Per-diem interest calculator

What this calculator computes

Per-diem interest is what a note earns between two specific dates. It is the number behind every payoff quote — a payoff is stated good through a specific date, and the per diem shown beside it is informational, an estimate of the daily change; if the closing date moves, the correct figure is a fresh accrual computed through the new date, which is exactly what this calculator does — and it is what decides how a late or early payment applies. This calculator takes a balance, a rate, and two dates and computes the accrual in one step, exactly as the OwnerNote engine does: balance × rate × days over the year basis, rounded to the cent once. It never computes a rounded daily dollar rate and multiplies it by a day count — that shortcut lands on the wrong cent often enough to make histories fail to tie out.

What each input means

Balance is the outstanding principal the interest runs on. Annual rate is the note's fixed yearly rate, accepted to six decimal places. From date is the day the balance was last paid to — interest starts running the day after money was last applied — and to date is the day the accrual runs through, typically a payment's effective date or a payoff's good-through date.

Day count convention is an explicit input because the two conventions count the same span differently, and a silently wrong convention is unrecoverable — every payment applied under the wrong one allocates the wrong interest, and the error compounds in the balance from then on. Under Actual/365 fixedthe accrual runs on real calendar days divided by 365; under30/360 Bond Basis every month counts as thirty days regardless of the calendar. The note's payment clause controls. Choose the convention the instrument states, and the result shows which one was used.

A worked example

Take a $150,000.00 balance on a note at 8.00% fixed, accruing under Actual/365 fixed, paid to January 1, 2026. Suppose the February 1 payment actually arrives eleven days late, on February 12, 2026. The span from January 1 to February 12 is 42 actual days, so the accrual is $150,000 × 8% × 42 ÷ 365 =$1,380.82.

Compare the on-time case. Received on the February 1 due date, the span is 31 days and the accrual is $1,019.18. The eleven days of lateness cost $361.64 of additional interest — and because interest is always paid first, every dollar of it stands ahead of principal when the payment arrives. On a payment smaller than the accrual, nothing reaches principal at all and the shortfall carries as accrued unpaid interest. Nobody assessed a penalty; the clock did. That is why per-diem accrual rewards early payments the same way it punishes late ones, and why a payoff quote always states its good-through date and its per diem: the figure is only correct for the exact span it was computed over.