How to Calculate Per Diem Interest on a Mortgage

Calculating per diem interest is a crucial step in understanding your mortgage closing costs. This daily interest charge covers the period from your loan closing date through the end of the month before your first regular payment is due. While it's a standard part of the home-buying process, it often surprises buyers at the closing table. This guide will walk you through the simple formula, provide clear examples, and show you how to manage this cost effectively.

For a complete overview of conventional loan options, see our main guide. If you want to instantly calculate your cost, try our per diem interest calculator here.

What Is Per Diem Interest and Why Does It Matter?

Per diem, a Latin term meaning "per day," refers to the interest your lender charges for each day between your loan closing and the first day of the next month. Lenders charge this to collect the interest that accrues during this short initial period. It ensures that your subsequent payments are processed smoothly and on the standard schedule. This cost is a standard line item on your Closing Disclosure and represents a key part of your upfront cash requirement. To reduce your out-of-pocket expenses, consider asking for seller concessions on conventional loans.

How to Calculate Per Diem Interest: A Step-by-Step Guide

The calculation is straightforward. You just need your loan amount, annual interest rate, and the number of days from your closing date to the end of the month.

Step 1: Determine Your Daily Interest Rate

Divide your annual interest rate (as a decimal) by the number of days in the year. Most mortgages use a 365-day calendar. For a 3.5% interest rate, the formula is 0.035 ÷ 365 = 0.00009589.

Step 2: Calculate Your Daily Interest Charge

Multiply your daily interest rate by your total loan amount. For a $200,000 loan, the calculation is 0.00009589 × $200,000 = $19.18. This is your per diem amount.

Step 3: Count the Days Until the End of the Month

Count the days from your closing date through the last day of that month. The day of closing is counted. If you close on June 15th, you count 16 days (June 15th - June 30th).

Step 4: Calculate Your Total Per Diem Interest

Multiply your daily interest charge by the number of days. In our example: $19.18 × 16 = $306.88.

Example: How to Calculate Per Diem Interest for a $250,000 Mortgage

Let's apply this to a real-world scenario. You are closing on a $250,000 mortgage at a 4.5% annual interest rate on June 20th.

• Daily Rate: 0.045 ÷ 365 = 0.00012329
• Daily Charge: 0.00012329 × $250,000 = $30.82
• Days from June 20th to June 30th: 11 days
• Total Per Diem Interest: $30.82 × 11 = $339.02

This $339.02 is due at your closing table. The exact amount will be detailed in your Closing Disclosure. To see how your rate influences this calculation, check today's mortgage rates.

How Per Diem Interest on a Mortgage Payoff Works

The calculation for a mortgage payoff is nearly identical. When you refinance or sell your home, you will need to calculate the per diem interest from the date of your last regular payment up to your payoff date. Your lender will provide an exact payoff quote that includes this amount, ensuring you pay the precise total to satisfy your loan. For refinance options, explore our cash-out refinance guide.

Per Diem Interest Table for Common Loan Amounts

Use this table to quickly estimate your per-diem cost for standard loan sizes and rates. These are estimates; your actual cost will depend on your exact closing date.

Frequently Asked Questions About Per Diem Interest

Here are answers to some of the most common questions we receive about per diem interest.

Is per diem interest the same as advance interest?

Yes, these terms are often used interchangeably. Both refer to the interest paid in advance from your closing date to the end of the month to cover that initial period before your first regular payment.

Can I roll the per diem interest into the loan?

No, for most standard purchase loans, per diem interest is a cash cost that must be paid at closing. Some refinance options may allow it, but it's best to assume you'll need to cover it with cash. A larger down payment lowers your loan amount, which in turn lowers your per diem interest. Use our affordability calculator to plan your down payment strategy.

Is per diem interest tax-deductible?

Generally, yes. Per diem interest paid at closing is treated as mortgage interest and may be tax-deductible. You should consult with a tax advisor to understand how this applies to your personal situation.

Does PMI (Private Mortgage Insurance) affect per diem interest?

No, PMI is a separate insurance premium and is not part of the per diem interest calculation. However, if you have PMI, it is a significant part of your overall monthly payment. Learn more about when conventional loans require PMI.

Key Takeaways & Next Steps

• Understand the Cost: Per diem interest is a predictable closing cost that you can calculate yourself.
• Plan Your Closing Date: If possible, schedule your closing at the end of the month to minimize your per-diem payment.
• Use a Tool: For a precise estimate, use our free per diem interest calculator.
• Budget Accordingly: Ensure you have these funds available at closing.

For a complete suite of financial planning tools, visit our mortgage calculators hub. You can also explore other related topics like mortgage escrow accounts and cash-out refinance options.

Remember, this information is for educational purposes. Always consult with a licensed mortgage professional to get figures specific to your loan and closing situation.