Daily Periodic Rate Formula and Calculator

The daily periodic rate is the annual percentage rate (APR) divided by the number of days in the year used by the account: daily periodic rate = APR ÷ 365 when the agreement uses a 365-day basis. Enter an APR below to calculate its daily rate as a percentage and decimal; the result is a rate per day, not the amount of interest charged.

Formula

Daily periodic rate = APR ÷ D, where APR is the stated annual percentage rate expressed as a percentage and D is the number of days in the annual basis. This calculator assumes D = 365.

• APR: annual percentage rate, entered as a percent per year. For example, enter 18 for 18%, not 0.18.
• Daily periodic rate: the rate for one day, displayed as a percentage per day.
• Daily rate as a decimal: the same daily rate in decimal form, suitable for multiplying by a balance. A decimal rate of 0.00049315 is equivalent to 0.049315%.

The formulas in the calculator are APR / 365 for the percentage-per-day result and APR / 36500 for the decimal result. Dividing by 100 in the second expression converts the APR from percent to decimal form.

Worked Example

Using the calculator’s default APR of 18%:

1. Daily periodic rate in percent: 18 ÷ 365 = 0.049315068…% per day.
2. Rounded to six decimal places, the displayed rate is 0.049315% per day.
3. Daily rate as a decimal: 18 ÷ 36,500 = 0.00049315068….
4. Rounded to eight decimal places, the displayed decimal rate is 0.00049315.

For illustration, applying the unrounded decimal rate to a constant $1,000 balance for one day gives $1,000 × 0.00049315068… = about $0.4932 of interest, before any account-specific rounding or balance changes. The calculator itself reports the rate, not this interest amount.

Assumptions

• The calculation uses a 365-day annual basis.
• APR is treated as a nominal annual rate and divided evenly across the assumed days.
• The calculation is for the rate per day only; it does not model payment timing or compound interest over multiple days.
• Fees, taxes, account balance changes, grace periods, and lender-specific calculation conventions are not included.

What the Daily Periodic Rate Means

A periodic rate is an interest rate for one defined period. A daily periodic rate applies to a day. It lets an issuer or lender calculate interest for the days a balance is outstanding, often by applying the daily rate to a daily balance and accumulating the resulting interest over a billing cycle. The exact procedure depends on the account terms.

As a rate, the number is not itself a dollar charge. To get a simple one-day interest estimate, multiply a balance by the rate in decimal form. At 18% APR on a 365-day basis, the daily decimal rate is approximately 0.00049315. Applied to $1,000 for one day, that is roughly 49 cents. If the balance changes from day to day, or the account compounds accrued interest, the total will differ from multiplying one fixed balance by the rate once.

The displayed daily percentage can look small because it represents only one day. For comparisons, check that both rates use the same annual basis and that you are comparing like measures—such as nominal APR with nominal APR, not an effective annual yield with a periodic rate.

365-Day and 360-Day Bases

“Divide by 365” is a common interpretation of the daily periodic rate, but it is not universal. Some agreements or financial calculations use a 360-day year, or specify another convention. In a 360-day calculation, the formula would be APR ÷ 360. At 18%, that equals 0.05% per day, slightly more than 18% ÷ 365.

The difference comes from the denominator, not a different APR. For an accurate account calculation, read the rate and interest method in the agreement or statement. This page’s calculator deliberately uses 365 and does not offer a day-count selector, so its answer should not be treated as the account’s exact rate when the account uses a different basis.

How It Relates to Interest and APR

APR describes an annualized rate, while the daily periodic rate expresses a per-day rate under a stated day-count convention. For a rough estimate when the balance stays constant and interest is not compounded during the period, multiply:

Estimated interest = balance × daily rate as a decimal × number of days.

For example, at the default 18% APR, a constant $1,000 balance for 30 days gives approximately $1,000 × (0.18 ÷ 365) × 30 = $14.79. This is an arithmetic illustration, not a statement of what any particular account will charge. Actual interest may be based on daily balances, transaction dates, payment credits, rounding, compounding, and the agreement’s day-count rules.

Do not confuse dividing the APR by 365 with calculating an effective annual rate. An effective rate accounts for repeated compounding over a year. If a decimal daily rate is compounded daily, the effective annual rate under that simplified model is (1 + daily rate)^365 − 1. Account terms may use different methods, so this relationship is explanatory rather than a substitute for the disclosed APR or account calculation.

Common Errors and Limits

• Entering 18% as 0.18: the input expects percent units, so type 18 for an 18% APR. Typing 0.18 means 0.18% APR.
• Using the percentage as a decimal: 0.049315% is not 0.049315 as a decimal. Divide the percentage by 100; the corresponding decimal is about 0.00049315.
• Assuming every account uses 365: the contract may use 360 or another convention. Verify the stated method before relying on a calculation.
• Rounding too early: use the unrounded rate for intermediate calculations where possible. Rounding the daily rate before multiplying over many days can introduce small differences.
• Calling the result a charge: a daily periodic rate must be applied to a balance under the account’s rules to estimate interest. It does not include fees or other charges.

This simple formula does not determine an account’s complete APR disclosure, minimum payment, grace period, compounding method, or final interest charge. The APR input should come from the relevant agreement or disclosure; the example default is only for demonstrating the arithmetic.

Frequently Asked Questions

How do I calculate a daily periodic rate?
Divide the APR by the number of days in the applicable annual basis. For a 365-day basis, an 18% APR gives 18 ÷ 365 = 0.049315% per day.

Is the daily periodic rate the same as APR?
No. APR is an annualized percentage rate. A daily periodic rate is the rate assigned to a day under a particular day-count convention.

Do I divide APR by 365 or 360?
Use the basis specified by the relevant agreement or calculation method. This calculator assumes 365 days; some financial calculations use 360.

How do I convert a daily rate percentage to a decimal?
Divide the percentage by 100. For example, 0.049315% per day becomes approximately 0.00049315 as a decimal.

How can I estimate daily interest on a balance?
Multiply the balance by the daily rate expressed as a decimal. For a changing balance or a multi-day period, the account’s daily-balance, payment, rounding, and compounding rules matter.

The result is an estimate of the rate conversion; the lender’s disclosure gives the exact figures and calculation method for its account.

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in apr | APR | % | 18 | 0.01 | 100
out dpr | Daily periodic rate (365-day basis) | % per day | apr / 365 | 6
out dec | Daily rate as a decimal | - | apr / 36500 | 8