Per Diem Interest Calculator

Mortgage closings collect interest from closing day through end of month as a prepaid. This calculator sizes the per-diem amount so you can reconcile it on the Closing Disclosure.

Loan amount

Mortgage rate

Closing day of month

Days in closing month

Day-count basis

Per-diem interest at closing

$641

included in prepaids

Daily interest

$71.20

Days collected

How the math works

Per-diem interest covers the gap between closing and the start of the first full month. Mortgages conventionally have the first payment due the month AFTER the first full month — so a June 22 closing has per-diem through June 30, then a first mortgage payment August 1 covering July interest.

Closing early in the month collects more per-diem upfront but produces the same first-payment timing. Some buyers pay "interim interest" by closing late in the month to minimize out-of-pocket at closing. The tradeoff is a sooner first payment.

Editorial noteMaintained by EveryCalc - Reviewed June 2026

EveryCalc calculators are designed for fast, practical estimates with transparent inputs and no required account. We use plain formulas, visible assumptions, and related tools so visitors can check the result from more than one angle.

Results are informational only. For financial, tax, legal, medical, construction, or other high-impact decisions, verify the output against primary sources or a qualified professional.

Learn more about our review process on the EveryCalc methodology page.

How this calculator works

What this page estimates

This Per Diem Interest Calculator is built to give a quick, browser-based estimate for per diem interest. Mortgage closings collect interest from closing day through end of month as a prepaid. This calculator sizes the per-diem amount so you can reconcile it on the Closing Disclosure. The inputs stay on the page during normal use, and the result should be treated as an estimate for planning, comparison, or education rather than professional advice.

Calculation approach

The calculator applies the standard relationship implied by the inputs, then formats the answer so it can be checked and reused. For finance tools, the most important step is using consistent units, rates, time periods, and assumptions before comparing the result with another calculator or outside quote.

Example workflow

For example, start with a realistic value you already know, change one input at a time, and watch how the answer moves. That makes it easier to tell whether the result is being driven by the main amount, the rate, the time period, or a unit conversion.

Practical checks

Use current, real-world numbers when the result affects money, health, tax, or legal decisions.
Run a low, base, and high case when the inputs are estimates.
Check the related calculators below when the next decision depends on a different assumption.

How to interpret the per diem interest result

Best use

Use the result as a planning number for comparing payments, rates, returns, tax reserves, or cash-flow choices before you request a quote or make a commitment.

Cross-check

Compare the answer with the contract, lender estimate, tax form, brokerage statement, payroll record, or invoice that will control the real-world outcome.

Watch for

Do not rely on a single optimistic rate, return, or fee assumption. Money pages work best when you run low, base, and high cases and keep professional advice separate from the estimate.

This page belongs to the Finance calculator library, so the answer should be read in the context of the decision you are modeling rather than as a universal rule.

Before relying on this per diem interest estimate

Most calculator mistakes come from the inputs, not the arithmetic. Use this short audit before you reuse the answer in a spreadsheet, quote, application, or important conversation.

Confirm source numbers

Match balances, rates, fees, taxes, income, and payment dates against the lender quote, payroll record, tax form, statement, invoice, or contract.

Separate cash flow from total cost

A lower monthly payment can still cost more over time if fees, interest, taxes, or a longer term are hidden in the structure.

Run conservative cases

Test at least one higher-cost or lower-return case before using the output for a purchase, refinance, investment, loan, or tax decision.

Rerun this page when the rate, price, term, fee, tax rule, income, expense, or expected holding period changes.

How to Use

Enter loan amount and rate.
Enter the closing day of the month.
Enter days in the closing month.
Pick day-count basis — Actual/365 for most residential mortgages.

Frequently Asked Questions

Why is per-diem so high when I close early in the month?

More days to month-end means more interest. Closing on the 2nd collects ~28 days; closing on the 28th collects ~2 days. Late-month closings reduce cash needed at closing but accelerate the first payment.

Is per-diem negotiable?

The amount isn't, but the closing date is. If cash-tight, negotiate a late-month closing to reduce per-diem. If cash-flush, close early to use seller proceeds efficiently and stretch to the next first payment.