How to Calculate Per Diem Interest on a Mortgage
Per diem interest is a daily interest charge you pay at mortgage closing for the days between your closing date and the end of that month. If you close on June 20th, for example, you'll owe per diem interest for June 20–30. This upfront cost is a standard closing expense that compensates your lender for the initial days your loan is outstanding, before your first full mortgage payment is due.
What Is Per Diem Interest at Closing?
Per diem interest is calculated as a daily amount based on your loan balance and interest rate. Unlike your regular monthly mortgage payment—which includes principal, interest, taxes, and insurance (PITI)—per diem interest is purely interest-only for a partial month. It appears as a separate line item on your Closing Disclosure and must be paid in full at closing. The amount varies based on three factors: your loan amount, your annual interest rate, and the number of days from closing through month-end.
Per Diem Interest Calculation: Step-by-Step Formula
Calculating per diem interest requires four simple steps. You'll need your total loan amount, annual interest rate, and closing date. Follow this process:
Step 1: Find Your Daily Interest Rate
Divide your annual interest rate by 365 days. If your interest rate is 6%, the calculation is: 0.06 ÷ 365 = 0.000164 (or 0.0164% per day). This daily rate is the foundation for all per diem calculations.
Step 2: Calculate Your Daily Interest Charge
Multiply your daily rate by your total loan amount. For a $300,000 mortgage at 6%, the daily interest charge is: 0.000164 × $300,000 = $49.20 per day. This is the amount of interest accruing every single day your loan is outstanding.
Step 3: Count the Days From Closing Through Month-End
Count the number of days from your closing date (inclusive) through the last day of that calendar month. If you close on June 15th, count June 15, 16, 17… through June 30th. That's 16 days. If you close on the 25th, count only 6 days (25–30). The timing of your closing date dramatically impacts your per diem cost.
Step 4: Multiply to Find Total Per Diem Interest
Multiply your daily interest charge by the number of days you calculated in Step 3. Using our $300,000 example at 6%, closing on June 15th: $49.20 × 16 days = $787.20. If you closed on June 25th instead: $49.20 × 6 days = $295.20—a savings of $492 just by closing 10 days later.
Per Diem Interest Examples for Different Loan Amounts and Rates
Let's look at how per diem interest varies by loan amount and interest rate to give you a better sense of typical costs:
Scenario 1: $250,000 loan at 5.5%, closing June 15th
Daily rate: 0.055 ÷ 365 = 0.0001507 | Daily charge: 0.0001507 × $250,000 = $37.68
Days from June 15–30: 16 days | Total: $37.68 × 16 = $602.88
Scenario 2: $400,000 loan at 6.5%, closing June 20th
Daily rate: 0.065 ÷ 365 = 0.0001781 | Daily charge: 0.0001781 × $400,000 = $71.24
Days from June 20–30: 11 days | Total: $71.24 × 11 = $783.64
Scenario 3: $350,000 loan at 7%, closing June 25th
Daily rate: 0.07 ÷ 365 = 0.0001918 | Daily charge: 0.0001918 × $350,000 = $67.13
Days from June 25–30: 6 days | Total: $67.13 × 6 = $402.78
Why Is Per Diem Interest Charged at Closing?
Mortgage payments are made in arrears, meaning you pay interest for the previous month during the following month. Here's how it works: If you close in June, the interest you accrue from your closing date through June 30th is paid at closing (the per diem charge). Your first full monthly payment, due August 1st, will cover interest for July. This system ensures a clean, predictable payment schedule and avoids having awkward partial payments or complicated pro-rations.
How Per Diem Interest Affects Your Total Closing Costs
Per diem interest is itemized on your Closing Disclosure as a separate charge, distinct from loan origination fees, title insurance, appraisals, and other closing costs. While this charge doesn't increase the total interest you'll pay over the life of your loan—it's simply prepaid interest—it does increase your cash needed at closing. For many buyers, per diem interest ranges from $300 to $1,500, depending on loan size, rate, and closing date. Strategic timing of your closing can reduce this expense significantly.
How to Minimize Per Diem Interest at Closing
Close Later in the Month
The simplest way to reduce per diem interest is to schedule your closing for the end of the month. Closing on the 28th versus the 5th can save you hundreds of dollars. Work with your real estate agent and lender to coordinate timing that minimizes per diem costs without disrupting your purchase timeline.
Negotiate Seller Concessions
Seller concessions—funds the seller contributes toward your closing costs—can help offset per diem interest. If you're unable to close late in the month due to timing constraints, discuss seller concessions with your real estate agent. Many sellers are willing to contribute up to a certain percentage of the purchase price toward buyer closing costs.
Request a Detailed Pre-Closing Estimate
Before you arrive at the closing table, ask your loan officer for a specific, written calculation of your per diem interest. This prevents surprises and gives you time to plan your cash. Don't hesitate to ask questions if the number seems unusually high.
Per Diem Interest for Refinances and Payoffs
When you refinance a mortgage or sell your home, per diem interest works slightly differently. For a payoff, you'll owe interest from the date of your last regular payment through your payoff date. For a refinance, your old loan is paid off on the close date of your new loan, and you'll owe per diem interest on your new loan from close through month-end. Timing is critical in refinances—you want your new loan to fund on a date that minimizes the overlap and double-interest between your old and new loans.
Is Per Diem Interest on the Closing Disclosure?
Yes, per diem interest is always itemized on your Closing Disclosure, the official summary of all closing costs provided three days before closing. It appears in the closing costs section and is labeled clearly so you know exactly what you're paying. If you don't see it, or if the amount seems incorrect, ask your closing agent or loan officer to explain the calculation before signing.
Key Takeaways: Per Diem Interest at Mortgage Closing
Per diem interest is an unavoidable but manageable closing cost. It's calculated as your daily interest rate multiplied by the number of days from closing through month-end. By understanding how it works and choosing a strategic closing date, you can reduce this expense and walk to the closing table with confidence. Use our per diem interest calculator for a personalized estimate, and review your Closing Disclosure carefully to understand all costs involved in your mortgage.
Frequently Asked Questions About Per Diem Interest
What is per diem interest on a mortgage?
Per diem interest is the daily interest charge you pay from your mortgage closing date through the last day of that month. Since mortgage interest is paid in arrears (for the previous month), this upfront charge ensures your lender is compensated for those initial days before your first regular payment is due.
How do I calculate per diem interest?
Follow these four steps: (1) Divide your annual interest rate by 365 to find your daily rate. (2) Multiply the daily rate by your loan amount to get your daily interest charge. (3) Count the days from closing through the last day of the month. (4) Multiply your daily charge by the number of days. For example, a $300,000 loan at 6% closing on June 20th would be: (0.06 ÷ 365) × $300,000 × 11 days = $542.52.
How much per diem interest will I pay?
Per diem interest depends on three factors: your loan amount, your interest rate, and your closing date. Closing later in the month significantly reduces this cost. For example, closing on the 25th of a month means you'll only pay for 6 days of interest, compared to 25 days if you close on the 1st. Use our per diem calculator for an exact estimate based on your specific loan details.
Why do lenders charge per diem interest?
Mortgage interest is paid in arrears, meaning you pay interest for the previous month. This standard practice ensures your first full mortgage payment is due on the first day of the second month after closing, keeping your monthly payments predictable and consistent. Per diem interest compensates the lender for the days between closing and month-end.
Can I avoid per diem interest at closing?
You cannot completely avoid per diem interest, but you can minimize it by closing later in the month. Closing on the 28th costs far less than closing on the 5th. Additionally, you may be able to negotiate seller concessions to help cover this closing cost. Discuss your closing date timing with your loan officer to reduce this expense.
Is per diem interest included in my closing disclosure?
Yes, per diem interest is always itemized on your Closing Disclosure as a separate line item under closing costs. It's part of your total cash needed at closing. While it doesn't increase the total interest you'll pay over the life of the loan, it does affect your upfront expenses, so it's important to understand this cost before signing.
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