Daily Periodic Interest Rate Calculator and Formula

The daily periodic interest rate is the annual percentage rate (APR) divided by the number of days used for a year in the account’s calculation. Enter an APR and the day-count basis to calculate the daily rate; the calculator also shows the equivalent percentage and a simple estimate of one day’s interest on a balance.

Formula

The daily periodic rate is the APR, expressed as a decimal, divided by the number of days in the rate year:

Daily rate = APR ÷ 100 ÷ D

• APR is the annual percentage rate entered as a percentage per year.
• D is the number of days used as the denominator, commonly 365 or, in some calculations, 360.
• Daily rate is a decimal rate per day. Multiply it by 100 to express it as a percentage per day.

The calculator’s first result displays the daily rate as a percentage: APR ÷ D. The second displays the same rate as a decimal: APR ÷ (100 × D). To estimate interest for one day on a balance, multiply the balance by the decimal daily rate. This last result assumes the balance stays unchanged for that day.

Worked Example

Using the default APR of 18%, a 365-day basis, and a $1,000 balance:

1. Daily rate as a percentage: 18 ÷ 365 = 0.049315% per day. Rounded to six decimal places, the calculator shows 0.049315%.
2. Daily rate as a decimal: 18 ÷ (100 × 365) = 0.0004931507 per day. Rounded to eight decimal places, the result is 0.00049315.
3. Estimated interest for one day: $1,000 × 0.18 ÷ 365 = $0.49315, or $0.49 rounded to cents.

The percentage and decimal are two formats for the same rate: 0.049315% is approximately 0.00049315 as a decimal. Use the decimal form when multiplying by a balance.

Assumptions

• The APR is treated as a fixed annual rate for this calculation.
• The calculation estimates one day of interest on the entered balance; it does not model payment timing or changes in the balance during a billing period.
• The daily rate is calculated by dividing APR by the selected day-count basis. This is not monthly compounding; an account may apply daily balance and compounding rules specified in its terms.
• Fees, taxes, payment credits, grace periods, promotional rates, and other account-specific terms are not included.

How the Daily Rate Is Used

A periodic rate is the rate applied for one period. If the period is a day, the daily periodic rate helps determine the interest attributable to a balance for each day. In a simplified daily calculation, interest for a day is the balance multiplied by the daily rate. Across several days, the balance may change because of purchases, payments, credits, or interest posting, so the actual calculation can require a separate daily balance for each day.

For example, at an 18% APR using 365 days, the daily rate is about 0.049315%. On a constant $1,000 balance, that is about 49 cents for one day. Multiplying by 30 gives about $14.79, but that is only a simple estimate: it assumes the same balance every day and does not account for the way interest may accrue or post under the account’s method.

Some accounts calculate interest using a daily periodic rate and a daily balance, while others define their calculation differently. The agreement or statement explains the applicable APR, day-count convention, balance method, and posting rules. Use those details when reconciling a real charge.

Choosing 360 or 365 Days

The denominator matters. Dividing by 360 produces a slightly higher daily rate than dividing the same APR by 365. For an 18% APR, the respective rates are 0.05% and about 0.049315% per day. That difference can affect an estimate over many days, but the correct basis is not a personal preference: it depends on the terms and method used for the particular calculation.

A 366-day denominator can also appear in a calculator when the method calls for the actual number of days in a leap year. Do not switch between 360, 365, and 366 simply to match a desired result. Check the account documentation or the calculation instructions for the applicable basis.

Common Mistakes and Limits

• Mixing up percent and decimal: 18% is 0.18 as a decimal. Dividing 18 by 365 gives a percentage rate; dividing 0.18 by 365 gives a decimal rate.
• Using the wrong day count: A 360-day basis and a 365-day basis produce different daily rates. Use the method relevant to the account.
• Treating the one-day estimate as a bill prediction: The calculator holds the balance constant and excludes account-specific rules, transactions, and fees.
• Rounding too early: Keep the full rate during intermediate calculations when estimating a longer period. Rounding the daily rate prematurely can accumulate error.

The tool calculates a daily rate from an APR; it does not determine whether a quoted APR includes every cost or explain how a particular issuer calculates a statement balance. A lender or account disclosure is the source for the exact method and terms.

Daily Rate Compared With Monthly Rate

A daily periodic rate and a monthly periodic rate describe different time periods. A simple monthly rate estimate divides the APR by 12, while a daily rate divides it by the selected number of days. These are useful conversions, but they do not by themselves reproduce the effect of daily accrual, compounding, changing balances, or payment dates. Do not multiply a daily rate by the number of days and assume that every account will charge precisely that amount; the account’s stated method controls the calculation.

Frequently Asked Questions

How do I calculate a daily periodic interest rate?
Convert APR to a decimal by dividing by 100, then divide by the applicable number of days: APR ÷ 100 ÷ D. Multiply by 100 if you want the result as a percentage per day.

Is the daily periodic rate the same as APR?
No. APR is an annual rate expressed as a percentage; a daily periodic rate is the rate for one day derived using a specified day-count basis. The daily rate alone does not describe all account fees or calculation rules.

Should I divide APR by 360 or 365?
Use the denominator specified for the account or calculation. The results differ, and the calculator cannot determine which convention applies without the relevant terms.

How much interest is charged each day?
A basic estimate is balance × APR ÷ (100 × days in the rate year). Actual daily charges may differ if the balance changes or the account uses additional calculation rules.

Does the daily rate include compounding?
The rate conversion itself does not model compounding. It calculates a rate per day; how interest accrues, compounds, and posts depends on the account’s terms.

This result is an estimate; the lender’s disclosure gives the exact figures.

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in apr | Interest rate (APR) | % | 18 | 0.01 | 100
in days | Days in rate year | days | 365 | 1 | 366
in bal | Balance | $ | 1000 | 0 | 100000000
out daily | Daily periodic interest rate | % | apr / days | 6
out dec | Daily rate as a decimal | - | apr / (100 * days) | 8
out charge | One day's estimated interest | $ | bal * apr / (100 * days) | 2