Mortgage Per Diem Calculator
Mortgage per diem interest is the dollar amount of interest your loan accrues each day. Lenders charge it whenever a loan starts or ends mid-month: as prepaid “odd-days” interest at the closing table, and from your last payment to the payoff date when you refinance or sell. Enter the balance, rate and the two dates below to get the daily figure, the day count and the total. (This is mortgage daily interest for closings and payoffs, not the military PCS travel per diem, which is a completely different allowance.)
The total is computed from the unrounded daily figure, then rounded to cents, so it can differ by a cent or two from multiplying the rounded daily amount by the day count. Lenders may also use a 366-day divisor in leap years; confirm the exact figure on your closing disclosure or payoff statement.
Quick Answer
On a $300,000 loan at 6%, daily interest is $49.32 per day ($300,000 × 0.06 ÷ 365). Over the 10 days from January 20 to January 30, 2020, that totals $493.15 in per diem interest, with a monthly equivalent of $1,501.15. Every number is a real run of the calculator above, so enter the same inputs and you get the same figures.
Key takeaways
- Per diem interest is the daily cost of holding the loan: balance × annual rate ÷ 365. It is collected at closing (prepaid interest) and at payoff (refinance or sale).
- The day count uses calendar days, weekends included. A Friday-to-Monday funding gap is 3 days of interest, not 1.
- The 365-day basis is the residential standard; the 360-day “bank method” gives a slightly higher daily figure and mostly appears on commercial loans.
- This tool multiplies the unrounded daily figure by the day count, then rounds. Multiplying the rounded $49.32 by 10 days gives $493.20, but the exact total is $493.15: know which one your lender uses before disputing pennies.
- Quick Answer
- How to use the mortgage per diem calculator
- The per diem formula, and the 365 vs 360 choice
- Worked example, checked against this exact tool
- 365 vs 360: the day count that quietly changes your number
- Where per diem interest lands at closing
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the mortgage per diem calculator
Enter the balance, the rate and the two dates, then click Calculate per diem. The results stay hidden until you calculate.
- Enter the loan amount: the balance the interest accrues on. At closing this is the new loan amount; for a payoff it is the outstanding balance.
- Enter the annual interest rate: your note rate, for example 6 for 6%.
- Set the start and end dates: for prepaid interest at closing, the funding date to the first of the next month; for a payoff, the last payment date to the payoff date.
- Pick the day-count basis: 365 days (Actual/365) for residential mortgages, 360 days (Actual/360) if your loan documents specify it.
- Read the five lines: daily interest, number of days, daily interest rate, total per diem interest and the monthly interest equivalent.
The calculator is pre-filled with the worked example ($300,000 at 6%, January 20 to January 30, 2020). Blank or invalid dates now get a clear error message instead of a nonsense answer.
The per diem formula, and the 365 vs 360 choice
One day of mortgage interest is the annual interest for a year divided into days. For residential loans the standard divisor is 365, which is the convention settlement agents use when they figure prepaid interest at the closing table.
Total per diem interest = Daily interest × number of days
The 360-day basis works the same way with 360 as the divisor. Because you divide by a smaller number, the daily figure is slightly higher: on the same balance and rate, the 360 basis charges about 1.4% more per day. Commercial loans commonly use it (the “bank method”); if your residential lender quotes a per diem that looks a little high, the basis is the first thing to check. The monthly equivalent line (daily interest × 30.44, the average month length) is there for budgeting: it answers “what would this daily burn rate cost me as a month of interest?”
Worked example, checked against this exact tool
The pre-filled example: $300,000 at 6%, January 20 to January 30, 2020, on the 365-day basis. The calculator returns 10 days, a daily rate of 0.016438%, daily interest of $49.32, a total of $493.15 and a monthly equivalent of $1,501.15. The table below runs the same tool on more scenarios, and every cell reproduces exactly.
| Scenario | Days | Per day | Total |
|---|---|---|---|
| $300,000 at 6%, Jan 20–30, 2020 (365) | 10 | $49.32 | $493.15 |
| $432,000 at 5%, Sep 12 – Oct 1, 2024 (365) | 19 | $59.18 | $1,124.38 |
| $200,000 at 5%, May 15 – Jun 1, 2024 (365) | 17 | $27.40 | $465.75 |
| $200,000 at 5%, May 15 – Jun 1, 2024 (360) | 17 | $27.78 | $472.22 |
| $300,000 at 6%, Fri Jun 7 – Mon Jun 10, 2024 (365) | 3 | $49.32 | $147.95 |
| $300,000 at 6%, Feb 28 – Mar 1, 2024 (365, leap year) | 2 | $49.32 | $98.63 |
The weekend row is worth a second look: Friday to Monday counts 3 days because interest accrues on calendar days, exactly as settlement agents figure it. And the second row matches the standard settlement example ($432,000 at 5%, September 12 to October 1 = 19 days at $59.18/day) to within four cents; the published example multiplies the rounded $59.18 by 19 to get $1,124.42, while this tool multiplies the unrounded daily figure for $1,124.38, which is the more exact total.
365 vs 360: the day count that quietly changes your number
Same loan, same dates, different basis. The $200,000 at 5% rows above show the gap plainly: $465.75 on 365, $472.22 on 360, a $6.47 difference over just 17 days.
Actual/365 (residential standard)
- Daily rate = annual rate ÷ 365.
- Used for virtually all residential mortgage prepaid interest and payoffs.
- On $200,000 at 5% for 17 days: $465.75.
Actual/360 (bank method)
- Daily rate = annual rate ÷ 360, about 1.4% higher per day.
- Common on commercial loans; over a year it effectively lifts a 5% rate to about 5.07%.
- On $200,000 at 5% for 17 days: $472.22.
If a lender’s per diem looks slightly higher than your own math, check the basis before anything else. The loan documents name it, and this calculator lets you switch between the two so you can reproduce either figure.
Where per diem interest lands at closing
Per diem interest shows up on your settlement statement as prepaid interest (sometimes labelled “odd-days interest”). Because mortgage payments are made in arrears, your first regular payment covers a full month, but the days between funding and the first of the next month belong to no payment period, so you prepay them at the closing table. It is one of the few closing figures that changes every day the closing slips, which is why timing your closing date matters: closing on the 28th means a couple of days of prepaid interest, while closing on the 3rd means nearly a full month of it. Either way the total interest over the loan barely changes; it just moves between the closing table and the first payments. Prepaid interest sits inside your overall closing costs, and if you are refinancing rather than buying, the same daily math prices the payoff of your old loan: interest from your last payment to the payoff date, which the cost to refinance calculator treats as part of the true refinance cost.
What competing calculators miss
Per diem explainers are everywhere, but working tools with honest math are rare:
- Prose-only explainers walk through the formula with a worked example but give you nothing to type your own numbers into. One widely copied example even gets its own arithmetic wrong, claiming $47.22 for a 17-day stretch that actually costs $472.22.
- Generic loan per diem tools compute the daily figure but stop there: no date-to-date day count, no 365/360 selector, no monthly equivalent, no closing context.
- The gap this page fills: real start and end dates with a calendar-day count (weekends included, as lenders do it), both day-count bases, the unrounded-math total disclosed honestly against the rounded shortcut, and the closing-table context that explains why you are paying it. Plus a clear fence around the topic: this is mortgage daily interest, not military travel per diem.
Frequently asked questions
The amount of interest your mortgage accrues per day, from the Latin “per day”. This page covers mortgage daily interest charged at closing or payoff, not military PCS per diem, which is a travel allowance for service members and a completely different topic.
Daily interest = loan balance × (annual rate ÷ 365). On $300,000 at 6%, that is $300,000 × (0.06 ÷ 365) = $49.32 per day. Multiply by the number of days: 10 days costs $493.15. The calculator does this with your exact dates and lets you switch to a 360-day basis.
Use 365 for residential mortgages; it is the standard convention. The 360-day “bank method” is mostly a commercial-loan convention and produces a slightly higher daily figure: on $200,000 at 5% over 17 days, the 360 basis gives $472.22 versus $465.75 on 365. Check your loan documents or ask your lender which one they use, because mixing them up explains most small disagreements.
Yes. Interest accrues on calendar days, so a Friday-to-Monday gap is 3 days, not 1. The calculator counts every calendar day between your two dates, which matches how lenders figure prepaid interest at closing.
This calculator divides by 365 (or 360) exactly as selected, all year round. Some lenders divide by 366 in leap years, which makes the daily figure a touch smaller. If your lender uses 366 and this tool uses 365, expect a tiny difference; confirm the convention on your closing disclosure.
Prepaid mortgage interest is generally treated as mortgage interest for tax purposes, and the IRS covers the deduction rules for home mortgage interest in Publication 936. The details depend on your situation, so confirm with a tax professional before relying on it.
Three usual reasons: the lender may multiply a rounded daily figure by the day count (this tool multiplies the unrounded figure, then rounds), use a 366-day divisor in a leap year, or use the 360-day basis. A few cents is normal; a few dollars means the inputs differ.
Twice in a loan’s life: at closing, as prepaid “odd-days” interest from the funding date to the first of the next month, collected with your closing costs; and at payoff, from your last regular payment to the payoff date, when you refinance or sell.
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References
- Consumer Financial Protection Bureau. What fees or charges are paid when closing on a mortgage and who pays them? Accessed October 23, 2026.
- Consumer Financial Protection Bureau. What is the difference between a mortgage interest rate and an APR? Accessed October 23, 2026.
- Consumer Financial Protection Bureau. How does paying down a mortgage work? Accessed October 23, 2026.
- Internal Revenue Service. Publication 936, Home Mortgage Interest Deduction. Accessed October 23, 2026.
- Internal Revenue Service. Topic No. 505, Interest Expense. Accessed October 23, 2026.
- Freddie Mac. What Is Amortization? Accessed October 23, 2026.
- U.S. Department of Housing and Urban Development. Buying a Home Accessed October 23, 2026.
- Board of Governors of the Federal Reserve System. A Consumer’s Guide to Mortgage Refinancings. Accessed October 23, 2026.
- Docketman Settlement Services. Prepaid Interest at Closing: Calculating the Per Diem. Accessed October 23, 2026.
- U.S. Department of Housing and Urban Development. Let FHA Loans Help You. Accessed October 23, 2026.