Per Diem Interest Calculator

About this calculator

Per diem interest — also called 'daily interest' or 'odd-days interest' — is the amount of loan interest that accrues for a single day, used to prorate charges between a loan's closing date and the start of its first full payment period. Lenders calculate it by taking the loan's annual interest rate, dividing by 360 or 365 days depending on the day-count convention used, and multiplying by the outstanding principal.

This figure shows up on nearly every mortgage or loan closing statement: if you close mid-month, you typically owe per diem interest for the days between closing and the end of that month before your regular amortization schedule begins. Title companies, escrow officers, and loan officers use it to make sure the borrower pays exactly for the days the loan was actually outstanding — no more, no less. This calculator takes your loan amount, annual rate, and day-count basis to return the daily interest charge.