How interest accrual is worked out
The per diem is the balance × the note rate ÷ 365 (or 360). Multiply by the days between the dates to get the interest accrued. Days are counted from the start date up to, but not including, the end date.
Worked example
$20,000 at 9% accrues $4.93 a day. From November 1 to December 1, 2026, 30 days, that's $147.95. On a 360-day basis it's $150.00.
365 or 360 days?
Most consumer loans divide by 365. Many commercial, bridge and private loans use 360, which charges a little more for the same rate.
What this calculator assumes
Simple interest on a fixed balance, with 365 days even in leap years. Your payoff amount adds this interest to the principal.
Borrowing in the UK?
The country choices above the inputs open separate local pages. UK loans usually divide by 365 and use the yearly interest rate, not the APR. Choosing United Kingdom opens that page and resets the inputs; amounts are not converted.
For the UK version, use the UK loan accrued interest calculator in pounds.
Inputs for the US loan interest accrual calculator
Loan balance
The amount owed, in US dollars.
Yearly interest rate
The note rate from your contract.
Days in the year
365 for most consumer loans; 360 for many commercial loans.
From
When interest starts accruing, such as your last payment.
To
The date you want the interest up to.
CALCULATOR QUESTIONS
US loan interest accrual calculator questions
How do I calculate daily interest on a loan?
Balance × note rate ÷ 365. $20,000 at 9% is $4.93 a day.
What is per diem interest?
The interest charged for one day, used to work out payoff amounts.
Why is 360-day interest higher?
Dividing by fewer days makes each day's interest larger.
Does interest accrue on weekends?
Yes. Interest accrues every calendar day.