is calculated daily on the original principal only; it does not compound. Daily compound interest means each day’s interest is added to the principal, and the next day’s interest is calculated on the new, larger balance. Savings accounts typically use daily compounding, while some loans use daily simple interest.

How accurate are daily interest calculations?

A daily interest calculator is accurate when you enter the correct principal, annual rate, number of days, and compounding frequency. Real-world results may vary slightly due to rounding, whether the lender uses a 365- or 360-day basis, and possible transaction timing. Always confirm with your bank or creditor for precise numbers.