Interest Per Day Calculator

Daily interest equals principal multiplied by the annual rate, divided by the year divisor of your day-count convention: P × r ÷ 365 for Actual/365, or P × r ÷ 360 for Actual/360 and 30/360. For example, $10,000 at 12% under Actual/365 accrues $3.2877 of interest per day.

Actual/365 divides the annual rate by 365 calendar days, while Actual/360 divides it by 360, which makes daily interest about 1.39% higher. Actual/365 is common for consumer loans and mortgages; Actual/360 is the money-market convention used for many commercial and bridging loans.

30/360 assumes every month has 30 days and the year has 360 days, simplifying bond and fixed-income interest math. For an explicit day count it behaves like Actual/360 (divide by 360), but when you enter start and end dates, days are counted on a 30-day-month basis with the standard US (NASD) end-of-month adjustments.

No. Actual/365 (fixed) always divides by 365, even in a leap year, so a full 366-day leap year accrues slightly more than one year's worth of interest. The ISDA Actual/Actual convention is different: it uses 366 for days falling in a leap year, so a full leap year accrues exactly the annual rate amount.

Per diem interest is simply interest charged per day, the same thing this calculator computes. The term is most common in mortgages, where per diem interest accrues from the loan funding date until the first payment date, and is often collected at closing.

Rearrange the formula: annual rate = daily interest × divisor ÷ principal × 100. For example, $5 of daily interest on a $20,000 balance under Actual/365 means a rate of $5 × 365 ÷ $20,000 × 100 = 9.125% per year. Use the reverse mode above to compute it.

$3.2877 per day under Actual/365 ($10,000 × 0.12 ÷ 365) or $3.3333 per day under Actual/360 or 30/360 ($10,000 × 0.12 ÷ 360). Over 90 days that totals $295.89 under Actual/365.

This calculator uses simple (non-compounding) daily interest: each day accrues the same amount on the original principal. That matches bridging loans, late-payment charges, and per-diem mortgage interest. If your product compounds daily, balances grow slightly faster — see our compound interest calculator for that case.

Many contracts charge overdue balances at an annual rate applied per day, for example 12% per annum on the unpaid invoice amount. Enter the overdue amount as principal, the contract rate, the Actual/365 convention (the usual default), and the number of days late to get the late fee.

Multiply the daily rate by the convention's divisor: daily rate × 365 for Actual/365, or × 360 for Actual/360 and 30/360. A daily rate of 0.0329% under Actual/365 equals 0.0329 × 365 = 12% per year.

Bridging and many commercial loans follow the money-market Actual/360 convention, which yields slightly higher daily interest than Actual/365 for the same quoted rate. On $100,000 at 8%, Actual/360 accrues $22.22 per day versus $21.92 under Actual/365 — always check which convention your loan agreement specifies.

Yes. Enter optional start and end dates and the calculator derives the day count for you: actual calendar days for Actual/365 and Actual/360, or 30-day-month counting (US/NASD rules) for 30/360. Leave the dates blank to enter the number of days directly.