How do I calculate a daily periodic rate?

Divide the annual rate by 365. For example, an 18% annual rate is about 0.049315% per day on a simple 365-day basis.

How do I calculate the rate for a custom period?

Multiply the annual rate by the number of days in the period, then divide by 365.

Is this the same as APY or EAR?

No. APY and EAR include compounding. This calculator converts simple rates on a 365-day basis.

Does this calculate the interest amount?

No. It only converts the rate. To calculate an interest amount, you also need the principal and the applicable interest method.

Periodic Interest Rate Calculator

Convert an annual interest rate to a periodic rate, or convert a known period rate back to an annual simple-rate equivalent on a 365-day basis.

Known value

Annual rate, %

Period length, days

This converts a rate, not an interest amount. It uses a simple 365-day basis without compounding; lender, tax, and contract calculations may use different rules.

What periodic interest rate means

A periodic interest rate is the rate for a specific period, such as one day or a custom number of days. This page converts rates, not dollar interest amounts.

The calculator uses a simple 365-day basis. It does not calculate APY, EAR, lender disclosures, taxes, fees, or contract-specific interest.

Rate conversion formulas

r_{d a y} = \frac{r _{ann u a l}}{365}

r_day is the daily periodic rate and r_annual is the annual rate.

r_{p er i o d} = r_{ann u a l} \cdot \frac{D}{365}

r_period is the rate for D days on a simple 365-day basis.

r_{ann u a l} = r_{p er i o d} \cdot \frac{365}{D}

Use this when the period rate is known and the annual simple-rate equivalent is needed.

When this is enough

Need	Use this page?
Daily periodic rate from an annual rate	Yes
Rate for a custom number of days	Yes
Annual equivalent of a known period rate	Yes
APY or EAR with compounding	No
Actual interest amount in dollars	No
Loan or credit-card disclosure compliance	No

Limits

The day-count basis is fixed at 365 days for this page.
The model is simple interest and does not compound within the period.
Some contracts and financial institutions use 360-day, 365-day, actual/actual, or other day-count conventions.
Use a dedicated APR, APY, compound interest, loan, or savings calculator when compounding, fees, cash flows, or legal disclosure rules matter.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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