Daily Interest Calculator

How daily interest works

Daily interest, also called per diem interest, is one day's share of a year's interest. Per diem is Latin for per day. Multiply your balance by the yearly rate, then divide by the days in your lender's year. On $200,000 at 6.5%, a year's interest is $13,000, so one day of a 365-day year is $35.62. The interest you owe at payoff is that daily amount times the days since interest was last paid.

Daily interest formulas

interest per day = balance × annual rate ÷ days in the lender's year (365 or 360; 366 for a leap year on Actual/Actual)
days = payoff date − interest paid-to date, + 1 when both dates count
30/360 days = 360 × years + 30 × months + (D2 − D1), where D1 is the paid-to day and D2 the payoff day; a D1 of 31 becomes 30, and a D2 of 31 becomes 30 only when D1 is 30 or 31
Actual/Actual interest = balance × rate × (days in other years ÷ 365 + days in leap years ÷ 366)
interest = balance × annual rate × days ÷ days in the year, rounded to the cent once
payoff = balance + interest + fees
rate on a 365-day year = rate × 365 ÷ 360, for a 360-day count

Worked examples

Exact and ordinary interest. Find the exact interest on a loan of $32,000 at 9% for 30 days. Exact interest uses a 365-day year: $32,000 × 9% × 30 ÷ 365 is $236.71. Ordinary interest uses a 360-day year and gives $240.00, $3.29 more. That is the same as a rate of 9.125% on a 365-day year.

Exact time and approximate time. Exact time counts the real days, and approximate time counts every month as 30 days. From January 31 to April 30, 2026, exact time is 89 days and approximate time is 90. On the same $32,000 at 9%, exact time with ordinary interest, called the Banker's rule, gives $712.00. Exact time with exact interest gives $702.25.

A payoff that crosses into a leap year. An Actual/Actual note divides each day of 2028 by 366. From December 15, 2027 to January 15, 2028, 17 days fall in 2027 and 14 in 2028. On $200,000 at 6.5% the interest is $1,102.75, which is $1.36 less than on a 365-day year.

Assumptions and limits

• The balance and the rate stay the same from the paid-to date to the payoff date. A payment in between lowers the balance, so work each part of the period on its own.
• Interest is simple: it is not added to the balance before the payoff. Cards and savings that compound daily are covered above.
• Days are whole calendar days. On 30/360 a paid-to date on the 31st counts as the 30th. A payoff date on the 31st counts as the 30th only when the paid-to date is the 30th or 31st. Other forms of 30/360 treat the 31st or the end of February differently.
• The payoff is the principal, the interest and the fees you enter. A statement may also add other fees you have been charged and have not yet paid, such as late charges, or a prepayment penalty.
• Interest is rounded to the cent once, at the end. Your servicer may round the per diem first, as shown above.
• The $200,000 balance, the 6.5% rate and the November 2026 dates are an example, not today's market. The car and card rates are the Federal Reserve's for August 2026.
• The form takes balances up to $10,000,000, rates up to 50% and payoff dates up to 10 years after the paid-to date.

These figures are estimates for learning and planning, not financial, tax, legal or investment advice. Terms of use.

What to do before you pay off

Ask your servicer in writing for a payoff statement for the day you plan to pay. Check its per diem against the one here, $35.62 on the default inputs, and its days with the date difference calculator.

Make sure your money arrives by the good-through date. On the default inputs each day after it adds another day's interest, and waiting from November 20 to December 1 adds $391.78. If you plan to pay the loan down rather than off, the mortgage payoff calculator shows how extra payments shorten it.

On a car or personal loan whose simple interest is figured by the day, pay on or before the due date. Paying 7 days early on $30,000 at 7.54% on a 365-day year moves $43.38 from interest to your balance. The loan extra payment calculator shows what steady extra payments do.

If a loan divides its rate by 360, compare offers by what that rate comes to over a full calendar year. A 7.2% rate charged that way comes to 7.3%. The loan amortization calculator shows the monthly payment at that rate.

Frequently asked questions