How to Calculate a Mortgage Payment: Formula and Examples

 

To calculate the principal-and-interest payment on a fully amortizing, fixed-rate mortgage, you need the amount borrowed, the fixed annual interest rate, and the number of monthly payments.

Use our Mortgage Payment Calculator for an estimate under a standard monthly-rate model. The result excludes property taxes, homeowners insurance, mortgage insurance, HOA fees, and other charges.

The mortgage payment formula

For a positive monthly interest rate:

M = P x r / (1 - (1 + r)^-n)

• M is the monthly principal and interest payment.
• P is the loan principal.
• r is the monthly rate as a decimal.
• n is the total number of monthly payments.

In this model, divide the nominal annual percentage rate by 1,200 to obtain r. Multiply the loan term in years by 12 to obtain n. Payments are assumed at the end of each monthly period.

Step 1: use the amount actually borrowed

The loan principal is not automatically the purchase price of the property. For example, a $375,000 purchase with a $75,000 down payment leaves $300,000 to finance before considering any financed fees or other adjustments.

Use the principal in the applicable loan estimate or documents. The calculator does not calculate a down payment, eligibility, or financing fees automatically.

Step 2: convert the fixed annual interest rate

For an illustrative fixed nominal annual rate of 6%, the monthly rate is 6 / 1,200 = 0.005, or 0.5% per month under this convention.

The note interest rate is not necessarily the same as an APR that incorporates other borrowing costs. Do not substitute a fee-inclusive APR into this formula without understanding the loan convention.

Step 3: convert the term to monthly payments

A 30-year monthly loan has 30 x 12 = 360 payments. A 15-year monthly loan has 180 payments.

The formula assumes full amortization over that period. Interest-only loans, balloon payments, adjustable rates, and other arrangements need different models.

Example: $300,000 at 6% over 30 years

Using P = 300,000, r = 0.005, and n = 360:

M = 300,000 x 0.005 / (1 - 1.005^-360)

The estimated monthly principal and interest payment is $1,798.65. Using the unrounded theoretical payment, total principal and interest over 360 months is approximately $647,514.57. Subtracting the $300,000 principal gives approximately $347,514.57 in interest.

These are illustrative calculations, not current rate offers or lender quotes. Multiplying the displayed cent-rounded payment by 360 will not exactly reproduce totals calculated from the unrounded payment.

What if the interest rate is zero?

At 0%, divide the principal by the number of payments:

M = P / n

For $300,000 over 360 payments, that is approximately $833.33 per month, with no interest in the model. A real payment schedule may adjust its final payment for cent rounding.

The full housing payment can be higher

Principal and interest alone do not represent every housing expense. Depending on the property and loan, your budget may also include:

• Property taxes.
• Homeowners insurance.
• Mortgage insurance.
• HOA or other association fees.
• Other loan charges, maintenance, and property expenses.

Some amounts may be collected through escrow and others paid separately. Use a lender's estimate and applicable documents to compare complete costs; this calculator does not determine those charges.

Why lender figures can differ

Actual amounts can depend on rounding, payment dates, daily interest, rate conventions, fees added to the principal, and the loan's specific terms. A fixed principal-and-interest payment does not mean that taxes or insurance remain fixed.

The calculator uses equal theoretical payments and totals based on the unrounded payment. It does not generate a lender-certified schedule or determine a final payoff quote.

Monthly, biweekly, and twice-monthly payments

Our current calculator models monthly payments only. Every two weeks commonly means 26 payment occasions per year, while twice monthly means 24. A half-monthly amount sent on each occasion does not have the same annual total under those two calendars.

How early or additional amounts affect interest and principal depends on the lender's application rules and loan terms. Do not assume a generic savings figure applies to your loan.

Frequently asked questions

Does a longer term reduce interest?

At the same principal and positive fixed rate, a longer amortization term generally lowers the required monthly payment but increases total interest if the loan runs to term with no extra payments. Compare both the payment and total cost.

Does the tool calculate affordability or approval?

No. It does not evaluate income, debt, credit, lending rules, or whether borrowing is suitable.

Can I use an adjustable rate?

The result assumes one fixed rate for the full term. It does not simulate future adjustments.

Estimate your monthly principal and interest

Enter the amount borrowed, fixed note rate, and term in the Mortgage Payment Calculator. Confirm the full payment components and loan terms with your lender.

This guide provides general mathematical examples, not personalized mortgage, financial, or tax advice, a loan approval, or a lender quote.